This application claims priority to U.S. application Ser. No. 15/905,295, filed on Feb. 26, 2018, and U.S. application Ser. No. 15/918,003, filed on Mar. 12, 2018, each of which is incorporated by reference in its entirety.
The present disclosure relates to high electron-mobility transistors (HEMTs) and in particular to anti-barrier-conduction (ABC) spacers for HEMTs.
Electrons are described by quantum mechanics for length scales on the order of the thicknesses of the channel and the barrier of HEMTs known in the art. A typical example of an HEMT 10 in the prior art is shown in
As shown in
Based on quantum mechanics and statistical mechanics, electrons have wave-like behavior at such sub-micron scales, which can be described by a wavefunction representing their probability of being located at a certain point. Electrons in a potential “well” can take one of a set of discrete or quantized energy “levels”, each corresponding to a specific wavefunction. The wavefunctions corresponding to discrete energies are called “bound states”, and such bound states and their energies are found by solving the Schrodinger wave equation. Solutions to the Schrodinger wave equation show that the deeper the potential well, the higher in energy are its bound states.
The charge density p at a point z is proportional to the sum of the squares of the absolute value of the wavefunctions, weighted by a factor that depends on how far the bound state energy is from a reference level called the “Fermi level”. The charge density p at any point z along the quantized direction may be written as:
in which kB is the Boltzmann constant equaling 1.38×10−23 Joule/Kelvin, q is the charge of an electron, T is the temperature, m* is the “effective” mass of an electron in the potential well, h is Planck's constant equaling 6.626×10−34 Joule-second, Ψi is the bound state indexed by i, Ei is the corresponding bound state energy, and EF is the energy at the Fermi level. The summation is to be executed over all states, bound or otherwise.
As described below, the energy EF at the Fermi level will be taken to be 0 electron-volts in energy. Based on the above equation, the lower the bound-state in energy compared to the energy EF of the Fermi level, the bigger the assigned weight to that state, and the larger its contribution to the charge density p.
As shown in
Bound states may be formed in the channel 24 and/or the barriers 20, 26 of the HEMT 10. Bound states in the barriers 20, 26 are formed due to the shape of the conduction band profile caused by the introduction of intentional impurities, which are called electron donors in the exemplary HEMT 10 in
HEMTs in the prior art may include one or more delta (or pulse) doping layers in one or more barriers. Delta doping layers with large sheet concentrations between 1×1012 cm2 and 4×1012 cm2 may need to be placed in one or more barriers of the HEMTs in order to meet device performance specifications such as transconductance, current handling capability, and linearity.
HEMTs in the prior art also display what is known as the “kink effect”, in which a current in the drain rises uncontrollably above a saturated value beyond a certain drain voltage. The kink effect is believed to be caused by hole accumulation in the channel near the source end. Therefore, it is desirable to suppress hole transmission across the barriers 20, 26 in an HEMT 10 as in
The following presents a simplified summary of some embodiments of the invention in order to provide a basic understanding of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some embodiments of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention enables the fabrication of HEMTs with highly doped HEMT barriers and with a reduced electron charge density in the barriers to reduce parasitic conduction through the nominally insulating barriers of the HEMT, even in highly doped barriers. Such reductions in electron charge density are caused by having the bound-state energy Ei be as far above the Fermi energy level EF as possible.
The present invention keeps electrons confined to the channel when the device is in the ON-state by reducing the resonant-tunneling mechanism which causes the ON-state leakage of channel electrons through the barriers and into the gate and/or the substrate.
By keeping the electrons confined to the channel, the HEMT of the present invention has an increased speed of operation by reducing scattering by donor impurities in the barrier, thus increasing the mobility.
By reducing the tunneling as well as thermionic emission of channel electrons as well as electron-holes through the front barrier into the gate, the present invention reduces OFF-state (or sub-threshold) gate leakage, and thus also reduces device noise.
The present invention also reduces the kink-effect caused by electron-holes accumulating near the source end of the channel due to tunneling or thermionic emissions across the front and/or back barriers, and increasing the drain current, sometimes abruptly through an avalanche breakdown process.
The present invention also increases the effective Schottky barrier height, which enables enhancement-mode operation of the HEMT, as described in U.S. application Ser. No. 15/918,003, filed on Mar. 12, 2018, which is incorporated by reference in its entirety.
The present invention includes placing one or more thin layers of wide-bandgap (WBG) materials, such as AlAs, on either side of the potential well, with such thin WBG layers being anti-barrier-conduction (ABC) spacers.
