Channel Heterostructure Field Effect Transistors Research Articles

Polarization engineering via InAlN/AlGaN heterostructures for demonstration of normally-off AlGaN channel field effect transistors

Normally-off AlGaN channel heterostructure field effect transistors (HFETs) have been proposed and investigated numerically by taking advantage of the polarization engineering perspective in III-nitrides. The utilization of polarization-matched InAlN/AlGaN heterostructures shifted the threshold voltage to approximately 1.1 V for an Al0.3Ga0.7N channel HFET. Compared to the AlGaN/GaN HFEF with a low breakdown voltage, the normally-off InAlN/AlGaN HFETs show substantially enhanced breakdown characteristics. Besides, the two-dimensional electron gas (2DEG) mobility in the InAlN/AlGaN HFETs exhibits much less reduction with the increasing temperature than that in the AlGaN/GaN HFET, according to the calculation of electron mobility dominated by alloy disorder scattering and polar optical phonon scattering. As a result, the temperature dependences of the power figures of merit based on the conduction loss consideration in terms of 2DEG mobility variation and Johnson figure of merit show superior potential for polarization-matched InAlN/AlGaN HFETs in high-power and high-frequency electronics applications particularly operating at elevated temperatures. This work provides a useful way for demonstration of normally-off AlGaN channel HFETs and is also helpful for design of future devices, which can be beneficially exploited from polarized III-nitride heterostructures.

Current collapse in high-Al channel AlGaN HFETs

We report the first study of current collapse in ultra-wide bandgap high-Al AlGaN channel heterostructure field-effect transistors (HFETs) using short-pulse biasing. Our results show that, under applied pulsed gate and drain voltages, the current collapse results from increased resistance of the source- gate and gate-drain regions but not from the channel under the gate. We also show that passivation of access regions of the high-Al channel HFET with SiNx results in significant reduction in current collapse.

Si–Ge interdiffusion under oxidizing conditions in epitaxial SiGe heterostructures with high compressive stress

Si–Ge interdiffusion under oxidizing and inert conditions has been studied in epitaxial relaxed Si1−xGex/compressive Si1−yGey/relaxed Si1−xGex heterostructures. The interdiffusion was measured by secondary ion mass spectroscopy (SIMS) and studied using simulations. Within the SIMS accuracy, the measured Ge profiles show that oxidation has a small effect, if any, on the Si–Ge interdiffusion of these structures. These results suggest that oxidation does not accelerate Si–Ge interdiffusion significantly, which lessens process integration constraints for SiGe devices such as high mobility dual channel metal oxide semiconductor field effect transistors and heterostructure tunneling field effect transistors.

Electron mobility in InGaN channel heterostructure field effect transistor structures with different barriers

InGaN possesses higher electron mobility and velocity than GaN, and therefore is expected to lead to relatively better performances for heterostructure field effect transistors (HFETs). However, the reported mobilities for AlGaN∕InGaN HFETs are lower than GaN channel HFETs. To address this issue, we studied the effect of different barriers on the Hall mobility for InGaN channel HFETs grown by metal organic chemical vapor deposition. Unlike the conventional AlGaN barrier, the AlInN barrier can be grown at the same temperature as the InGaN channel layer, alleviating some of the technological roadblocks. Specifically, this avoids possible degradation of the thin InGaN channel during AlGaN growth at high temperatures; and paves the way for better interfaces. An undoped In0.18Al0.82N∕AlN∕In0.04Ga0.96N HFET structure exhibited a μH=820cm2∕Vs, with a ns=2.12×1013cm−2 at room temperature. Moreover, with an In-doped AlGaN barrier, namely, Al0.24In0.01Ga0.75N, grown at 900°C, the μH increased to 1230cm2∕Vs with a ns of 1.09×1013cm−2 for a similar InGaN channel. Furthermore, when the barrier was replaced by Al0.25Ga0.75N grown at 1030°C, μH dropped to 870cm2∕Vs with ns of 1.26×1013cm−2 at room temperature. Our results suggest that to fully realize the potential of the InGaN channel HFETs, AlInN or AlInGaN should be used as the barrier instead of the conventional AlGaN barrier.

Study of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors

The device properties of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors (DDDCHFETs) are comprehensively analyzed and demonstrated. Based on the variations of delta-doped densities of InGaAs double channels and GaAs spacer thickness, the dc and rf characteristics are compared and studied. Due to the employed InGaAs DDDC structure and Schottky behaviors of InGaP “insulator,” good pinch-off and saturation characteristics, higher and linear transconductance, and good rf performances are obtained. Experimentally, for comparison, a practical DDDCHFET with good device properties is fabricated successfully. It is known that the experimental results are very consistent with theoretical simulation data.

