This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-052989 filed on Mar. 6, 2009, the disclosure of which is incorporated by reference herein.
1. Technical Field
The present invention relates to a gallium nitride high electron mobility transistor (HEMT), and in particular to a gallium nitride-HEMT suppressing shifts in threshold voltages, capable of setting with a high forward voltage value, and capable of increasing the saturation output voltage.
2. Related Art
Gallium nitride-HEMT's (sometimes referred to below as GaN HEMT's) have both high breakdown voltages and high saturated electron velocities. AlGaN/GaN hetero structure HEMT's utilizing these characteristics have drawn attention as high speed devices (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2002-359256 and 2004-228481).
In order to clarify the purpose of the present invention, a discussion follows of the operation characteristics of conventional configuration GaN HEMT's. First, a configuration of a GaN HEMT of conventional configuration is explained, with reference to
Explanation follows regarding the operation characteristics of a conventional GaN HEMT, with reference to
In
In
In a conventional GaN HEMT, the value of the forward voltage Vf is about 0.8V. The forward voltage Vf is defined, in the Igs-Vgs characteristics, as the value of the gate voltage Vgs applied as the gate voltage that is attained when the value of the gate current Igs in the forward direction is made to be 1 mA/mm. In
Therefore, it is difficult to set the gate voltage value any higher than this value, since on the Ids-Vds characteristics curve shown in
However, if the forward voltage Vf could be made higher, it would also be possible to increase the drain current Ids. Explanation follows with reference to
From the standpoint of operation of the GaN HEMT as a power device, if the saturated output power is denoted Psat, then Psat approximates to the following Equation (1).
P
sat≈(ΔVds×ΔIds)/8 (1)
Wherein ΔVds is the amplitude of change in drain voltage and ΔIds is the amplitude of change in the drain current. The constant ⅛ appearing in Equation (1) is for computation of the saturation power output Psat utilizing effective values of the drain voltage and drain current, respectively. Namely, since effective values of the drain voltage and the drain current are each respectively 1/(2×21/2) of the alternating current amplitude values, the constant ⅛ appears as the product of {1/(2×21/2)}×{1/(2×21/2)}=⅛.
Assuming class-A operation in a high frequency band for the GaN HEMT, for example, if the reference voltage value is 50V, then the value of ΔVds is 100V, twice that of the reference voltage value 50V. The value of the ΔIds is equivalent to Ids-max, this being the maximum value of the drain current Ids. As explained above, the drain current Ids is limited by the forward voltage Vf, and the higher the forward voltage Vf, enables an increase in the Ids-max, this being the maximum value of the drain current Ids, and hence an increase in the saturation power output Psat is also enabled. By being able to increase the saturation power output Psat, the current density of the drain current can be increased, and as a result this contributes to miniaturization of the device.
By employing a Metal-Insulator-Semiconductor (MIS) structure using a SiN film (see, for example, JP-A No. 6-334176), the absolute value of the forward voltage Vf can be made greater, however, in techniques using a SiN film, the absolute value of the threshold voltage Vth of negative potential also becomes greater, with the problem that the mutual conductance gm is reduced. When the mutual conductance gm is reduced, then this has an effect on the size of the cut-off frequency fT, one of the high-frequency characteristics of the element.
The threshold voltage Vth here refers to the minimum gate voltage value at which current starts flowing between the source and the drain (in the case of a normally-off HEMT), or the minimum gate voltage value at which current stops flowing between the source and the drain (in the case of a normally-on HEMT).
The cut-off frequency fT is defined as the frequency value when the amplification ratio becomes 1, namely the minimum frequency at which an amplification effect is not obtained at this frequency or above, and is given by the following Equation (2).
f
T
=g
m/(2πCgs) (2)
Cgs here is a value of the parasitic capacitance between the gate and the source induced by the gate electrode structure (inter gate-source parasitic capacitance). In Equation (2), it can be seen that when the mutual conductance gm is decreased, the cut-off frequency fT also decreases.
As described above, it can be seen that by employing the conventional technique of an MIS structure using an SiN film as described above, although a shift in the threshold voltage Vth is suppressed, it is difficult to increase the absolute vale of the forward voltage Vf.
In order to solve the issue described above, it has been discovered, as a result of research by the inventors of this application, that by making the thickness of the cap layer 18 thicker than 11 nm, the forward voltage Vf can be increased linearly, with hardly any change in the threshold voltage Vth. It has been found that by utilizing this characteristic, the Ids-max, this being the maximum value of the drain current Ids, can be increased without changing the threshold voltage Vth, enabling the saturation power output Psat to be increased.
