Patent Application
20130099245
The present invention relates to a field effect transistor, a method for producing the same, and an electronic device.
Field effect transistors (FETs) are widely used in various electronic devices. Examples of field effect transistors include field effect transistors disclosed in Patent Documents 1 to 3.
A field effect transistor (FET) disclosed in Patent Document 1 includes: a channel portion made of a first GaN-based semiconductor material; and first and second electron supply portions each made of a second GaN-based semiconductor material with a band gap greater than the first GaN-based semiconductor material, joined to the channel portion, and located separately from each other. The channel portion is formed so as to electrically connect to source and drain electrodes. An insulating film is formed on the surface of the channel portion, and a gate electrode is formed on the insulating film. According to the disclosure of Patent Document 1, this FET is of a normally-off type, has an extremely small on-state resistance during an operation, and can achieve a large-current operation.
Patent Document 2 discloses a metal-insulating film-semiconductor (MIS) type FET including: a GaN buffer layer; an AlGaN electron supply layer; and a GaN stress relaxation layer, being laminated in this order. This structure is to suppress generation of carriers caused by a polarization effect by neutralizing polarization charges through making the buffer layer and the stress relaxation layer of the same material of GaN and to enable the FET to achieve a normally-off operation.
Patent Document 3 reports a recess gate-type FET including: a GaN carrier travel layer; an AlGaN barrier layer; and a GaN threshold value control layer, being laminated in this order. This structure also is to suppress generation of carriers caused by a polarization effect by neutralizing polarization charges through making the carrier travel layer and the threshold value control layer of the same material of GaN and to enable the FET to achieve a normally-off operation. According to Patent Document 3, the polarization charges are compensated by the structure, so that an electric field is not generated in the direction perpendicular to a substrate of the threshold value control layer in the pinch-off state, and even when the recess depth, i.e., the thickness of the remaining threshold value control layer in the recess portion is changed, it is difficult to change the threshold voltage Vth.
Patent Document 4 discloses a FET including: a lower barrier layer made of AlGaN; and a channel layer that is laminated on the lower barrier layer, has a band gap less than the lower barrier layer, and is made of an InGaN layer having a compressive strain. Patent Document 5 discloses a FET including: a lattice-relaxed AlGaN lower barrier layer; an InGaN channel layer having a compressive strain, and an AlGaN contact layer, being laminated in this order. These FETs exert superior characteristics of being capable of achieving a normally-off operation and the like.
Patent Document 1: WO 2003/071607
Patent Document 2: JP 2004-335960 A
Patent Document 3: JP 2007-067240 A
Patent Document 4: WO 2009/081584
Patent Document 5: WO 2009/113612
In order to increase an electric power and decrease a loss (save energy) in an electronic device (electronic equipment) using a field effect transistor (FET), the field effect transistor (FET) needs to achieve both of a high threshold voltage and a low on-state resistance. However, none of the field effect transistors (FETs) of Patent Documents 1-3 achieves both of a high threshold voltage and a low on-state resistance. Even though a high threshold voltage and a low on-state resistance can be obtained according to the field effect transistors (FETs) of Patent Documents 4-5, they need to further improve performance thereof in order to further increase an electric power and decrease a loss (save energy) in an electronic device (electronic equipment).
Hence, the present invention is intended to provide a field effect transistor which can achieve both of a high threshold voltage and a low on-state resistance, a method for producing the same, and an electronic device.
In order to achieve the aforementioned object, the first field effect transistor of the present invention is a field effect transistor including: a substrate; a buffer layer; a channel layer; a barrier layer; a spacer layer; a gate insulating film; a gate electrode; a source electrode; and a drain electrode, wherein the buffer layer is formed of lattice-relaxed AlxGa1-xN (0≦x<1), the channel layer is formed of AlxGa1-xN (0≦x<1) with the same composition as the buffer layer, the barrier layer is formed of AlzGa1-zN (x<z≦1) with an Al composition ratio greater than the buffer layer, the spacer layer is formed of AluGa1-uN (0≦u<z) with an Al composition ratio less than the barrier layer, at least one of the semiconductor layers formed below the gate electrode is a p-type layer, each of the upper surfaces of the buffer layer, the channel layer, the barrier layer, and the spacer layer is a Ga plane or an Al plane that is perpendicular to a (0001) crystal axis, the buffer layer, the channel layer, the barrier layer, and the space layer are laminated on the substrate in this order, the gate insulating film is arranged on the spacer layer, the gate electrode is arranged on the gate insulating film, and the source electrode and the drain electrode are electrically connected to the channel layer directly or via another component.
The second field effect transistor of the present invention is a field effect transistor including: a substrate; a buffer layer; a channel layer; a barrier layer; a spacer layer; a gate electrode; a gate insulating film; a source electrode; and a drain electrode, wherein each of the buffer layer, the channel layer, the barrier layer, and the spacer layer is formed of a group-III nitride semiconductor, each of the upper surfaces of the buffer layer, the channel layer, the barrier layer, and the spacer layer is a group-III atomic plane that is perpendicular to a (0001) crystal axis, the buffer layer and the channel layer are lattice-relaxed, the barrier layer has a tensile strain, the buffer layer, the channel layer, the barrier layer, and the spacer layer are laminated on the substrate in this order, the gate insulating film is arranged on the spacer layer, the gate electrode is arranged on the gate insulating film, and the source electrode and the drain electrode are electrically connected to the channel layer directly or via another component.
A method for producing a field effect transistor, the method including the steps of: laminating semiconductor layers including a buffer layer, a channel layer, a barrier layer, and a spacer layer on a substrate in this order; forming a gate insulating film on the spacer layer; forming a gate electrode on the gate insulating film; and forming a source electrode and a drain electrode so as to electrically connect to the channel layer directly or via another component, wherein in the step of forming semiconductor layers, each of the buffer layer, the channel layer, the barrier layer, and the spacer layer is grown on a Ga plane or an Al plane that is perpendicular to a (0001) crystal axis, the buffer layer is formed of lattice-relaxed AlxGa1-xN (0≦x<1), the channel layer is formed of AlxGa1-xN (0≦x<1) with the same Al composition ratio as the buffer layer, the barrier layer is formed of AlzGa1-zN (x<z≦1) with an Al composition ratio greater than the buffer layer, the spacer layer is formed of AluGa1-uN (0≦u<z) with an Al composition ratio less than the barrier layer, and at least one of the semiconductor layers formed below the gate electrode is a p-type layer.
