Heterostructure Insulated-gate Field-effect Transistors Research Articles

III-nitride microwave control devices and ICs

The growing complexity and functionality of modern microwave systems put increasing demands on the performance and reliability of RF control devices. Low channel sheet resistance in the on-state and small leakage currents in the off-state, extremely high breakdown voltages, chemical inertness and planar structure make III-nitride heterostructure-based devices uniquely suitable for the next-generation switching devices. The AlInN-based devices and devices using low-conducting layers for controlling surface field and surface charges are expected to further boost the performance. This paper presents a review of III-nitride control devices based on heterostructure field-effect transistors (HFETs), insulated gate HFETs (MISHFETs) and novel capacitively coupled contact and frequency configurable electronics technologies.

Power Stability of AlGaN/GaN HFETs at 20 W/mm in the Pinched-Off Operation Mode

High power-added efficiency (PAE) (ap74%) and rf-power (20 W/mm) operation of Schottky and insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) is reported at 2 GHz. In the pinched-off mode of operation, the PAE increases from a value of 55% to 74% when the drain bias is changed from 35 to 60 V. While both the Schottky and the insulated HFETs show high powers and PAE values, only the insulated-gate devices are stable at 20-W/mm output powers during a 60-h continuous wave rf-stress test. Their power drop of less than 0.1 dB is much smaller than the 0.8-dB drop for identical geometry Schottky-gate HFETs. The superior stability of the insulated-gate HFETs is attributed to the low forward gate currents

Performance stability of high‐power III‐nitride metal‐oxide semiconductor‐heterostructure field‐effect transistors

Record high RF power densities up to 30 W/mm have been recently achieved using AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs). These results exhibit the promise of III-Nitride HFETs for high-power microwave applications. However the output RF power of these devices was found to degrade with the operation time. Gate leakage current in conventional HFETs is a major contributor to device instability and failure. We previously reported on the development of Metal-Oxide-Semiconductor Insulated gate HFETs (MOSHFETs) with gate leakage currents four to six orders of magnitude lower than those of the HFETs. The MOSHFETs has demonstrated a significant performance improvement as compared to conventional HFETs resulting in higher channel currents and RF powers. In this study, we present the first comparative experimental evaluation of the HFET and MOSHFET lifetime at room and elevated temperatures. The room temperature HFET stability lifetime as high as 2.4 years is estimated. We also present the detailed study of the HFETs stabilization and failure mechanisms. The pronounced similarity of the mechanisms of device degradation and current collapse is demonstrated. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Superior suppression of gate current leakage in Al2O3/Si3N4 bilayer-based AlGaN/GaN insulated gate heterostructure field-effect transistors

AlGaN/GaN-based metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayer as insulator have been investigated in detail, and compared with the conventional HFET and Si3N4-based MIS-HFET devices. Al2O3/Si3N4 bilayer-based MIS-HFETs exhibited much lower gate current leakage than conventional HFET and Si3N4-based MIS devices under reverse gate bias, and leakage as low as 1×10−11 A/mm at −15 V has been achieved in Al2O3/Si3N4-based MIS devices. By using ultrathin Al2O3/Si3N4 bilayer, very high maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been obtained in the MIS-HFET device with gate length of 1.5 µm, a reduction less than 5% in maximum transconductance compared with the conventional HFET device. This value was much smaller than the more than 30% reduction in the Si3N4-based MIS device, due to the employment of ultra-thin bilayer with large dielectric constant and the large conduction band offset between Al2O3 and nitrides. This work demonstrates that Al2O3/Si3N4 bilayer insulator is a superior candidate for nitrides-based MIS-HFET devices.

Comparison of AlGaN/GaN Insulated Gate Heterostructure Field-Effect Transistors with Ultrathin Al2O3/Si3N4 Bilayer and Si3N4 Single Layer

Device performances have been compared between two types of AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayers and a Si3N4 single layer. Al2O3/Si3N4 bilayer-based MIS-HFETs have much lower gate current leakage than Si3N4-based MIS devices by more than 3 orders of magnitude under reverse gate biases. An ultralow gate leakage of 1×10-11 A/mm at -15 V has been achieved in the Al2O3/Si3N4 bilayer-based MIS devices though higher maximum drain-source current has been obtained in the Si3N4-based MIS devices. A maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been achieved in the ultrathin Al2O3/Si3N4 bilayer-based MIS-HFET device with a gate length of 1.5 µm, which is much higher than that of less than 130 mS/mm in the Si3N4-based MIS devices. The reduction in the transconductance of Al2O3/Si3N4 bilayer-based devices was much smaller than that in the Si3N4-based MIS devices due to the employment of ultrathin bilayers with a large dielectric constant.This work demonstrates that an Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices.