Consequently, the present invention is made in consideration of the above circumstances and an objective thereof is to provide a GaN HEMT that suppresses shift in the threshold voltage Vth, is capable of setting with a large positive value for the forward voltage Vf, and is capable of increasing the saturation power output Psat.
In consideration of the above circumstances, the present invention provides a GaN HEMT of the following configuration, in accordance with the spirit of the present invention.
The GaN-HEMT of the present invention is a GaN-HEMT including: a channel layer that lets a carrier transit at high velocity; a carrier supply layer that generates the carrier; and a cap layer, disposed on the carrier supply layer and functioning to prevent oxidation of the carrier supply layer, to reduce gate leakage current, and to increase voltage withstand to gate voltage, wherein the thickness of the cap layer is set at a minimum as thicker than 11 nm.
In the GaN-HEMT of the present invention, preferably the thickness of the carrier supply layer is set in a range enabling the threshold voltage to adopt a positive value, in order to realize a GaN-HEMT of normally-off operation.
Furthermore, the cap layer is preferably an AlxInyGa1-x-yN crystal layer, doped with Si in a range from 0 to 5×1018 cm−3.
Furthermore, the cap layer is an AlxInyGa1-x-yN crystal layer where the values of x and y, giving composition ratios, are set in a range that satisfies 0≦x<0.1, and 0≦y<0.1.
Furthermore, the carrier supply layer is preferably formed from AlxGa1-xN crystal, with the thickness of the carrier supply layer set at values of 15 nm or less, 10 nm or less, and 4 nm or less for values of the composition ratio x of the AlxGa1-xN set at 0.15, 0.20 and 0.25, respectively.
According to the GaN-HEMT of the spirit of the invention as described above, corresponding to an increase in the thickness of the cap layer, the forward voltage Vf increases with hardly any effect on the threshold voltage Vth. Consequently, by making the thickness of the cap layer 18 thicker than 11 nm, realization is enabled of a GaN-HEMT in which shift in the threshold voltage Vth can be suppressed as far as possible, the forward voltage value is capable of being set high, and the saturation power output Psat is capable of being increased.
Furthermore, in the GaN-HEMT, by making the thickness of the cap layer thicker than 11 nm, and also setting the thickness of the carrier supply layer so the threshold voltage can be a positive value, realization is enabled of a normally-off operating GaN-HEMT in which the forward voltage value can be made greater, and the saturation power output Psat can be increased.
Furthermore, by making the Si doping amount to the cap layer 0 to 5−×1018 cm−3, a reduction in current collapse effect is expected.
Furthermore, when the composition ratios x and y of the AlxInyGa1-x-yN forming the cap layer are set to satisfy 0≦x<0.1, and 0≦y<0.1, respectively, large shifts in the threshold voltage Vth can be suppressed.
Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:
Explanation follows regarding an exemplary embodiment of the present invention, with reference to the figures. Note that
GaN HEMT of First Exemplary Embodiment
Explanation follows regarding the configuration of a GaN HEMT of a first exemplary embodiment of the present invention, with reference to
The GaN HEMT of the first exemplary embodiment of the present invention is configured with a source electrode 122, a gate electrode 124, and a drain electrode 126 formed on an epitaxial growth substrate. The epitaxial growth substrate is a buffer layer 112, a channel layer 114, a carrier supply layer 116, and a cap layer 118 formed, in sequence, on a crystal substrate 110 by an epitaxial growth method.
An element isolation layer 134 is provided adjacent to the source electrode 122 and to the drain electrode 126, and a passivation film 120 is provided between the source electrode 122 and the gate electrode 124, and between the drain electrode 126 and the gate electrode 124. The passivation film 120 is formed from a SiN crystal film that functions as a covering film between electrodes.
The relationship between the source electrode 122 and the drain electrode 126 is a relationship determined by selection of one or other as the high potential side, namely it is a relationship in which the carrier supply side is the source electrode and the carrier destination is the drain electrode, with the electrode structures employing the same structures. In GaN-HEMT's, since the carrier is electrons, the electrode selected as the high potential side is the drain electrode. Since the carrier supply layer is a layer supplying electrodes it is sometimes referred to as an electron supply layer. The channel layer is a layer through which electrons, these being the carrier, transit with high mobility, and is also sometimes referred to as a carrier transit layer or electron transit layer.
The buffer layer 112 (AlN layer) has the role of growth nucleation when the channel layer 114 (GaN layer) is being formed by epitaxial growth. Generally it is preferable, when forming an epitaxial growth layer on a single crystal substrate, for the lattice constant of the substrate crystal and the lattice constant of the epitaxial growth crystal to be values that are close to each other. Furthermore, it is preferable for the crystal substrate face for epitaxial growth to be formed with an ordered array, without defects in the crystal lattice, however, it is difficult to expect crystal substrate faces formed by polishing to have crystal lattices with sufficiently ordered arrays.