The method for producing a second field effect transistor of the present invention is a method for producing a field effect transistor, the method including the steps of: laminating semiconductor layers including a buffer layer, a channel layer, a barrier layer, and a spacer layer on a substrate in this order; forming a gate insulating film on the spacer layer; forming a gate electrode on the gate insulating film; and forming a source electrode and a drain electrode so as to electrically connect to the channel layer directly or via another component, wherein in the step of forming semiconductor layers, each of the buffer layer, the channel layer, the barrier layer, and the spacer layer is grown on a group-III atomic plane that is perpendicular to a (0001) crystal axis, the buffer layer and the channel layer are formed so as to be lattice-relaxed, the barrier layer is formed so as to have a tensile strain, and at least one of the semiconductor layers formed below the gate electrode is formed as a p-type layer.
The electronic device of the present invention includes the first or second field effect transistor of the present invention.
The present invention can provide a field effect transistor which can achieve both of a high threshold voltage and a low on-state resistance, a method for producing the same, and an electronic device.
In the field effect transistor of the present invention, the “on-state resistance” is an electric resistance between a side to which a positive bias is applied and a side to which a negative bias is applied (e.g., between a source electrode and a drain electrode, or between an anode and a cathode) when a voltage is on (a voltage is applied). The “contact resistance” is an electric resistance between an ohmic electrode and a two-dimensional electron gas (2DEG).
In the present invention, unless otherwise indicated, a state of being “on the upper side” is not limited to a state of being directly in contact with the upper surface and includes a state of being indirectly in contact with the upper surface, i.e., being above the upper surface, via any other component. Similarly, unless otherwise indicated, a state of being “on the lower side” may be a state of being directly in contact with the lower surface or a state of being indirectly in contact with the lower surface, i.e., being below the lower surface, via any other component. A state of being “on the upper surface” indicates a state of being directly in contact with the upper surface. Similarly, a state of being “on the lower surface” indicates a state of being directly in contact with the lower surface.
In the present invention, when the n-type impurity (donor impurity) concentration, a p-type impurity concentration, or the like is represented by the volume density (cm-3 or the like), it is represented by the volume density with respect to the number of atoms unless otherwise indicated. Similarly, when the effective dose or the like of n-type impurity ions is represented by the area density (cm-2 or the like), it is represented by the area density with respect to the number of atoms unless otherwise indicated. The “effective dose” is an actual dose which has reached the upper surface of an electron absorption layer after subtracting a loss caused by being absorbed in the through film and the like.
In the present invention, the concentration of an ionized impurity is the concentration in the state where a voltage is not applied to any of electrodes in a field effect transistor unless otherwise indicated.
In the present invention, the “composition” represents a quantitative relationship among the numbers of atoms of elements composing a semiconductor layer or the like. The “composition ratio” is a relative ratio of the number of atoms of a specific element composing the semiconductor layer or the like to the number of atoms of the other elements. For example, in a semiconductor layer represented by the composition of AlxGa1-xN, the numerical value of x is an “Al composition ratio”. In the present invention, when the composition or the composition ratio of a semiconductor layer is defined, an impurity (dopant) which causes conductive properties and the like to exert is not considered as an element composing the semiconductor layer. For example, a p-type GaN layer and an n-type GaN layer contain different impurities (dopants) however have the same composition. Moreover, for example, in the case of an n-type GaN layer and an n+GaN layer with an impurity concentration greater than the n-type GaN layer, they have the same composition.
In the present invention, the “main surface” of a substrate, a semiconductor layer, or the like is a surface with the largest area and is, for example, the upper surface or the lower surface, or the front side or the back side.
In the present invention, the “threshold voltage” is a gate voltage at the critical point at which the carrier density in the channel layer becomes positive from 0. The threshold voltage may be represented by a symbol Vth.
In figures, structures of components described below may be simplified as a matter of convenience of illustration, and dimensional ratios of the respective components may differ from the actual ratios. Moreover, mathematical formulae, graphs, and descriptions thereof are based on theoretical calculations and qualitatively or approximately represents actual phenomena occurred in a FET and the like.
The inventors of the present invention independently examined a threshold voltage and an on-state resistance of a field effect transistor (FET) by theoretical calculations and the like and found the following.
The cross-sectional view of
The graph of
As shown in
Next, the dependency of the gate voltage on the carrier density when the thickness of Al2O3 as the gate insulating film 94 is changed in the range from 30 to 70 nm in the FET of
The Vth of the FET is determined according to the difference between the Schottky barrier height and the conduction band offset at the interface between the insulating film and the channel layer. Therefore, for example, as shown in
In the FET of Patent Document 2, the mobility of 2DEG generated at the interface between the AlGaN electron supply layer and the GaN buffer layer is from 1000 to 2000 cm2/Vs which is high, so that it becomes possible to reduce the on-state resistance. However, the polarization charges are compensated. Thus, an electric field in the direction perpendicular to the substrate is not generated in the gate insulating film in the pinch-off state. Therefore, even when the thickness of the gate insulating film is changed, the change in Vth is small.
In the FET of Patent Document 3, the mobility of 2DEG generated at the interface between the AlGaN barrier layer and the GaN carrier travel layer is from 1000 to 2000 cm2/Vs which is high, so that it is possible to reduce the on-state resistance. However, the small change in Vth by the change in thickness of an epitaxial layer is equivalent to the small design flexibility of Vth, so that it is difficult to increase the Vth.
According to the results of the examinations conducted by the inventors of the present invention, it is difficult to make the threshold voltage Vth to be greater than 2 V in any of the FETs of Patent Documents 1-3. Moreover, the FETs of Patent Documents 1-3 cannot achieve both of a high threshold voltage and a low on-state resistance.