INSULATED GATE III-N HETEROSTRUCTURE FIELD-EFFECT TRANSISTORS

Unique materials properties of GaN -based semiconductors that make them promising for high-power high-temperature applications are high electron mobility and saturation velocity, high sheet carrier concentration at heterojunction interfaces, high breakdown field, and low thermal impedance (when grown over SiC or bulk AlN substrates). The chemical inertness of nitrides is another key property. An AlGaN / GaN Heterostructure Field Effect Transistor (HFET) has been a topic of intensive investigations since the first report in 1991 [1]. Several groups demonstrated high power operation of AlGaN / GaN HFETs at microwave frequencies [2,3,4], including a 100 W output power single chip amplifier developed by Cree, Inc. and devices with 100 GHz cut-off frequency reported in [5]. However, in spite of impressive achievements, the potential of nitride based HFETs has not been fully realized as yet. The RF powers expected from the fundamental properties of nitride based materials significantly exceed the experimental data. One of the key problems limiting the HFETs RF characteristics is high gate leakage currents causing DC and RF parameter degradation. When the gate voltage goes positive the forward leakage current shunts the gate-channel capacitance and thus limits the maximum device current. When the gate voltage is negative, high voltage drop between the gate and the drain causes premature breakdown and thus limits maximum applied drain voltage. In addition, gate leakage currents increase the device sub-threshold currents, which decrease the achievable amplitude of the RF output. These limitations become even more severe at high ambient temperatures. The characteristics of III-N HFETs can be considerably improved by implementing a new approach, which results from the demonstration of good quality of SiO 2/ AlGaN and Si 3 N 4/ AlGaN interfaces. This approach opens up a way to fabricate insulated gate heterostructure field-effect transistors (IGHFETs), which have the gate leakage currents several orders of magnitude below those of regular HFETs, and exhibit better linearity and higher channel saturation currents. In this chapter, we describe design, characterization and applications of these novel devices.

Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors

We have systematically investigated effects of plasma processing, formation of Si-based dielectrics, and formation of a thin Al2O3 film on the chemical and electronic properties of GaN and GaN/AlGaN heterostructure surfaces. The surface treatment in H2-plasma excited by electron-cyclotron-resonance (ECR) source, produced nitrogen-vacancy-related defect levels at GaN and AlGaN surfaces, while the ECR-N2-plasma treatment improved electronic properties of the surfaces. The deposition of a SiO2 film on GaN and AlGaN surfaces was found to induce high-density interface states, due to unexpected and uncontrollable oxidation reactions on the surfaces during the deposition process. In comparison, the SiNx/GaN passivation structure prepared by ECR-plasma assisted chemical vapor deposition showed good interface properties with the minimum Dit value of 1×1011 cm−2 eV−1. However, excess leakage currents governed by Fowler–Nordheim tunneling were observed in the SiNx/Al0.3Ga0.7N structure, due to a relatively small conduction band offset of 0.7 eV between SiNx and Al0.3Ga0.7N. A novel Al2O3-based passivation structure was proposed and fabricated by molecular beam deposition of Al and subsequent ECR O2-plasma oxidation. In situ x-ray photoelectron spectroscopy showed successful formation of the Al2O3 layer with a thickness of 3.5 nm and a large conduction band offset of 2.1 eV between Al2O3 and A0.3Ga0.7N. The GaN/AlGaN insulated-gate heterostructure field-effect transistors (IG HFETs) having the Al2O3-based passivation structure showed a good gate control of drain currents up to VGS=+3 V and achieved drain saturation current of 0.8 A/mm. The observed maximum gm value is 120 mS/mm. No current collapse was observed in the Al2O3 IG HFETs, indicating a remarkable advantage of the present Al2O3-based passivation structure.

