Antimonide-based compound semiconductors for electronics

InAs-Channel High-Electron-Mobility Transistors (HEMTs)

J. Brad Boos, brad.boos@nrl.navy.mil

The material properties that have the greatest impact on the high-speed performance of HEMTS are the sheet charge density in the two-dimensional electron gas and the effective electron velocity. In recent years, this has led to the use of In_xGa_1-xAs channel HEMTs with increasing mole fractions of InAs. For relatively small values of x (< 0.2), GaAs substrates are suitable. For x values near 0.5, InP substrates are nearly lattice matched to the In_xGa_1-xAs. For higher InAs mole fractions, however, lattice mismatch becomes a problem. For this reason, the Electronic Materials Branch, in collaboration with the Microwave Technology Branch, Code 6850, is investigating HEMTs with InAs channels and AlGaSb barriers. Advantages of this material system include the high electron mobility (30,000 cm²/V-s) and velocity (4 x 10⁷ cm/s) of InAs, and a large conduction band offset between InAs and AlGaSb (1.3 eV). As a result, single quantum wells of InAs clad by AlGaSb are of interest for application to high-speed, low-voltage HEMTs. Promising HEMT characteristics have been achieved, with an intrinsic unity-current-gain cut-off frequency, f_T, of 90 GHz at a drain-source voltage of only 100 mV for a 0.1 µm gate length.

As part of the DARPA antimonide-based compound semiconductors (ABCS) and integrated sensor is structure (ISIS) programs, NRL has been collaborating with Northrop Grumman Corporation (Redondo Beach, CA). This joint effort has resulted in the fabrication of HEMTs with 0.1 µm T-gates. These transistors have values of f_T and f_max above 250 GHz. They provide equivalent high-speed figure-of-merit performance at 5 to 10 times lower power dissipation as compared to state-of-the-art InAlAs/InGaAs/InP HEMTs with the same gate length. Low-power MMIC LNA circuits were demonstrated in the X and W bands. In addition, a narrowband amplifier showed ultra-low power consumption in the S-band.

2011

• Enhancement Mode Antimonide Quantum Well MOSFETs With High Electron Mobility and GHz Small-Signal Switching Performance, IEEE Electron Device Letters 32, 11689-11691 (2011).
• Experimental Determination of Quantum and Centroid Capacitance in Arsenide-Antimonide Quantum-Well MOSFETs Incorporating Non-Parabolicity Effect, IEEE Transactions on Electron Devices 58. 1397-1403 (2011).

2010

• AlGaSb buffer layers for Sb-based transistors, Journal of Electronic Materials 39, (2010) 2196-2202.
• Fermi level unpinning of GaSb (001) using plasma enhanced atomic layer deposition of Al₂O₃ dielectric, Applied Physics Letters 97, 143502 (04 Oct 2010).
• Advanced Composite High-κ Gate Stack for Mixed Anion Arsenide-Antimonide Quantum Well Transistors, Proc. IEDM, Dec. 2010.

2008

• Sb-based n- and p-channel FETs for High-Speed, Low-Power Applications, IEICE Transactions on Electronics, E91-C, 1050-1057 (July 2008).

2007

• InAlSb/InAs/AlGaSb quantum well heterostructures for high-electron-mobility transistors, Journal of Electronic Materials 36, 99-104 (Feb 2007).

2006

• InAs HEMT narrowband amplifier with ultra-low power dissipation, Electronics Letters 42, 29-30 (08 June 2006).

2005

• Antimonide-Based Compound Semiconductors for Electronic Devices: A Review, Solid State Electronics 49, 1875-1895 (Dec 2005).
• A W-band InAs/AlSb Low-Noise/Low-Power Amplifier, IEEE Microwave and Wireless Component Letters 15, 208-210 (April 2005).

2004

• Materials growth for InAs high electron mobility transistors and circuits, J. Vac. Sci. Techn. B 22, 688-694 (March/April 2004).