Study of InGaP/InGaAs double doped channel heterostructure field-effect transistors (DDCHFETs)

InGaP/InGaAs double doped channel heterostructure field-effect transistors (DDCHFETs) with δ-doped channels and uniformly doped channels are comprehensively studied and demonstrated. From the simulation results, based on a two-dimensional simulator of Atlas, the band diagrams, DC and RF characteristics of studied devices are compared and studied. The better drain current drivability, higher transconductance and microwave performances are obtained in the studied device with δ-doped channel. In addition, experimentally, the DDCHFET with δ-doped quantum well channel is fabricated successfully. Due to the employed InGaAs double δ-doped channel structure and Schottky behaviours of InGaP ‘insulator’, good DC properties including pinch-off and saturation characteristics, higher and linear transconductance, and good RF properties are obtained. Moreover, the experimental results are consistent with simulated data.

Design of composite channels for optimized transport in nitride devices

Heterostructure field effect transistors (HFETs) based on AlGaN / GaN structures have shown good performance as high power and high frequency devices. Theoretical simulations of transport in short channel HFETs show that there are several areas where considerable improvements in mobility, etc. can be made if thin composite structures (as considered in this work) can be utilized. We examine transport in a metal / AlGaN / InN / GaN composite structure. The InN region is very thin (∼ 15 Å) and is introduced to improve the low field transport without significantly impacting the device breakdown properties. Our model is capable of examining any other type of composite structure as well. Our simulation method consists of charge control solution of the heterostructure wave functions, followed by Monte Carlo simulation of scattering and free flight events. We present comparison of results on i) metal / AlGaN / GaN structure, and ii) a metal / AlGaN / GaN structure with a thin InN channel of the order of a few mono-layers. We find that mobility in the channel can improve considerably with very little effect on the mobility - charge product. Indeed, the charge density induced in the thin InN channel region is ∼ 1013cm∼2. While the peak velocities in a metal / AlGaN / InN / GaN structure exhibit an increase of nearly 30% over the values in metal / AlGaN / GaN structures, the low field mobilities are also increased. Low-field mobilities of ∼ 2500cm2(V ·s)∼ are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface disorder (∼ 70Å), we have found good agreement with experimental Hall mobility data for similar structures. At higher electric fields, we find that most electron population transfers to higher valleys or other sub-bands that lie in AlGaN or GaN. This ensures that high field breakdown of low band gap InN layer is also suppressed.

Characteristics of δ-doped InP heterostructures using In0.34Al0.66As0.85Sb0.15 Schottky layer

Single and coupled δ-doped InP channel heterostructure field-effect transistors using high barrier height (>0.73 eV) and wide energy band gap (∼1.8 eV) In0.34Al0.66As0.85Sb0.15 Schottky layer have been successfully grown by low-pressure metalorganic chemical vapor deposition. Comprehensive material characterizations have been conducted and investigated for various growth specifications, including the Hall measurement, secondary ion mass spectrometry, double crystal x-ray diffraction technique, and photoluminescence measurement. Excellent gate-source breakdown characteristics (>40 V), low leakage current (111 μA/mm), improved gate voltage swing, improved carrier mobility, good saturation and pinch-off properties, and low output conductance (0.5 mS/mm) have been achieved by using the high barrier height In0.34Al0.66As0.85Sb0.15 Schottky layer and InP channels. The proposed structure provides good potential for high-power circuit applications.

Investigation of temperature-dependent characteristics of an n/sup +/-InGaAs/n-GaAs composite doped channel HFET

The temperature-dependent characteristics of an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -InGaAs/n-GaAs composite doped channel (CDC) heterostructure field-effect transistor (HFET) have been studied. Due to the reduction of leakage current and good carrier confinement in the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -InGaAs/n-GaAs CDC structure, the degradation of device performances with increasing the temperature is insignificant. Experimentally, for a 1 x 100 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device, the gate-drain breakdown voltage of 24.5 (22.0) V, turn-on voltage of 2.05 (1.70) V, off-state drain-source breakdown voltage of 24.4 (18.7) V, transconductance of 161 (138) mS/mm, output conductance of 0.60 (0.60) mS/mm, and voltage gain of 268 (230) are obtained at 300 (450) K, respectively. The shift of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> from 300 to 450 K is only 13 mV. In addition, the studied device also shows good microwave performances with flat and wide operation regime.