Reduced short-channel effects in submicron N-HIGFET technology using sidewalls

In this letter, 0.35 /spl mu/m gate length pseudomorphic AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect-transistors (HIGFETs) have been fabricated on GaAs. The short-channel effects have been reduced by using a sidewall technology. A high current density and a high transconductance were obtained, reflectively, 510 mA/mm and 550 mS/mm, in addition to a high value of extrinsic current gain cutoff frequency F/sub T/=44 GHz. The dependencies of subthreshold current, threshold voltage, and output conductance on gate length have been emphasised.

Simulation of the Piezoelectric Effect on the Device Characteristics of AlGaN/GaN Insulated-Gate Heterostructure Field Effect Transistors

The piezoelectric effect (PE) on the device characteristics due to lattice mismatch in the heterostructure was simulated by solving Poisson and Schrodinger equations self-consistently for GaN-based field effect transistors (FET). By comparing the simulation results obtained with and without PE, we found the following. (1) PE shifts the threshold voltage (Vth) of FET; this Vth shift widens the flat region in transconductance–gate voltage (Gm–Vgs) and cutoff frequency–gate voltage (fT–Vgs) characteristics. (2) Gm is higher with PE than without PE, while fT values are nearly the same for both with and without PE. (3) The equilibrium two-dimensional channel electron gas (2DEG) concentration (Nch) increases considerably with PE. This 2DEG concentration greatly reduces the parasitic resistance between the gate and the source/drain. We concluded that PE improves FET performance, and we determined the optimal FET structure with PE from the viewpoint of charge control.

Current/voltage characteristic collapse in AlGaN/GaNheterostructure insulatedgate field effect transistors at high drain bias

The authors describe the current/voltage characteristic collapse under a high drain bias in AlGaN/GaN heterostructure insulated gate field effect transistors (HIGFETs) grown on sapphire substrates. These devices exhibit a low resistance state and a high resistance state, before and after the application of a high drain voltage, respectively. At room temperature, the high resistance state persists for several seconds. The device can also be returned into the low resistance state by exposing it to optical radiation. Electron trapping in the gate insulator near the drain edge of the gate is a possible mechanism for this effect, which is similar to what has been observed in AlGaAs/GaAs HFETs at cryogenic temperatures.

Gate current in AlInAs/GaInAs heterostructure insulated-gate field-effect transistors (HIGFETs)

A self-aligned WSi gate heterostructure insulated-gate field-effect transistor (HIGFET) with a gate length of 1 mu m was fabricated using an AlInAs/GaInAs heterostructure grown by atmospheric pressure metal-organic chemical vapor deposition (MOCVD). The gate current is investigated experimentally and theoretically. The measured gate current was found to be about two orders of magnitude higher than predicted by theory. The origin of this increase is unclear. However, the theoretical result suggests the possibility of reducing the gate current in AlInAs/GaInAs HIGFETs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Subthreshold and above threshold gate current in heterostructure insulated gate field-effect transistors

Experimental data are presented which show that the gate current at zero drain bias in HIGFETs changes qualitatively when the gate voltage is varied from below to above threshold. Above threshold, the gate current flows from the entire channel, and below threshold, the gate current flows from the gate edges. A new independent method for deducing threshold voltage is introduced.

In/sub 0.52/Al/sub 0.48/As/In/sub x/Ga/sub 1-x/As (0.53&amp;gt;or=x&amp;gt;or=0.70) lattice-matched and strained heterostructure insulated-gate FETs

The DC and microwave properties of In/sub 0.52/Al/sub 0.48/Al/In/sub x/Ga/sub 1-x/As (0.53<or=x<or=0.70) heterostructure insulated gate field-effect transistors (HIGFETs) with a quantum well channel design are presented. DC and microwave transconductances (g/sub m/) are enhanced as the In content is increased in the InGaAs channel. An intrinsic microwave g/sub m/ value of 428 mS/mm and a K-factor of 1140 mS/mm-V have been obtained for 1.0- mu m gate length with the 65% In channel devices. The sheet charge density, drift mobility, transconductance, current-gain cutoff frequency (f/sub T/), and maximum oscillation frequency (f/sub max/) all show a continuous improvement up to 65% In content (f/sub T/=22.5 GHz with 53% and f/sub T/=27 GHz with 65% In; the corresponding f/sub max/ change is from 6.5 to 8 GHz). The device performance degrades as the In content is increased to 70%. DC and microwave characteristics show the presence of negative differential resistance (NDR) up to 2.7 GHz.<<ETX>>

Measurement of source and drain series resistances of HIGFETs using a bias-scan method

In this work, we present a bias-scan method to evaluate the source and drain series resistances of heterostructure field-effect transistors, which offers a simple, direct, fast, and reliable technique for on-wafer probing, and can be applied to automatic characterization systems. The source and drain resistances of AlAs/GaAs heterostructure insulated-gate field-effect transistors (HIGFETs) with both undoped and doped channels are studied using this method.