(In)GaSb p-channel Field-Effect Transistors

Dr. Brian R. Bennett, brian.bennett@nrl.navy.mil

NRL has been the leader in developing n-channel, Sb-based field-effect transistors (FETs). Amplifier circuits have been demonstrated in the S-band, X-band, and W-band with several times lower power consumption than conventional GaAs- or InP-based circuits [see B.R. Bennett et al., Solid-State Electronics 49, 1875 (2005)]. Sb-based complementary circuits are needed for a variety of digital logic applications. This technology will require p-channel FETs with high hole mobility. We applied band-structure engineering to enhance hole mobility in compound semiconductors. Quantum wells of InGaSb clad with AlGaSb were grown with compressive strain to split the degenerate heavy- and light-hole bands, resulting in lower effective mass and higher hole mobility. The hole density in the quantum well was controlled by modulation doping with beryllium. The composition of the well (and hence the strain), the thickness of the well, the carrier density, and the interface formation were varied to optimize the room-temperature hole mobility. Values as high as 1500 cm²/V-s were achieved with a strain of 2%. This represents a factor of two improvement over the previous state-of-the-art. Low-temperature (77K) mobilities of 10,000 cm²/V-s have been achieved which is also a record. In addition, hole effective masses of 0.10 m₀ were measured, compared to 0.40 m₀ for unstrained GaSb and InSb.

Device processing techniques were developed, and FETs were fabricated with a source-drain spacing of 1 µm, a gate width of 30 µm, and a gate length of 0.2 µm. They exhibited a maximum transconductance of 165 mS/mm, compared to the previous record of 51 mS/mm in this technology. Microwave measurements yielded a value of 16 GHz for the unity-current-gain cutoff frequency and 34 GHz for the unity-power-gain cutoff frequency. This work has demonstrated that the band structure of InGaSb can be favorably altered by strain and produce the theoretically-predicted reduction in effective mass and increase in mobility. The high microwave cutoff frequencies represent the first reported measurements of microwave performance in Sb-based p-channel FETs, and are superior to what has been reported for GaAs-based p-channel FETs. These results, combined with previous work on n-channel Sb-based transistors, should enable complementary logic circuits with extremely low power consumption. Such circuits could be essential for several potential military and commercial applications including analog-to-digital conversion circuits, military application-specific integrated circuits, and microprocessors.

This work was carried out as a joint project with the Microwave Technology Branch, Code 6850.

2012

• Enhancing hole mobility in III-V semiconductors, Journal of Applied Physics 111, 103706 (15 May 2012).
• Atomic layer deposition of Al2O3 on GaSb using in situ hydrogen plasma exposure, Applied Physics Letters 101, 231601 (03 Dec 2012).

2011

• Optimization of the Al₂O₃/GaSb Interface and a High-Mobility GaSb, IEEE Transactions on Electron Devices 58, 3407 (2011).
• Device quality Sb-based compound semiconductor surface: A comparative study of chemical cleaning, Journal of Applied Physics 109, 114908 (2011).
• Shubinov-de Haas oscillations in compressively-strained (In)GaSb quantum wells, Solid-State Electronics 62, 138-141 (2011).
• In_xGa_1-xSb channel pMOSFETS: Effect of strain on heterostructure design, Journal of Applied Physics 110, 014503 (2011)
• Hole Mobility Enhancement through <110> Uniaxial Strain in In_0.41Ga_0.59Sb Quantum-Well Field Effect Transistors, Applied Physics Letters 98, 053505 (2011).
• Molecular beam epitaxial regrowth of InGaSb, J. Electronic Materials 40, 6-10 (2011)

2010

• Scaling Projections for Sb-Based p-Channel FETs, Solid-State Electronics 54, 1349-1358 (Nov 2010).
• Development of high-? dielectric for Antimonides & a sub 350°C III-V pMOSFET outperforming Ge, Proc. IEDM, Dec. 2010.

2009

• Compound semiconductors for low-power p-channel field-effect transistors, MRS Bulletin 34, 530-536 (July 2009).
• Demonstration of high-mobility electron and hole transport in a single InGaSb well for complementary circuits, Journal of Crystal Growth 312, 37-40 (15 Dec 2009).

2008

• Strained GaSb/AlAsSb quantum wells for p-channel field-effect transistors, Journal of Crystal Growth 311, 47-53 (15 Dec 2008).

2007

• Mobility enhancement in strained p-InGaSb quantum wells, Applied Physics Letters 91, 042104 (23 July 2007).
• High mobility p-channel HFETs using strained Sb-based materials, Electronics Letters 43, 834-835 (19 July 2007).

0000

• Atomic layer deposition of Al2O3 on GaSb using in situ hydrogen plasma exposure, Applied Physics Letters 101, 231601 (03 Dec 2012).