A purpose of the present invention is, as mentioned above, to provide a field effect transistor (FRT) which can achieve both of a high threshold voltage and a low on-state resistance.
The embodiments of the present invention are described below. It is, however, to be noted that the following embodiments are mere examples, and the present invention is not limited thereby. As described above, actual phenomena occurred in the FET and the like of the present invention may not completely agree with the theoretical descriptions based on the mathematical formulae, graphs, and the like. In the present invention, in the case where the present invention is specified by numerical limitations, it may be strictly specified by the numerical value or may be roughly specified by the numerical value. For example, when the Al composition ratio is “0.4 or more”, it may be strictly 0.4 or more or may be about 0.4 or more.
The cross-sectional view of
As shown in
As shown in
The cross-sectional view of
In the FET of the present invention, “lattice-relaxation” represents the state where the lattice constant of a thin film (a semiconductor layer composing the FET) agrees with the lattice constant of its bulk materials. In a semiconductor crystal, the “bulk material” is a semiconductor crystal in the state where the effects of the surface, the interface, and the edge thereof can be ignored. In the state where a lattice constant agree with the lattice constant of a bulk material, the lattice constant does not need to strictly agree with the lattice constant of the bulk material and may agree with it with an error of ±0.1% or less, for example. The error is preferably ±0.03% or less, more preferably ±0.01% or less, and ideally 0. Moreover, in the present invention, the “lattice-relaxed” layer may not be a layer which has been entirely lattice-relaxed, and it is only necessary that a part of the layer has been lattice-relaxed. For example, the buffer layer is, as mentioned above, a lattice-relaxed layer. When the lattice constant of the substrate differs from that of the buffer layer, and there is no layer having a lattice relaxation effect between them, it is necessary that the buffer layer has a function to release strain energy by generating a dislocation and a sufficient thickness to reduce the effect of the dislocation. When the buffer layer has a sufficient thickness and no component is laminated thereon, the lattice constant of the topmost surface (that is the lattice plane located on the top and represents the same as the “upper surface”) of the buffer layer agrees with that of its bulk semiconductor with the same composition as the buffer layer. When a thin film semiconductor layer with the same composition as such a buffer layer is epitaxially grown on the buffer layer, generation of new dislocation is suppressed. In contrast, when the lattice constant of the substrate is equal to that of the buffer layer, the effect of the dislocation may be ignored, and it is, however, necessary that the buffer layer has a relatively large thickness in order to suppress the effects of a crystal defect and an interface state at the substrate-buffer layer interface. The appropriate thickness of the buffer layer depends on the difference in lattice constant between the substrate and the buffer layer and the state of the substrate-buffer layer interface and is, however, from 0.1 to 10 μm in general.
A general fact (physical law) on generation of interface charges caused by the polarization effect in an AlGaN heterojunction is described below.
When an AlaGa1-aN layer (a<x) with an Al composition ratio less than AlaGa1-xN is grown on an AlxGa1-xN layer having a lattice-relaxed (0001) plane, a compressive strain is generated in the AlaGa1-aN layer, so that interface charges are generated by piezopolarization. Moreover, the difference in spontaneous polarization between the layers are added as interface charges, so that negative charges with the surface density (−σa) are generated on the substrate side of the AlaGa1-aN layer, and positive charges with the surface density (+σa) are generated on the surface side (side opposite to the substrate). The absolute value σa of the surface density of polarization charges is increased in approximate proportion to the difference (x−a) in composition ratio. That is, the σa is approximately represented by the following mathematical formula (1). In the following mathematical formula (1), q represents the elementary charge, q=1.60219×10-19C. The same applies to the following formulae unless otherwise indicated.
σa/q[cm-2]=5.3×1013×(x−a) (1)
Even when another semiconductor layer is inserted between the AlxGa1-xN layer and the AlaGa1-aN layer, the same interface charges are generated unless the semiconductor layer has been lattice-relaxed.
On the other hand, when an AlbGa1-bN layer (x<b) with an Al composition ratio greater than AlxGa1-xN is grown on an AlxGa1-xN layer having a lattice-relaxed (0001) plane, a tensile strain is generated in the AlbGa1-bN layer, so that interface charges are generated by piezopolarization. Moreover, the difference in spontaneous polarization between the layers are added as interface charges, so that positive charges with the surface density (+σb) are generated on the substrate side of the AlbGa1-bN layer, and negative charges with the surface density (+σa) are generated on the surface side (side opposite to the substrate). The absolute value σb of the surface density of polarization charges is increased in approximate proportion to the difference (b−x) in composition ratio. That is, the σb is approximately represented by the following mathematical formula (2).
σb/q[cm-2]=6.4×1013×(b−x) (2)
Even when another semiconductor layer is inserted between the AlxGa1-xN layer and the AlbGa1-bN layer, the same interface charges are generated unless the semiconductor layer has been lattice-relaxed.
Based on these facts, an example of formation of interface charges in the FET of the present invention is described below with reference to
That is, when an AlxGa1-xN layer 113 with the same Al composition ratio as an AlxGa1-xN buffer layer 112 having a lattice-relaxed (0001) plane is grown on the buffer layer 112, polarization charges are compensated at the heterointerface. Thus, interface charges are not generated. When an AlzGa1-zN layer 114 (x<z) with an Al composition ratio greater than the buffer layer 112 is grown on this AlxGa1-xN layer 113, positive charges with the surface density (+σ2) are generated on the substrate side of the AlGaN layer 114, and negative charges with the surface density (−σ2) are generated on the surface side based on the mathematical formula (2). When an AluGa1-uN layer 115 (u<x) with an Al composition ratio less than the buffer layer 112 is grown on this AlzGa1-zN layer 114, negative charges with the surface density (−σ3) are generated on the substrate side of the AlGaN layer 115, and positive charges with the surface density (+σ3) are generated on the surface side (side opposite to the substrate) based on the mathematical formula (1). When the Al composition ratio of the AluGa1-uN layer 115 is equal to that of the buffer layer 112 (u=x), interface charges are not generated (σ3=0). Furthermore, when an AlvGa1-vN layer 116 (x<v) with an Al composition ratio greater than the buffer layer 112 is grown on this AluGa1-uN layer 115, positive charges with the surface density (+σ4) are generated on the substrate side of the AlGaN layer 116, and negative charges with the surface density (−σ4) are generated on the surface side based on the mathematical formula (2).