In one embodiment, the present invention is a field effect transistor (FET) including: a substrate, a back barrier disposed on the substrate, a channel disposed on the back barrier, and a front barrier disposed on the channel, wherein at least one of the front barrier and the back barrier includes an anti-barrier-conduction (ABC) spacer. The ABC spacer is grown by a fabrication method selected from molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD), thermal evaporation, and sputtering. The ABC spacer is grown by a fabrication method selected from a lattice-matched growth, pseudo-morphic growth and a metamorphic growth. The ABC spacer may be disposed adjacent to the channel. The ABC spacer causes a conduction-band offset in the range of +0.1 eV to +10 eV relative to and above at least one of the front barrier and the back barrier. The ABC spacer is composed of a wide-bandgap (WBG) material. The FET further includes a source, a drain, and a gate, and the ABC spacer may be disposed between the gate and the front barrier.
The ABC spacer reduces parasitic conduction on a path from the source to the drain through at least one of the front barrier and the back barrier. A pair of one of the barrier materials/WBG material is selected from AlGaAs/AlAs, AlGaAs/GaP, AlGaAs/InGaP, InP/In(Ga)AlAs, In(Ga)AlAs/Al(Ga)AsSb, InP/Al(Ga)AsSb, InGaAlAs/InAlAs, AlGaAsSb/AlAsSb and AlGaSb/AlSb. The ABC spacer reduces ON-state leakage into the gate caused by resonant tunneling from the channel, reduces thermionic emission of electrons over one at least of the front and back barriers, reduces thermionic emission of electron-holes over at least one of the front and back barriers, reduces tunneling of electrons through at least one of the front and back barriers, reduces tunneling of electron-holes through at least one of the front and back barriers, improves the OIP3 figure of merit for linearity, reduces gate leakage, reduces substrate leakage, and reduces gate noise.
In another embodiment, the present invention is a high-electron mobility transistor (HEMT) including: a substrate, a back barrier disposed on the substrate, a channel disposed on the back barrier, a front barrier disposed on the channel, a pulse-doping layer disposed in at least one of the front barrier and the back barrier, and at least one of the front barrier and the back barrier includes an anti-barrier-conduction (ABC) spacer. The ABC spacer is composed of a wide-bandgap (WBG). The HEMT further includes: a source and a drain, and the ABC spacer reduces parasitic conduction on a path from the source to the drain through at least one of the front barrier and the back barrier. In another embodiment, the present invention is a high-electron mobility transistor (HEMT) including: a substrate, a back barrier disposed on the substrate, a channel disposed on the back barrier, a front barrier disposed on the channel, a pulse-doping layer disposed in at least one of the front barrier and the back barrier, and at least one of the front barrier and the back barrier includes an anti-barrier-conduction (ABC) spacer. The channel is a compositionally graded alloy such that the composition of one of the alloy constituents is varied in a piecewise linear or piecewise quadratic manner versus distance in the growth direction. The grading imparts high linearity to the HEMT, as quantified by the OIP3 figure-of-merit. The ABC spacer is composed of a wide-bandgap (WBG) material. The HEMT further includes: a source and a drain, and the ABC spacer reduces parasitic conduction on a path from the source to the drain through at least one of the front barrier and the back barrier.
A pair of one of the barrier materials/WBG material is selected from AlGaAs/AlAs, AlGaAs/GaP, AlGaAs/InGaP, InP/In(Ga)AlAs, In(Ga)AlAs/Al(Ga)AsSb, InP/Al(Ga)AsSb, InGaAlAs/InAlAs, AlGaAsSb/AlAsSb and AlGaSb/AlSb. The ABC spacer reduces ON-state leakage into the gate caused by resonant tunneling from the channel, reduces thermionic emission of electrons over one at least of the front and back barriers, reduces thermionic emission of electron-holes over at least one of the front and back barriers, reduces tunneling of electrons through at least one of the front and back barriers, reduces tunneling of electron-holes through at least one of the front and back barriers, improves the OIP3 figure of merit for linearity, reduces gate leakage, reduces substrate leakage, and reduces gate noise.