Antimonide-based Heterojunction Bipolar Transistor

Dr. Richard Magno, richard.magno@nrl.navy.mil

The heterojunction bipolar transistor (HBT) is an important component in both high-speed analog and digital circuits in use today. They are used in analog-to-digital and digital-to-analog converters, mm- wave, and microwave receivers, and elecro-optical systems used in fiber optical communications. There is a continuing demand for higher frequency electronics operating at lower power, particularly for use in hand held and other portable military and aerospace applications. The goal of this work, that is carried out in collaboration with Code 6853 the High Speed / Low power Devices Section, is to make significant improvements in both speed and power by the development of HBTs composed of InAs, GaSb, AlSb and their ternary and quaternary alloys with a lattice constant between 6.2 and 6.3 Å. Estimates indicate that with these materials it is possible to realize devices operating up to four times faster at one tenth the power of those currently available in the InP and GaAs material systems. These improvements stem from the high electron and hole mobilities available in the InAs/GaSb/AlSb system and from the small bandgaps that allow low voltage operation. The availability of a wide assortment of conduction band and valence band offsets allows the use of bandgap engineering to optimize device performance.

A considerable amount of materials growth science is being developed at NRL in order to build this new HBT composed of an InGaSb base and InAlAsSb alloys for the emitter and collector. The procedures for growing the individual alloys were developed by using a variety of tools such as X-ray diffraction, photoluminescence (PL), Hall Effect, SIMS and atomic force microscopy to determine the composition, structure and quality of the materials. The X-ray indicates that the alloys have a lattice constant near 6.2 Å, and the PL indicates the bandgaps are near the desired values. The AFM data reveal good surface morphology with root-mean-square roughness of 1 nm over a 5µmx5µm area. Hall Effect measurements indicate the p-type base can be doped to at least 3x10¹⁹ cm^-3 with a mobility of 160 cm²/Vs. These values result in a sheet resistance for the base layer that is lower than those reported for other systems. This is important, as the base sheet resistance needs to be minimized to obtain high frequency operation.

The individual alloys have been combined to successfully grow the HBT structure illustrated in the figure. The common-emitter current-voltage characteristics for the HBT are also shown there. A good DC current gain of 20 has been measured for this, and breakdown was not observed with up to 3 V applied between the emitter and collector. The small emitter-base turn-on voltage of 0.2 eV is encouraging as it indicates a low power dissipation device is possible. The layer structure design of this HBT is very conservative, and considerable improvement is expected in the future. A very high mobility n-type InAsSb alloy with very low ohmic contact resistance is under development for use as the sub-collector. This will result in a significant reduction in series resistance. The ability to grow these alloys with a 6.2Å lattice constant on semi-insulating GaAs substrates, with a 5.65 Å lattice constant is the result of the development of a series of alloys that allow the relaxation of the large lattice mismatch. Semi-insulating substrates are essential for making complex circuits and for carrying out high frequency tests on single devices.

2010

• InAlAsSb/InGaSb Double-Heterojunction Bipolar Transistors with InAsSb Contact Layers, Electronics Letters 46, (2010). Also see Cover Story from Electronic Letters.

2007

• Low resistance, unannealed ohmic contacts to n-type InAs_0.66Sb_0.34, Electron. Lett. 43, (2007).

2006

• Narrow Bandgap InGaSb, InAlAsSb Alloys for Electronic Devices, J. Vac. Sci. Technol. B 24, 1622 (2006).
• Optical Characterization of In_0.27Ga_0.73Sb and In_xAl_1-xAs_ySb_1-y Epitaxial Layers for Development of 6.2 Å-based Heterojunction Bipolar Transistors, J. Vac. Sci. Technol. B 24, 1604 (2006).
• Low Resistance, Unannealed, Ohmic Contacts to p-type In_0.27Ga_0.73Sb, J. Vac. Sci. Technol. B 24, 2388, Sep 2006.
• Determination of Conduction Band Offsets in Type-II In_0.27Ga_0.73Sb/In_xAl_1-xAs_ySb_1-y Heterostructures grown by Molecular Beam Epitaxy, Phys. Rev. B74 235306, 5 Dec 2006.

2005

• InAlAsSb/InGaSb Double Heterojunction Bipolar Transistor, Electron. Lett. 41, 370 (2005).

2003

• Antimony-based Quaternary Alloys for High-Speed Low-Power Electronic Devices, Proceedings of the IEEE Lester Eastman Conference on High Performance Devices 2002, edited by R. E. Leoni III, IEEE, Piscataway, NJ, 2003, pp. 288-296.