As described above, positive charges as surface charges (+σ2) are generated at the interface between the AlGaN layer 113 and the AlGaN layer 114. Similarly, negative charges with the surface density (−σ2−σ3) are generated at the interface between the AlGaN layer 114 and the AlGaN layer 115, and positive charges as surface charges (+σ3+σ4) are generated at the interface between the AlGaN layer 115 and the AlGaN layer 116. Although negative charges as surface charges (−σ4) are generated at the topmost surface of the AlGaN layer 116, they are compensated by the interface state between the surface protective film 12 and the AlGaN layer 116.
The sum of interface charges between source-gate and between gate-drain under the electron supply layer 116 is (+σ2)+(−σ2−σ3)+(+σ3+σ4)=+σ4. That is, positive fixed charges are generated. In addition to these interface charges, positive fixed charges are generated by ionizing the n-type impurity added to the electron supply layer 116. Thus, negative fixed charges generated by ionizing the p-type impurity in the AlGaN channel layer 113 can be compensated. Therefore, 2DEG (17) is generated in each of the channel layer 113 and the spacer layer 115 each with an Al composition ratio less than the buffer layer 112. In contrast, the sum of interface charges under the gate electrode 15 is (+σ2)+(−σ2−σ3)=−σ3 which is negative. Therefore, the channel is depleted in the thermal equilibrium state (Vg=0 V), so that 2DEG is not formed.
Next, a method for producing a FET is described. The method for producing a FET of the present invention is not particularly limited. It is however preferred that the FET of the present invention is produced by the first or second method for producing a FET of the present invention. A method for producing a FET shown in
First, a nucleation layer 111 (200 nm) made of a superlattice obtained by laminating alternately undoped AlN and undoped GaN, a buffer layer 112 (1 μm) made of undoped GaN, a channel layer 113 made of p-type GaN, a barrier layer 114 made of undoped AlN, a spacer layer 115 made of undoped GaN, and an electron supply layer 116 made of n-type Al0.2Ga0.8N are grown, in this order, on a silicon (Si) substrate 100 having a (111) plane by, for example, Metalorganic Chemical Vapor Deposition (MOCVD) (a step of laminating semiconductor layers). In this step, the crystal growth is growth on a Ga plane or an Al plane which is perpendicular to a (0001) crystal axis. The thickness of each of the AlN layer 114 and the Al0.2Ga0.8N layer 116 is less than the critical thickness at which a dislocation occurs on the GaN buffer layer. Thus, the occurrence of dislocation is suppressed, and favorable crystal quality can be obtained.
Magnesium (Mg) or zinc (Zn) is used as the p-type impurity, for example. Si is used as the n-type impurity, for example. An appropriate n-type impurity concentration of the electron supply layer 116 is, for example, about 1×1017 cm-3 or more to about 1×1020 cm-3 less. Undoped semiconductor layers are used as semiconductor layers 112, 114, and 115. The semiconductor layers 112, 114, and 115 may be, for example, p-type or n-type semiconductor layers each with an impurity concentration of about 1×1017 cm-3 or less.
Further, metals such as titanium (Ti)/aluminum (Al)/nickel (Ni)/gold (Au) are deposited on the electron supply layer 116, which is then subjected to alloy treatment, for example. Thus, a source electrode 161 and a drain electrode 162 are formed and are in ohmic contact with the channel layer 113 (a step of forming a source electrode and a drain electrode). Then, for example, 50 nm of a surface protective film 12 made of an insulator such as silicon nitride (Si3N4) is deposited using Plasma-Enhanced Chemical Vapor Deposition (PECVD), for example. An opening portion is formed in a part sandwiched between the source electrode 161 and the drain electrode 162 by etching the surface protective film 12 using reactive gas such as sulfur hexafluoride (SF6). Thereafter, for example, a recess portion 13 is formed by removing parts of the AlGaN electron supply layer 116 and GaN spacer layer 115 under the opening portion by etching using reactive gas such as boron chloride (BCl3), for example. Then, a gate insulating film 14 such as Al2O3 is deposited so as to be filled in the recess portion 13, using, for example, atomic layer deposition (ALD) (a step of forming a gate insulating film). Furthermore, for example, metals such as Ni/Au are deposited on the gate insulating film 14, and lift-off procedures are performed, so that a gate electrode 15 is formed (a step of forming a gate electrode).
By such a method for producing a FET, a FET having the structure of
As shown in
AN forming the barrier layer 114 and GaN forming the channel layer 113 are flat at the atomic layer level. Therefore, scattering at the interface is suppressed, so that the mobility of channel electrons is from 1000 to 2000 cm2/Vs which is high. Thus, a low on-state resistance can be obtained. In the present embodiment, the sum (−σ3/q) of fixed charges present on the substrate side of the gate insulating film 14 is calculated to be −5.3×1012 cm-2. An electric field directed from the surface of the gate insulating film 14 toward the substrate is generated in the gate insulating film 14 in pinch-off (Vg=0 V) due to this negative fixed charges. Therefore, the Vth can be increased by increasing the thickness of the gate insulating film 14.
In the first FET of the present invention, the channel layer and the buffer layer are formed of AlxGa1-xN (0≦x<1) with the same composition as mentioned above. The impurity concentration, the conductivity type (p-type, n-type, i-type, or the like), and the like of the channel layer may be identical to or different from those of the buffer layer, for example. The channel layer and the buffer layer may be formed as different layers which are distinguishable from each other by setting the impurity concentration, the conductivity type (p-type, n-type, i-type, or the like), and the like of the channel layer to be different from those of the buffer layer, for example. The channel layer and the buffer layer may be formed integrally (as a single layer). In the second FET of the present invention, the composition of the channel layer may be identical to or different from that of the buffer layer. In the second FET, the channel layer and the buffer layer may be formed as different layers which are distinguishable from each other by setting the impurity concentration, the conductivity type (p-type, n-type, i-type, or the like), and the like of the channel layer to be different from those of the buffer layer, for example. The channel layer and the buffer layer may be formed integrally (as a single layer).