In a further embodiment, the present invention is a method including: disposing a back barrier on a substrate, disposing a channel on the back barrier, disposing a front barrier on the channel, and disposing an anti-barrier-conduction (ABC) spacer in relation to at least one of the front barrier and the back barrier. The ABC spacer may be disposed adjacent to the channel. Alternatively, the ABC spacer is disposed within at least one of the front barrier and the back barrier. A source and a drain may be disposed above the front barrier, and the ABC spacer reduces parasitic conduction on a path from the source to the drain through at least one of the front barrier and the back barrier. The ABC spacer is composed of a wide-bandgap (WBG) material.
A pair of one of the barrier materials/WBG material is selected from AlGaAs/AlAs, AlGaAs/GaP, AlGaAs/InGaP, InP/In(Ga)AlAs, In(Ga)AlAs/Al(Ga)AsSb, InP/Al(Ga)AsSb, InGaAlAs/InAlAs, AlGaAsSb/AlAsSb and AlGaSb/AlSb. The ABC spacer reduces ON-state leakage into the gate caused by resonant tunneling from the channel, reduces thermionic emission of electrons over one at least of the front and back barriers, reduces thermionic emission of electron-holes over at least one of the front and back barriers, reduces tunneling of electrons through at least one of the front and back barriers, reduces tunneling of electron-holes through at least one of the front and back barriers, improves the OIP3 figure of merit for linearity, reduces gate leakage, reduces substrate leakage, and reduces gate noise.
Gate and substrate leakage can be measured using current-voltage measurements, or through terminal noise measurements, or other techniques known in the art. The electron concentration and mobility in the channel and barrier may be deduced from Hall Effect measurements, or other methods as known in the art. Gate noise measurement techniques for HEMTs are well known in the art. Bound state energies and wave-functions in various regions of the device may be determined by simulation using Schrodinger-Poisson solvers and other techniques well known in the art.
The foregoing summary, as well as the following detailed description of presently preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
To facilitate an understanding of the invention, identical reference numerals have been used, when appropriate, to designate the same or similar elements that are common to the figures. Further, unless stated otherwise, the features shown in the figures are not drawn to scale, but are shown for illustrative purposes only.
Certain terminology is used in the following description for convenience only and is not limiting. The article “a” is intended to include one or more items, and where only one item is intended the term “one” or similar language is used. Additionally, to assist in the description of the present invention, words such as top, bottom, side, upper, lower, front, rear, inner, outer, right and left may be used to describe the accompanying figures. The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import.
The ABC spacers 140, 142, 144, 146 are formed from at least binary compounds or alloys which. may be grown by molecular beam epitaxy (MBE), by metal-organic chemical vapor deposition (MOCVD), by atomic layer deposition (ALD), by thermal evaporation, by sputtering, and/or by any known fabrication method. The ABC spacers 140, 142, 144, 146 may be grown in a lattice matched manner, or pseudo-morphically or metamorphically. The ABC spacers 140, 142, 144, 146 are formed in combination with another barrier material, or may be disposed either as a first barrier layer adjacent to the gate 114, or alternatively may be enclosed by other barrier material, or may be disposed adjacent to the channel 124. The ABC spacers 140, 142, 144, 146 are formed with a conduction-band offset in the range of, for example, +0.1 eV to +10 eV in electron energy relative to and above at least one other barrier material.
In an example embodiment, the HEMT 110 in
As shown in
However, several advantages accrue to the speed, noise and other electrical characteristics of a HEMT due the modification of the third and fourth lowest bound states when ABC spacers are included, when compared to HEMTs in the prior art.
With ABC spacers 140, 142, 144, 146 shown in
In addition, the ABC spacers 140, 142, 144, 146 in
Moreover, the ABC spacers 140, 142, 144, 146 confer a reduction of tunneling current simply by virtue of offering taller barriers to the electrons, even without the additional advantage of preventing resonant tunneling described above. Thus, the ABC spacers 140, 142, 144, 146 reduce gate electron currents in all regimes of HEMT operation, whether in the ON-state with the gate voltage higher than a certain threshold voltage, or in the OFF-state where the gate voltage is sub-threshold.
In implementing the present invention, it might appear at first glance that composing the entire barrier 120, 126 out of the WBG material would bring the same advantages as narrow WBG ABC spacers, such as the ABC spacers 140, 142, 144, 146 in
As described above, the WBG ABC spacers within and/or adjacent to the respective barriers enable high donor doping levels in the barriers, and hence high electron charge density in the channel without concomitantly high electron density in the barriers. The WBG ABC spacers within and/or adjacent to the respective barriers push bound states of the electrons in the barriers upwards in energy, reducing their charge density and hence reducing parasitic conduction. The WBG ABC spacers within the respective barriers allow the engineering of quantum bound states in the barriers to be off-resonance with the channel bound states, thus reducing leakage of channel electrons through resonant tunneling from the channel through barrier into the remainder of the device in the ON-state, thus confining electron wavefunctions to the channel, and reducing their overlap with scattering donor centers in the barriers, thus increasing HEMT channel electron velocity.