Heterostructure Barrier Varactors

Dr. James Champlain, james.champlain@nrl.navy.mil

A heterostructure barrier varactor is an electronic device that can be used in a circuit like that shown in the figure to produce small amounts of power at THz frequencies. The device needs a bias dependent capacitance, C, that is symmetric with respect to bias polarity along with a high resistance. The single barrier structure also shown in the figure accomplishes this as the high potential barrier acts to block the current flow, and the depletion layers in the InAs on either side of the barrier produce the bias dependent capacitance. InAs is used as the electrode material because it has a very high electron mobility, and it is easy to make low resistance ohmic contacts to it. These properties are important in lowering the parasitic series resistances, R_s, that would act to give a large R_sC product limiting the frequency response. The digital alloy barrier has been used as it is found to result in a higher barrier resistance than that found with an AlSb barrier. The digital alloy also relieves some of the concern about using a pure AlSb barrier that may easily decay due to oxidation.

Code 6853, the High Speed / Low power Devices Section, has fabricated HBVs and made current-voltage, and S-parameter measurements on them. The C-V data extracted from S-parameter measurements on a large area device is shown in the figure along with a curve generated by the Chalmer's model. A high C_max/C_min ratio is desirable, and our device exceeds the value expected by the model. Other features of our data that differ from the model are explained qualitatively by including accumulation layers at the InAs/barrier interfaces. Extrapolating our data to a micron size device leads to the expectation of operation at frequencies as high as 10 THz.

2008

• InAs-based Heterostructure Barrier Varactor Diodes with In_0.3Al_0.7As_0.4Sb_0.6 as the Barrier Material, Solid State Electronics 52, 1829 (2008).

Low-Power pN THz Mixer Diode

Dr. Richard Magno, richard.magno@nrl.navy.mil

Mixer diodes operating at THz frequencies are needed for use in imaging arrays, communication devices and in spectroscopic systems for the detection of chemical and biological agents. An example of a circuit using a mixer in an anti-parallel pair geometry is illustrated in the figure. In an array, many pairs of diodes would need to be driven by the local oscillator source, and a highly complicated set of wires would be required to route the signals. It is desirable to make these systems lightweight and small to be hand carried or used in a satellite. A major problem, particularly in a system containing an array with a large number of elements, is the availability of small lightweight local oscillator sources in the THz frequency range. GaAs Schottky diodes are available for use in the THz frequency range, but their saturation current densities are orders of magnitude smaller than the InGaSb/InAlAsSb pN diode being developed here. The small saturation current density results in the need for a much higher power local oscillator source than needed with this diode. The high saturation current for the pN diode is a result of the 0.5 eV bandgap of the InGaSb layer, and the bandgap could easily be made smaller by using a higher indium fraction in the InGaSb.

pN diodes are generally not used in high frequency mixer application because of the large diffusion capacitance associated with the minority carriers injected at forward bias. This problem is minimized by the design of the Sb-based diode developed here. There is a large valence band offset between the p InGaSb and the n InAlAsSb that prevents holes from being injected into the InAlAsSb at forward bias thereby eliminating the minority hole contribution to the diffusion capacitance. The minority electron concentration in the p InGaSb layer is minimized by using a very thin InGaSb layer with a high electron mobility that results in the minority electrons diffusing through the InGaSb and being removed at the ohmic contact.

The capacitance-voltage data extracted from S-parameter measurements, made by Code 6853 on devices fabricated by Code 6853, the High Speed / Low power Devices Section, are easily fit by a depletion model up to a forward bias of 0.3V and to that point show no sign of a diffusion capacitance. The resistance and capacitance data, also shown in the figure, from the S-parameter measurements indicate that the RC time constants are small enough to allow operation of the diode at THz frequencies. Modifications to the layer structure are under development to allow lower resistances and even higher frequency operation than indicated here.

2012

• High-frequency, 6.2 Å pN heterojunction diodes, Solid-State Electronics 67, 105 (2012)

2011

• Antimonide-based pN terahertz mixer diodes, Journal of Vacuum Science & Technology, Vol 29 Issue 3, 03C109 (2011).

2008

• Antimonide diodes for low-power THz mixers, Applied Physics Letters 92, 243501 (2008).

"Material contained herein is made available for the purpose of peer review and discussion and does not necessarily reflect the views of the Department of the Navy or the Department of Defense."