As can be seen from
As can be seen from
As can be seen from
As can be seen from
As can be seen from
In the first FET of the present invention, the Al composition ratio x of the channel layer and the Al composition ratio u of the spacer layer are only necessary to satisfy the above-described relational formulae (0≦x<z, 0≦u<z). However, from the viewpoint of reducing no electron mobility, the Al composition ratio x and the Al composition ratio u preferably are not too high. Specifically, the Al composition ratio x and the Al composition ratio u are preferably 20% (0.2) or less.
The cross-sectional view of
In such a FET, the magnitude relationship between the Al composition ratios of the AlGaN spacer layer 215 and the buffer layer 112 is opposite to that in the first embodiment. Thus, the direction of polarization occurred in the spacer layer 215 is reversed. That is, electric charges with the surface density (−σ2+σ3) are generated at the interface between the AlGaN layer 114 and the AlGaN layer 215, and electric charges as surface charges (−σ3+σ4) are generated at the interface between AlGaN layer 215 and the AlGaN layer 116. Therefore, the sum of interface charges under the gate electrode 15 is (+σ2)+(−σ2+σ3)=+σ3, i.e., positive fixed charges are generated. In order to conduct a normally off operation in which the channel is depleted in the thermal equilibrium state (Vg=0V), it is necessary to make the surface density of the p-type ions (ionized p-type impurity) in the channel layer 113 greater than that (σ3/q) of these positive electric charges. For example, when the channel layer 113 has a p-type ion concentration (ionized p-type impurity concentration) of 1×1018 cm-3 and a thickness of 0.2 μm, the surface density of the p-type ions (ionized p-type impurity) becomes 2×1013 cm 2. The amount (σ3/q) of polarization charges generated in the spacer layer 215 can be approximated as 6.4×1013×(u−x) [cm 2]using the Al composition ratio u of the spacer layer 215 and the Al composition ratio x of the buffer layer 112 based on the mathematical formula (2). The calculated condition for making the surface density of the p-type ions (ionized p-type impurity) exceed σ3/q is u−x<0.31. As can be seen from the above fact, for example, in the case where the buffer layer is GaN (x=0.0), it is possible to conduct a normally off operation as long as the spacer layer 215 has an Al composition ratio u of 31% or less. From the viewpoint of obtaining higher Vth, the spacer layer preferably has an Al composition ratio of 20% or less.
From the viewpoint of enabling the first or second FET of the present invention to conduct a normally off operation, it is preferred that the Al composition ratio x of the buffer layer and the Al composition ratio u of the spacer layer satisfy u>x, and the surface density of p-type ions in the p-type layer is 6.4×1013 cm-2×(u−x). In the first or second FET of the present invention, conditions under which a normally off operation can be conducted however are not limited to this.
The cross-sectional view of
The FET having the structure of
In the FET of
As described above, in the FET of the present invention, it is preferred that the n-type impurity containing region is formed in at least a part under the source electrode or the drain electrode and includes at least a part of the barrier layer. Moreover, it is more preferred that the n-type impurity containing region is formed from each of the lower surfaces of the source electrode and the drain electrode to at least the inside of the channel layer. The n-type impurity containing region has an n-type impurity concentration of, for example, 1017 cm-3 or more, preferably 1018 cm-3 or more, more preferably 1019 cm3 or more. The upper limit of the n-type impurity concentration in the n-type impurity containing region is, for example, 1022 cm-3 or less.
The cross-sectional view of
The structure of the FET according to the present embodiment is described below. That is, first, this EFT does not include an electron supply layer 116 as shown in
The FET of
First, a nucleation layer 111 (200 nm) made of a superlattice obtained by laminating alternately undoped AN and undoped GaN, a buffer layer 112 (1 μm) made of undoped GaN, a channel layer 113 made of p-type GaN (with an impurity concentration: 1×1018 cm-3), a barrier layer 114 made of undoped AlN, and a spacer layer 115 made of undoped GaN are grown, in this order, on a silicon (Si) substrate 100 having a (111) plane by, for example, MOCVD (a step of laminating semiconductor layers). In this step, the crystal growth is growth on a Ga plane or an Al plane which is perpendicular to a (0001) crystal axis. The thickness of the AlN layer 114 is less than the critical thickness at which a dislocation occurs on the AlGaN buffer layer. Thus, the occurrence of dislocation is suppressed, and favorable crystal quality can be obtained.
Undoped semiconductor layers are used as the semiconductor layers 112, 114, and 115. The semiconductor layers 112, 114, and 115 may be, for example, p-type or n-type semiconductor layers each with an impurity concentration of about 1×1017 cm-3 or less, for example.
Then, a resist pattern is formed in a semiconductor layer structure produced as described above by normal lithography, so that a region in which a gate electrode is formed is covered. Thereafter, for example, a part of the GaN spacer layer 115 is removed by etching with mixed gas of BCl3 and oxygen (O2), so that the upper surface of the AlN barrier layer 114 is exposed. Thus, an ohmic recess portion 43 is formed. The etching selectivity ratio between GaN and AlN can be 5 or more by appropriately selecting a gas flow ratio between BCl3 and O2, and the AlN layer 114 can be used as an etching stop layer.