The implementation of WBG ABC spacers within the respective barriers creates an energetically taller barrier for electrons which reduces thermionic emission as well as tunneling, and hence reduces sub-threshold OFF-state gate leakage. Furthermore, WBG ABC spacers within and/or adjacent to the respective barriers reduce tunneling and thermionic emission of electron-holes across the front and/or back barriers.
Therefore, HEMTs with highly doped HEMT barriers may be fabricated with a reduced electron charge density in the barriers to reduce parasitic conduction through the nominally insulating barriers of the HEMT, even in highly doped barriers. Such reductions in electron charge density are caused by having the bound-state energy Ei be as far above the Fermi energy level EF as possible. The present invention keeps electrons confined to the high-speed channel when the device is in the ON-state by reducing the resonant-tunneling mechanism which causes the ON-state leakage of channel electrons through the barriers and into the rest of the HEMT.
By keeping the electrons confined to the channel, the present invention has an increased speed of operation by reducing scattering by donor impurities in the barrier, thus increasing the electron mobility. This is achieved by having the bound states in the channel and barrier be off resonance.
The present invention also reduces the kink-effect caused by electron-holes accumulating near the source end of the channel due to tunneling or thermionic emissions across the barriers, and increasing the drain current, sometimes abruptly through an avalanche breakdown process.
The present invention improves linearity of the HEMT by enabling the utilization of heavily doped barriers, by improving channel electron mobility and reducing parasitic resistances and associated non-linearities, by reducing parasitic conduction across barrier(s), by reducing leakage through barrier(s).
The present invention also increases the effective Schottky barrier height, which enables enhancement-mode operation of the HEMT, as described in U.S. application Ser. No. 15/918,003, filed on Mar. 12, 2018, which is incorporated by reference in its entirety.
In an alternative embodiment, the present invention may apply ABC spacers in other types of FETs, not limited to HEMTs. For example, an ABC spacer may be disposed in a barrier of hole-channel (p-channel) FETs, in which the carriers of electrical current are holes rather than electrons. The ABC spacer would have similar band offset properties relative to the other materials in the barrier stack, except that the offsets would be in the valence band. The valence band-edge diagrams would be exact mirror images to the conduction band-edge diagrams presented above for n-channel FETs.
In another alternative embodiment, an ABC spacer may be disposed in a barrier of a FET that depletes a doped channel, i.e. a pre-existing bridge between the source and drain by applying a voltage opposite in polarity to the ionized impurities (dopants). Such FETs include Hetero-Junction FETs (HFETs), Junction Gate FETs (JFETs), and Metal-Semiconductor FETs (MESFETs).
In further alternative embodiments, ABC spacers may be disposed in a barrier of a FET in which the channel is a compositionally graded alloy such that the composition of one of the alloy constituents is varied in a piecewise linear or piecewise quadratic manner versus distance in the growth direction. The ABC spacer is composed of a wide-bandgap (WBG) material.
In further alternative embodiments, ABC spacers may be disposed in a barrier of an Enhancement-Mode FET or of a Depletion-Mode FET.
As described above, the present invention has been described in connection with a GaAs platform. That is, the HEMT 110 in
The inventive device may be distinguished from prior art using a variety of experimental and analytical techniques. Gate and substrate leakage can be measured using current-voltage measurements, or through terminal noise measurements, or other techniques known in the art. The electron concentration and mobility in the channel and barrier may be deduced from Hall Effect measurements, or other methods as known in the art. Gate noise measurement techniques for HEMTs are well known in the art. Bound state energies and wave-functions in various regions of the device may be determined by simulation using Schrodinger-Poisson solvers and other techniques well known in the art. Linearity may be quantified by the “OIP3” figure-of-merit (third order output intercept point) among other metrics, and may be measured using two-tone techniques and others well known in the art.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention, therefore, will be indicated by claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.
Number | Date | Country | |
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Parent | 15918003 | Mar 2018 | US |
Child | 16239909 | US | |
Parent | 15905295 | Feb 2018 | US |
Child | 15918003 | US |