Consequently, by forming the buffer layer 112 (AlN layer) on the surface of the crystal substrate 110 (SiC crystal substrate), favorable conditions are achieved for epitaxial growth of the channel layer 114 (GaN layer), and a state is realized in which the crystal lattice of the crystal substrate is an ordered array without defects, and with a smaller difference in size of the lattice constant.
The channel layer 114 (GaN layer) employs a GaN layer capable of letting electrons, which are the carrier, transit with high mobility.
During operation of the GaN HEMT, due to piezo polarization and spontaneous polarization in the carrier supply layer 116 (i-AlGaN layer), electrons accumulate in the upper portion of the channel layer 114 (GaN layer) formed as the layer below, and a two dimensional electron gas channel 128 is formed, capable of moving electrons at high speed. The two dimensional electron gas channel 128 is located at the position shown by the intermittent line on
The cap layer 118 (i-GaN layer), has the effect of preventing oxidation of the carrier supply layer 116 (i-AlGN layer) that includes the readily oxidized element Al, reducing gate leakage current, and raising withstanding to gate voltage.
The crystal substrate 110 uses a SiC crystal substrate. The buffer layer 112, the channel layer 114, the carrier supply layer 116, and the cap layer 118 are formed in sequence using a Metal Organic Chemical Vapor Deposition (MOCVD) method. The buffer layer 112 is an AlN epitaxial growth layer of thickness 10 nm to 200 nm, the channel layer 114 is a GaN epitaxial growth layer, and the carrier supply layer 116 is an AlxGa1-xN epitaxial growth layer of thickness 10 nm to 30 nm.
The carrier supply layer 116 is a non-doped AlxGa1-xN epitaxial growth layer, with the composition ratio x set as a value in the range from 0.15 to 0.30. The cap layer 118 is a GaN epitaxial growth layer having a thickness at the minimum that thicker than 11 nm. The cap layer 118 is an Un-Intentionally Doped (UID) GaN epitaxial growth layer.
Since the cap layer 118 is a layer formed by an epitaxial crystal growth method as described above, the upper practical limit value of the thickness thereof is several tens of nm. Namely, there are few positive merits technically even if a thickness of cap layer 118 greater than this was to be secured, and the value of the upper limit of the thickness of the cap layer 118 is of the order of several tens of nm from the perspective of the requirements for industrial applicability.
A UID-GaN epitaxial growth layer is formed as an n-type conductor even without performing intentional Si doping. However, intentional Si doping may be performed, since a reduction in so-called current collapse phenomenon can be expected, where the drain current falls off during operation at high power due to the influence of electron traps. However, if the doping amount of Si is 5×1018 cm−3 or greater, leakage current, between the source electrode 122 and the gate electrode 124, and between the drain electrode 126 and the gate electrode 124, occurs at a level that cannot be ignored. Furthermore, since the problem of a reduction in the forward voltage Vf also occurs, even if the GaN epitaxial growth layer of the cap layer 118 is doped with Si, the amount is preferably 5×1018 cm−3 or less.
The inventors of the present invention have experimentally confirmed that when Si doping is performed at 1×1019 cm−3 to the cap layer 118, the leakage current between the source electrode 122 and the gate electrode 124, and between the drain electrode 126 and the gate electrode 124, is too big, and is not practically usable. Furthermore, when Si doping is performed to the cap layer 118 in a similar manner at 2×1018 cm−3, the leakage current value is sufficiently small, and the forward voltage Vf is not lowered to such an extent as to cause problems. So, from these experimentally confirmed results, the Si doping amount to the cap layer 118 is preferably 5×1018 cm−3 or less.
The cap layer 118 may also be an AlxInyGa1-x-yN epitaxial growth layer (AlxInyGa1-x-yN crystal layer), instead of the GaN epitaxial growth layer. However, as a result of experimental confirmation, when the value of the Al composition ratio x and the value of the In composition ratio y are set at 0.1 or greater, respectively, large shifts in the threshold voltage value Vth are difficult to suppress.