Further, metals such as Ti/Al/Ni/Au are deposited on the AlN barrier layer 114 in the ohmic recess portions 43, which is then subjected to alloy treatment, for example. Thus, a source electrode 161 and a drain electrode 162 are formed and are in ohmic contact with the channel layer 113 (a step of forming a source electrode and a drain electrode). Then, for example, 50 nm of a surface protective film 12 made of an insulator such as silicon nitride (Si3N4) is deposited using PECVD, for example. An opening portion (opening portion to be filled) is formed on the GaN spacer layer 115 by etching the surface protective film 12 using reactive gas such as sulfur hexafluoride (SF6), for example. Then, about 50 nm of a gate insulating film 14 such as Al2O3 is deposited so as to be filled in the opening portion, using ALD, for example. Furthermore, metals such as Ni/Au are deposited, and lift-off procedures are performed, so that a gate electrode 15 is formed (a step of forming a gate electrode). As described above, the FET of
In such a FET, the layer structure under the gate electrode (under the gate) is exactly the same as that of the first embodiment shown in
In the method for producing a FET of the present invention, described in the first embodiment, a gate electrode is formed so as to be in contact with the spacer layer exposed by removing an electron supply layer by etching. According to such a method, the thickness of the spacer layer under the gate electrode is determined depending on the depth of the recess formed by etching, and the Vth may be changed depending on the fluctuation of the etching rate. In contrast, in the method for producing a FET of the present invention, described in the present embodiment, a gate electrode is formed on the topmost surface of the semiconductor layer structure, and ohmic electrodes are formed in the ohmic recess portion obtained by removing the spacer layer by etching. As described above, according to the structure of the FET according to the present embodiment, the thickness of the spacer layer under the gate electrode can be determined depending on only the epi structure (without the effect of the etching depth), and in-plane evenness and repeatability of the Vth can be further enhanced. The methods for producing a FET in the first embodiment and the present embodiment are mere examples and are not limited thereby. Moreover, the characteristics of the FETs of the first embodiment and the present embodiment are not limited by the above description. For example, a FET with superior in-plane evenness and repeatability of the Vth can be obtained by the means of appropriately controlling the etching rate or the like in the steps of producing the FET of the first embodiment.
In the present embodiment, an n-type impurity addition region may be formed in the channel layer 113 and the barrier layer 114 under each of the source electrode 161 and the drain electrode 162 as in the third embodiment. With the n-type impurity addition region, the contact resistance components caused by the conduction band barrier formed in the barrier layer are reduced, and the on-state resistance is further reduced as in the second embodiment.
The cross-sectional view of
The FET of the present embodiment is characterized in that the AlGaN channel layer 513 is undoped, and a p-type impurity is added to the AlGaN buffer layer 512. As the p-type impurity, magnesium (Mg), zinc (Zn), or the like is used, for example. An appropriate concentration of the p-type impurity is, for example, about 1×1017 cm-3 or more to about 1×1018 cm-3 or less as an ionic concentration (p-type ion concentration). The structure of the FET of
In such a FET, negative fixed charges are generated by ionizing the p-type impurity in the buffer layer. Therefore, the sum of the fixed charges under the gate electrode becomes negative as in the first embodiment. Thus, it becomes possible to conduct a normally-off operation. An impurity is not added to the channel of such a FET, so that the mobility of 2DEG is further improved, and the on-state resistance is further reduced.
In the present embodiment, a p-type impurity is added to the buffer layer 512. A p-type impurity may be added to at least one (at least a part) of semiconductor layers such as a barrier layer 114 and a spacer layer 115 as long as the semiconductor layers are below the gate electrode. That is, for example, in the FET of
In the present embodiment, an n-type impurity addition region (n-type impurity containing region) may be formed in the electron supply layer 116, the spacer layer 115, the barrier layer 114, the channel layer 513 under each of the source electrode 161 and the drain electrode 162 as in the third embodiment. With the n-type impurity addition region, the contact resistance components caused by the conduction band barriers formed in the electron supply layer and the barrier layer are reduced, and the on-state resistance is further reduced as in the third embodiment.
The sixth embodiment of the FET of the present invention is described below.
As the first to fifth embodiments, embodiments in each of which group-III elements contained in a laminate structure including the buffer layer, the channel layer, the barrier layer, and the spacer layer are gallium (Ga) and aluminum (Al) are described. As the following sixth to fifteenth embodiments, embodiments in each of which the laminate structure contains indium (In) are described. The sixth to fifteenth embodiments are embodiments of the second field effect transistor of the present invention.
Prior to the description of the sixth to fifteenth embodiments, an a axis length of the group-III nitride semiconductor represented by the composition of InxAlyGa1-x-yN shown in a graph (contour plot) of
a(x, y)=3.548x+3.112y+3.189 (1−x−y) (3)
The cross-sectional view of
1012: Inx1Alx2Ga1-x1-x2N buffer layer
1013: p-type Inx1Alx2Ga1-x1-x2N channel layer
1014: Inz1Alz2Ga1-z1-z2N barrier layer
1015: Inu1Alu2Ga1-u1-u2N spacer layer
1016: Inv1Alv2Ga1-v1-v2N electron supply layer
It is to be noted that the composition ratios are set so that the buffer layer 1012, the channel layer 1013, the barrier layer 1014, the spacer layer 1015, and the electron supply layer 1016 satisfy the following mathematical formulae (4) to (6) based on the mathematical formula (3) and
a(z1, z2)<a(x1, x2) (4)
a(z1, z2)<a (u1, u2) (5)
a(v1, v2)<a(x1, x2) (6)
The cross-sectional view of
1112: AlxGa1-xN buffer layer (a axis length: a (0, x))
1113: p-type AlxGa1-xN channel layer (a axis length: a (0, x))
1114: AlzGa1-zN barrier layer (a axis length: a (0, z))
1115: InuAl1-uN spacer layer (a axis length: a (u, 1−u))
1116: AlvGa1-vN electron supply layer (a axis length: a (0, v))
It is to be noted that the composition ratios are set so that the buffer layer 1112, the channel layer 1113, the barrier layer 1114, the spacer layer 1115, and the electron supply layer 1116 satisfy the mathematical formulae (7) to (10) (e.g., x=0.1, z=1.0, u=0.18, v=0.2).