DC and RF Characteristics of AlN/GaN Doped Channel Heterostructure Field Effect Transistor

First small signal, large signal and RF power characteristics of experimental 3 µm gatelength AlN/GaN devices between RT and 200°C are reported. A high drain breakdown of 42 V and the absence of dispersion effects indicate that this challenging heterostructure is indeed attractive for the development of high power/high speed devices.

DC and RF characteristics of AlN/GaN doped channelheterostructure field effect transistor

First small signal, large signal and RF power characteristics of experimental 3 µm gatelength AlN/GaN devices between RT and 200°C are reported. A high drain breakdown of 42 V and the absence of dispersion effects indicate that this challenging heterostructure is indeed attractive for the development of high power/high speed devices.

Impact Ionization Effects on the Microwave Performance of InAs Channel Heterostructure Field-Effect Transistors: the Role of Channel Quantization

The shrinking channel energy gaps in millimeter-wave heterostructure field-effect transistors (HFETs) make the devices more susceptible to impact ionization. Until recently, the effects of impact ionization on the microwave performance of narrow bandgap channel HFETs have largely been ignored. We studied the effects of impact ionization and quantum confinement on the microwave characteristics of narrow bandgap InAs/AlSb millimeter-wave HFETs. We systematically varied the level of impact ionization in HFETs through channel quantization and bias conditions, and compared the microwave behavior of InAs/AlSb HFETs (with different quantum well widths) to that of conventional GaAs metal semiconductor field effect transistors (MESFETs) fabricated to provide a low ionization benchmark. The observed trends in the S- and Y-parameter data enable us to identify the physical effects of impact ionization on the microwave behavior of narrow-gap channel HFETs. We found that field effect transistor (FET) equivalent circuit models are grossly inadequate when InAs channel HFETs are operated under high impact ionization bias conditions.

Scaling of gate length in ultra-short channel heterostructure field effect transistors

Ultra-short channel AlGaAs/GaAs modulation doped field effect transistors (MODFETs) have been fabricated with gate lengths ranging from 25 to 115 nm, using an electron beam lithography process, in order to examine the fundamental limitation of transistor scaling. For gate lengths in the tens of nanometer range, where the gradual channel approximation is no longer valid, it is observed that the transconductance is improved by electron velocity overshoot. Velocity overshoot starts to occur in our devices with sub-70 nm gate lengths, which is smaller than that expected from other published experimental results. This is due to the suppression of the velocity overshoot by a parasitic gate-fringing effect in a reduced gate aspect ratio. However, we find that the enhancement in transconductance by the velocity overshoot is limited with further scaling and the transconductance starts to drop at a gate length below 40 nm. Short-channel electron tunneling is suggested experimentally to explain the degradation of transistor performance in the shortest channel devices.

Flicker noise in GaN/Al/sub 0.15/Ga/sub 0.85/N doped channel heterostructure field effect transistors

We have investigated noise characteristics of novel GaN/Al/sub 0.15/Ga/sub 0.85/N doped channel heterostructure field effect transistors designed for high-power density applications. The measurements were carried out for various gate bias voltages V/sub GS/ and with the drain voltage V/sub DS/ varying from the linear to the saturation regions of operation V/sub DS/>5 V. Our results show that flicker, e.g., 1/f noise, is the dominant limiting noise of these devices; and the Hooge parameter is of the order of 10/sup -5/-10/sup -4/. The gate voltage dependence of 1/f noise was observed in the linear region for all examined V/sub GS/ and in the saturation region for V/sub GS/>0. These results indicating low values of the Hooge parameter are important for microwave applications.

Comparison of single- and double-barrier pseudomorphic InGaP/InGaAs HFETs

We report the growth and fabrication of pseudomorphic InGaP/InGaAs/GaAs doped-channel heterostructure field effect transistors (HFETs). A direct comparison between single and double barrier HFETs of otherwise nominally identical devices shows that the use of a double barrier structure reduces the output conductance and enhances both the current gain cutoff frequency f T and the maximum oscillation frequency f max. The maximum drain current reaches 630 mA/mm for highly strained In 0.30Ga 0.70As channel devices compared with 360 mA/mm for comparably doped In 0.15Ga 0.85As channel HFETs. Compared to that of In 0.15Ga 0.85As channel devices, the In 0.30Ga 0.70As channel HFETs yield a higher maximum oscillation frequency while having a comparable current gain cutoff frequency. Double barrier HFETs with an In 0.15Ga 0.85As channel yield f T=19 GHz and f max=63 GHz, while those with an In 0.30Ga 0.70As channel show a result of 18 and 78 GHz, respectively. These results demonstrate that the use of highly strained InGaAs channels and double InGaP barriers significantly improves device performance.