One of the characteristics of the GaN HEMT of the first exemplary embodiment of the present invention is that the thickness t1 of the cap layer 118 shown in
Explanation follows, with reference to
In the measurements of the Igs-Vgs characteristics shown in
Explanation follows of the experimental results regarding the relationship between the forward voltage Vf and the threshold voltage Vth, with reference to
It can be seen from
The forward voltage Vf is preferably as high as possible in applications of HEMT, and in particular, when operating with a threshold voltage Vth of positive value, when designing a HEMT of normally-off operation, it is preferably if a threshold voltage Vth of 1.5V or greater can be secured. Also, in order to also secure a sufficiently large drain current Ids, a large difference between the forward voltage Vf and the threshold voltage Vth is preferable, and it is preferable if a forward voltage Vf at +3V or greater can be secured. Therefore, as shown in
Fabrication Method of GaN HEMT of First Exemplary Embodiment
Explanation follows regarding a fabrication method of the GaN HEMT of the first exemplary embodiment of the present invention, with reference to
The protection film 130 is an insulating film that becomes the passivation film (inter-electrode covering film) 120 shown in
Since the cap layer 118 formed from GaN crystal has an extremely high resistance, even if both the source electrode and the drain electrode were to be formed on the cap layer 118, electrodes arising would have non-ohmic characteristics, with extremely high contact resistance. Consequently, the cap layer 118 and the carrier supply layer 116 are preferably completely removed at locations where the source electrode and the drain electrode are to be formed, and the recess portions 142 formed by dry etching down to locations where portions of the channel layer 114 are present. Note that contact is made when the electrodes are formed on the cap surface, even though there is a high resistance. Furthermore, contact with ohmic characteristics is made when excavation is not made down as far as the channel, and only the cap is removed, although the contact resistance is high. Therefore, there is no limitation to making the recess down as far as the channel.
Forming the recess portions 142 in this manner, and forming the source electrode and drain electrode as electrodes buried in the recess portions 142, enables an ohmic recess structure to be achieved. Due to the ohmic recess structure, the ohmic contact interfaces, between the source electrode and the drain electrode with the channel layer 114, come into direct contact with the two dimensional electron gas channel 128. As a result, the contact resistance of the ohmic contact interfaces, between the source electrode and the drain electrode with the channel layer 114, can be made smaller, and the electrical characteristics as an HEMT can be raised.
After removing the photoresist films 136 and 138 and the metal thin film 144 using the lift-off method, annealing is performed in a nitrogen gas atmosphere at 600° C. Due to this annealing, ohmic contact of the metal thin film 148, which will become the source electrode 122 and the drain electrode 126, is formed.
GaN HEMT of Second Exemplary Embodiment
The basic structure of the GaN HEMT of the second exemplary embodiment of the present invention is similar to that of the GaN HEMT of the first exemplary embodiment explained with reference to
In an HEMT employed, for example, in amplifiers of RF band, in order to ensure the safety of the device a normally-off HEMT is demanded in which, even when there is damage to the control circuit, there is no shorting between the source and the drain, namely, even though the gate voltage. Vgs=0V, a state is secured in which current does not flow between the source and the drain. However, in conventional GaN HEMT's, basically, normally-on HEMT's are easy to manufacture, since the value of the forward voltage Vf is about 0.8 to 1.8 V, however the situation is that the range of positive values of the value of the gate voltage Vgs cannot be made larger. Therefore, normally-off HEMT's have been designed with the assumption that a large Ids-max, the maximum value of the drain current Ids, cannot be achieved.
As explained with reference to
In order to design an HEMT of normally-off operation, namely having a value of 0V or greater for the threshold voltage Vth, that has basically the same structure as that of the first GaN HEMT, the thickness of the carrier supply layer 116 needs to be controlled. This is explained with reference to
Explaining first the case where the value of the composition ratio x of the AlxGa1-xN layer is 0.25, in order to achieve normally-off operation (operation with threshold voltage Vth being in a positive value range), the thickness t2 of the carrier supply layer 116 needs to be 4 nm or less (see the location of the upward facing arrow on the left hand side in
As explained above, in order to achieve a value of threshold voltage Vth that is a positive value, this means that the thickness t2 of the carrier supply layer 116 must be 15 nm or less, 10 nm or less, or 4 nm or less, when the composition ratio x of the AlxGa1-xN layer forming the carrier supply layer 116 is 0.15, 0.20 and 0.25, respectively.
Note that explanation above is related to the thickness t2 of the carrier supply layer 116 when no doping has been performed to the carrier supply layer 116 (AlxGa1-xN layer). When Si is doped to the carrier supply layer 116, it has been confirmed that there is a tendency for the threshold voltage Vth to be comparatively lower for the same thickness of the carrier supply layer 116. Namely, when the carrier supply layer 116 has been Si doped, the thickness t2 of the carrier supply layer 116 needs to be comparatively thinner than when no doping is performed. However, when the gate length is made shorter, the thickness of the AlGaN gets thicker. For example, if Lg is 0.1 μm, then for an Al composition of 25%, the thickness of the AlGaN may be 6 nm.
Number | Date | Country | Kind |
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2009-052989 | Mar 2009 | JP | national |