0≦x<1 (7)
x<z (8)
−0.177z+0.177<u (9)
x<v (10)
As can be seen from the mathematical formula (3) and
a(0, z)<a(0, x) (11)
a(0, z)<a(u, 1−u) (12)
a(0, v)<a(0, x) (13)
The cross-sectional view of
1212: AlxGa1-xN buffer layer (a axis length: a (0, x))
1213: p-type AlxGa1-xN channel layer (a axis length: a (0, x))
1214: AlzGa1-zN barrier layer (a axis length: a (0, z))
1215: InuGa1-uN spacer layer (a axis length: a (u, 0))
1216: AlvGa1-vN electron supply layer (a axis length: a (0, v))
It is to be noted that the composition ratios are set so that the buffer layer 1212, the channel layer 1213, the barrier layer 1214, the spacer layer 1215, and the electron supply layer 1216 satisfy the following mathematical formulae (14) to (17) (e.g., x=0.0, z=1.0, u=0.05, v=0.2).
0≦x<1 (14)
x<z (15)
0<u (16)
x<v (17)
As can be seen from the mathematical formula (3) and
a(0, z)<a(0, x) (18)
a(0, z)<a(u, 0) (19)
a(0, v)<a(0, x) (20)
The cross-sectional view of
1312: InxAl1-xN buffer layer (a axis length: a (x, 1−x))
1313: p-type InxAl1-xN channel layer (a axis length: a (x, 1−x))
1314: AlzGa1-zN barrier layer (a axis length: a (0, z))
1315: InuAl1-uN spacer layer (a axis length: a (u, 1−u))
1316: AlvGa1-vN electron supply layer (a axis length: a (0, v))
It is to be noted that the composition ratios are set so that the buffer layer 1312, the channel layer 1313, the barrier layer 1314, the spacer layer 1315, and the electron supply layer 1316 satisfy the following mathematical formulae (21) to (24) (e.g., x=0.18, z=1.0, u=0.23, v=0.2).
0<x<1 (21)
−0.177z+0.177<x (22)
−0.177z+0.177<u (23)
−0.177v+0.177<x (24)
As can be seen from the mathematical formula (3) and
a(0, z)<a(x,1−x) (25)
a(0, z)<a(u, 1−u) (26)
a(0, v)<a(x,1−x) (27)
The cross-sectional view of
1412: InxAl1-xN buffer layer (a axis length: a (x, 1−x))
1413: p-type InxAl1-xN channel layer (a axis length: a (x, 1−x))
1414: AlzGa1-zN barrier layer (a axis length: a (0, z))
1415: InuGa1-uN spacer layer (a axis length: a (u, 0))
1416: AlvGa1-vN electron supply layer (a axis length: a (0, v))
It is to be noted that the composition ratios are set so that the buffer layer 1412, the channel layer 1413, the barrier layer 1414, the spacer layer 1415, and the electron supply layer 1416 satisfy the following mathematical formulae (28) to (31) (e.g., x=0.18, z=1.0, u=0.05, v=0.2).
0<x<1 (28)
−0.177z+0.177<x (29)
0<u (30)
−0.177v+0.177<x (31)
As can be seen from the mathematical formula (3) and
a(0, z)<a(x, 1−x) (32)
a(0, z)<a(u, 0) (33)
a(0, v)<a(x, 1−x) (34)
The cross-sectional view of
1512: InxGa1-xN buffer layer (a axis length: a (x, 0))
1513: p-type InxGa1-xN channel layer (a axis length: a (x, 0))
1514: AlzGa1-zN barrier layer (a axis length: a (0, z))
1515: InuGa1-uN spacer layer (a axis length: a (u, 0))
1516: AlvGa1-vN electron supply layer (a axis length: a (0, v))
It is to be noted that the composition ratios are set so that the buffer layer 1512, the channel layer 1513, the barrier layer 1514, the spacer layer 1515, and the electron supply layer 1516 satisfy the following mathematical formulae (35) to (38) (e.g., x=0.05, z=1.0, u=0.1, v=0.2).
0≦x<1 (35)
0<z (36)
0<u (37)
0<v (38)
As can be seen from the mathematical formula (3) and
a(0, z)<a(x, 0) (39)
a(0, z)<a(u, 0) (40)
a(0, v)<a(x, 0) (41)
The cross-sectional view of
1612: InxAl1-xN buffer layer (a axis length: a (x, 1−x))
1613: p-type InxAl1-xN channel layer (a axis length: a (x, 1−x))
1614: InzAl1-zN barrier layer (a axis length: a (z, 1−z))
1615: InuAl1-uN spacer layer (a axis length: a (u, 1−u))
1616: InvAl1-vN electron supply layer (a axis length: a (v, 1−v))
It is to be noted that the composition ratios are set so that the buffer layer 1612, the channel layer 1613, the barrier layer 1614, the spacer layer 1615, and the electron supply layer 1616 satisfy the following mathematical formulae (42) to (45) (e.g., x=0.18, z=0.08, u=0.23, v=0.13).
0<x<1 (42)
z<x (43)
z<u (44)
v<x (45)
As can be seen from the mathematical formula (3) and
a(z, 1−z)<a(x, 1−x) (46)
a(z, 1−z)<a(u, 1−u) (47)
a(v, 1−v)<a(x, 1−x) (48)
The cross-sectional view of
1712: InxAl1-xN buffer layer (a axis length: a (x, 1−x))
1713: p-type InxAl1-xN channel layer (a axis length: a (x, 1−x))
1714: InzAl1-zN barrier layer (a axis length: a (z, 1−z))
1715: InuGa1-uN spacer layer (a axis length: a (u, 0))
1716: InvAl1-vN electron supply layer (a axis length: a (v, 1−v))
It is to be noted that the composition ratios are set so that the buffer layer 1712, the channel layer 1713, the barrier layer 1714, the spacer layer 1715, and the electron supply layer 1716 satisfy the following mathematical formulae (49) to (52) (e.g., x=0.18, z=0.08, u=0.05, v=0.13).