Electron mobility measurement in short-channel FET's using the cutoff frequency method

The cutoff frequency (FT) method is presented to measure electron mobility in short-channel field-effect transistors (FETs). This technique was used to study the electron mobilities in AlGaAs-GaAs self-aligned heterostructure insulated-gate field-effect transistors (HIGFETs), with both undoped and doped channels (E and D mode). The structures were molecular-beam-epitaxy (MBE) grown on

Observation of electron velocity overshoot in AlxGa1−xAs/GaAs heterostructure insulated-gate field-effect transistors

The factor γ=1+(RS+RD)/Rds is suggested to modify the expression of effective electron velocity in field-effect transistors (FETs), where RS and RD are the source and drain resistances, respectively, and Rds is the intrinsic drain-to-source resistance. Based on this modified expression ν′eff = 2πLfTγ, where L is the gate length and fT is the cut-off frequency, velocity overshoots were observed clearly at room temperature in AlxGa1−xAs/GaAs heterostructure insulated-gate FETs with both undoped and doped channels.

A 0.4- mu m gate-length AlGaAs/GaAs P-channel HIGFET with 127-mS/mm transconductance at 77 K

WN-gate, p-channel AlGaAs-GaAs heterostructure insulated-gate field-effect transistors (HIGFETs) fabricated on a metalorganic vapor-phase epitaxy (MOVPE) wafer are discussed. A self-aligned Mg ion implantation (80 keV, 6*10/sup 13/ cm/sup -2/) annealed at 850 degrees C in an arsine atmosphere and the control of the SiO/sub 2/ sidewall dimensions allow the fabrication of p-channel HIGFETs with a gate length smaller than 0.5 mu m with low subthreshold current. P-channel HIGFETs with 0.4- mu m gate lengths exhibit extrinsic transconductances as high as 127 mS/mm at 77 K and 54 mS/mm at 300 K. >

The importance of contact injection in undoped AlGaAs/GaAs heterostructures

A two-dimensional finite-difference analysis is used to simulate the current flow and charge distribution in undoped AlGaAs/GaAs MODFET structures called heterostructure insulated-gate field-effect transistors (HIGFETs). How an electron channel can be induced at the AlGaAs/GaAs interface when there is no doped supply layer is studied. The origins of the charge in this channel are studied by comparing its behavior in equilibrium with that when a current flows. By considering the different geometries of a Schottky gate and implanted source and drain contacts, the interrelationship between gate control and electron injection from the doped contact regions is shown and a graphic view of the origins of the interfacial charge in a HIGFET is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum-well p-channel AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors

The authors report experimental and theoretical results for self-aligned gate quantum-well p-channel pseudomorphic GaAs/InGaAs/AlGaAs heterostructure insulated-gate field-effect transistors (HIGFETs). High transconductances and transconductance parameters, 113 mS/mm and 305 mS/mm/V at room temperature for 0.8- mu m gate length, are reported. The authors discuss the effects of built-in strain on the valence bands, analyze the device data on the gate length dependence of the device parameters, and discuss the subthreshold characteristics and evidence for implant straggle. They also show the importance of the gate current for the device characteristics and propose to reduce the gate current and thus to increase the voltage swing by fabricating p-channel devices with p-doped InGaAs channels and n/sup +/ AlGaAs gates on the insulating AlGaAs layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low-frequency noise in p-channel heterostructure insulated-gate field-effect transistors (HIGFETs) at 77 K and drain current 1 mu A

Measurements of the low-frequency spectral intensity of the current fluctuations in p-channel GaAs/AlGaAs heterostructure insulated-gate field-effect transistors are discussed. The measurements were performed at 77 K and a drain current of 1 mu A. The spectra of two types of devices are compared, one grown directly on the substrate and the other embedded in an n-well. The latter type produced markedly less noise, its spectrum being almost perfect 1/f noise. The former type exhibited, in addition to the 1/f noise, a significant generation-recombination noise component in the spectrum.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>