A novel analytical model for short channel heterostructure field effect transistors

In this paper, we propose a novel analytical model for accurate and efficient simulation of integrated circuits with short channel heterostructure field effect transistors (HFETs). The proposed model for short channel HFETs is based on an analytical expression of the voltage-dependent two-dimensional electron gas density, which is either a linear or logarithmic function of the gate voltage. This compact model accurately describes the electrical characteristics of HFETs over the whole operation range including linear, saturation, and subthreshold operation without any discontinuity, which is known to be a detrimental factor for accurate modeling and simulation of analog and digital integrated circuits with field effect transistors. This new compact model has been verified with experimental data obtained from an Al 0.3Ga 0.7As/GaAs/In 0.13Ga 0.87As double heterojunction pseudomorphic HFET with a gate length L g=1 μm.

Substrate Bias Effects in AlGaN/GaN Doped Channel Heterostructure Field Effect Transistors Grown on Doped SiC Substrates

Substrate Bias Effects in AlGaN/GaN Doped Channel Heterostructure Field Effect Transistors Grown on Doped SiC Substrates

AlGaN/GaN doped channel heterostructure field effect transistors

AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) are expected to have a high transconductance and a high saturation current because of the relatively high electron mobility and high saturation velocity of the two dimensional electron gas (2DEG) in AlGaN/GaN modulation doped structures. Our experimental results revealed that, in the fabricated devices, the transconductance is limited by a large source series resistance, Rs, and by a small sheet carrier concentration, ns, of the 2DEG in the device channel. In order to increase ns, we recently proposed to use a Doped Channel HFET (DC-HFET) design, similar to that used in AlGaAs/GaAs DC-HFETs. In AlGaN/GaN structures, an additional impurity scattering caused by the channel doping has relatively less of an effect on the electron mobility than in AlGaAs/GaAs devices, due to a much higher electron effective mass in GaN. One micron gate AlGaN/GaN Doped Channel HFETs reached the cutoff frequency fT of 18.3 GHz. Short channel AlGaN/GaN Doped HFETs with 0.25 micron gates demonstrated the cutoff frequency fT up to 36.1 GHz and the maximum oscillation frequency, fmax ≈ 97.3 GHz. These values of fT and fmax are the highest value reported for any wide band gap semiconductor device. Our simulations of AlGaN/GaN Doped Channel HFETs show that transconductances in access of 1000 mS/mm may be obtained for very thin AlGaN layers, a large gate voltage swing, and for a large surface sheet density of the 2D electron gas.

Investigation of step-doped channel heterostructure field-effect transistor

A new heterostructure field-effect transistor (FET) with an InGaAs step-doped-channel (SDC) profile has been fabricated and demonstrated. The SDCFET studied provides the advantages of high current density, high breakdown voltage, wide gate voltage swing for high transconductance, and adjustable threshold voltage. A simple model is employed to analyse the performance of threshold voltage. For comparison two kinds of SDCFETs have been fabricated. For the 1 × 100 µm2 gated dimension, maximum drain saturation currents of 735 and 675 mA/mm, maximum transconductances of 200 and 232 mS/mm, gate breakdown voltages of 15 and 12 V, wide gate voltage swing of 3.3 and 2.6 V with transconductance gm higher than 150 mS/mm, and threshold voltage –3.7 and –1.8 V are obtained, respectively. These good performance figures show the SDCFET has good potential for high-speed, high-power circuit applications.

CW operation of short-channel GaN/AlGaN doped channel heterostructure field effect transistors at 10 GHz and 15 GHz

We report on a 0.15-/spl mu/m gate length AlGaN/GaN doped channel heterostructure field effect transistor (DC-HFET) with maximum frequency of oscillation in excess of 97 GHz. HFETs based on our doped channel design exhibited CW microwave operation up to 15 GHz with a maximum output power of approximately 270 mW/mm at 10 GHz. These values are still limited by parasitics and can be significantly improved by optimizing the device design.