0<x<1 (49)
z<x (50)
1.215z−0.215<u (51)
v<x (52)
As can be seen from the mathematical formula (3) and
a(z, 1−z)<a(x, 1−x) (53)
a(z, 1−z)<a(u, 0) (54)
a(v, 1−v)<a(x, 1−x) (55)
The cross-sectional view of
1812: InxGa1-xN buffer layer (a axis length: a (x, 0))
1813: p-type InxGa1-xN channel layer (a axis length: a (x, 0))
1814: InzAl1-zN barrier layer (a axis length: a (z, 1−z))
1815: InuGa1-uN spacer layer (a axis length: a (u, 0))
1816: InvAl1-vN electron supply layer (a axis length: a (v, 1−v))
It is to be noted that the composition ratios are set so that the buffer layer 1812, the channel layer 1813, the barrier layer 1814, the spacer layer 1815, and the electron supply layer 1816 satisfy the following mathematical formulae (56) to (59) (e.g., x=0.0, z=0.08, u=0.05, v=0.13).
0≦x<1 (56)
1.215z−0.215<x (57)
1.215z−0.215<u (58)
1.215v−0.215<x (59)
As can be seen from the mathematical formula (3) and
a(z, 1−z)<a(x, 0) (60)
a(z, 1−z)<a(u, 0) (61)
a(v, 1−v)<a(x, 0) (62)
The cross-sectional view of
1912: InxGa1-xN buffer layer (a axis length: a (x, 0))
1913: p-type InxGa1-xN channel layer (a axis length: a (x, 0))
1914: InzGa1-zN barrier layer (a axis length: a (z, 0))
1915: InuGa1-uN spacer layer (a axis length: a (u, 0))
1916: InvGa1-vN electron supply layer (a axis length: a (v, 0))
It is to be noted that the composition ratios are set so that the buffer layer 1912, the channel layer 1913, the barrier layer 1914, the spacer layer 1915, and the electron supply layer 1916 satisfy the following mathematical formulae (63) to (66) (e.g., x=0.1, z=0.0, u=0.15, v=0.05).
0<x<1 (63)
z<x (64)
z<u (65)
v<x (66)
As can be seen from the mathematical formula (3) and
a(z, 0)<a(x, 0) (67)
a(z, 0)<a(u, 0) (68)
a(v, 0)<a(x, 0) (69)
The sixth to fifteenth embodiments show examples each having the same layer structure as in
The present invention is described above with reference to the embodiments. The present invention, however, is not limited to these descriptions, and various changes can be made.
In each of the embodiments, Si is used as a substrate, for example. The substrate may be any of other substrates such as silicon carbide (SiC), sapphire (Al2O3), GaN, and diamond (C).
In each of the embodiments, a superlattice of AlN and GaN is used as a nucleation layer. A single layer such as AlN, AlGaN, or GaN may be used as the same.
In the first to fifth embodiments, GaN or AlGaN is used as each of materials for forming a buffer layer and a channel layer. In the second FET of the present invention, for example, any of other group-III nitride semiconductors such as gallium indium nitride (InGaN), aluminum indium nitride (InAlN), InAlGaN, and InN may be used as the same as in the sixth to fifteenth embodiments. In the second FET of the present invention, the material for forming a buffer layer may be identical to or different from the material for forming a channel layer.
In the first to fifth embodiments, GaN or AlGaN is used as a material for forming a spacer layer. In the second FET of the present invention, any of other group-III nitride semiconductors with a band gap less than a barrier layer may be used as the same. For example, any of other group-III nitride semiconductors such as InGaN, InAlN, InAlGaN, and InN may be used as each of the materials for forming a channel layer and a spacer layer as in the sixth to fifteenth embodiments.
In the first to fifth embodiments, AlGaN or AlN is used as each of materials for forming a barrier layer and an electron supply layer. In the second FET of the present invention, any of other group-III nitride semiconductors with a band gap greater than a buffer layer may be used as the same. For example, each of the materials for forming a barrier layer and an electron supply layer may be any of InGaN, InAlN, InAlGaN, and GaN as in the sixth to fifteenth embodiments. The material for forming a barrier layer may be identical to or different from the material for forming an electron supply layer.
In each of the embodiments, Al2O3 is used as a gate insulating film. Any of other insulators such as silicon dioxide (SiO2) and Si3N4 may be used as the same.
In each of the embodiments, Si3N4 is used as a surface protective film (insulator). Any of the other insulators such as Al2O3 and SiO2 may be used as the same.
In each of the embodiments, Ti/Al/Ni/Au is used as each of materials for forming a source electrode and a drain electrode. Any of other materials such as Ti/Al, Ti/Al/molybdenum (Mo)/Au, and Ti/Al/niobium (Nb)/Au may be used as the same.
In each of the embodiments, Ni/Au is used as a material for forming a gate electrode. Any of other materials such as Ni/palladium (Pd)/Au, Ni/platinum (Pt)/Au, Ti/Au, Ti/Pd/Au, and Ti/Pt/Au may be used as the same.
As described above, according to the present invention, a field effect transistor which can achieve both of a high Vth and a low on-state resistance can be obtained. The field effect transistor of the present invention can increase the off-state breakdown voltage and reduce the on-state resistant. Thus, the field effect transistor of the present invention can be used as, for example, a power semiconductor element which contributes largely to the reduction in loss (energy conservation) in an electronic device (electronic equipment) such as a switching power supply or an inverter circuit. As mentioned above, the electronic device of the present invention includes the semiconductor device of the present invention. The use of the electronic device of the present invention is not particularly limited, and it can be used widely in motor controllers (e.g., for an electric car, an air conditioner, and the like), power supply devices (e.g., for a computer and the like), inverter lighting, high-frequency power generators (e.g., for a microwave oven, an electromagnetic cooker, and the like), image displays, information recording/reproducing devices, and communication devices, for example. The field effect transistor of the present invention can contribute largely to the saving of power of the electronic devices (electronic equipment).
The invention of the present application is described above with reference to the embodiments. However, various changes that can be understood by those skilled in the art can be made in the configurations and details of the invention within the scope of the invention of the present application.
This application claims priority from Japanese Patent Application No. 2010-073880 filed on Mar. 26, 2010. The entire subject matter of the Japanese Patent Applications is incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-073880 | Mar 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/072591 | 12/15/2010 | WO | 00 | 1/9/2013 |
