HFET Structure Research Articles

High-current recessed gate enhancement-mode ultrawide bandgap Al x Ga1−x N channel MOSHFET with drain current 0.48 A mm−1 and threshold voltage +3.6 V

We report a recessed-gate enhancement-mode Al2O3-ZrO2/Al0.6Ga0.4N/Al0.4Ga0.6N metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) with drain current as high as 0.48 A mm−1 at a gate-source voltage of +12 V. This was enabled by a pseudomorphic HFET structure with graded back barrier for strain management and to screen the growth interface from the channel. The device exhibited a threshold-voltage (V TH) of 2.75 ± 0.57 V with absolute maximum V TH = 3.6 V, a +12.2 V shift from that for a depletion-mode MOSHFET fabricated on the same wafer. A 3-terminal breakdown voltage of 700 V was measured in the off-state, showing the viability of E-mode UWBG AlGaN for power electronics.

Impact of AlN/Si Nucleation Layers Grown Either by NH3‐MBE or MOCVD on the Properties of AlGaN/GaN HFETs

Epitaxial growth of group III nitrides on silicon substrates is challenging as large lattice and thermal expansion coefficient mismatches have to be accounted for. It is generally accepted that AlN is the best nucleation layer for GaN‐on‐Si growth. However, some open questions remain about this AlN nucleation layer. In this study, it is investigated how the choice of deposition method of the AlN nucleation layer affects the material and electrical properties of a full HFET structure. First, electrical characterization is performed on thin AlN layers deposited on two inch p‐type silicon substrates grown by NH3‐MBE. CV characteristics indicate that electron injection from an inversion‐like layer at the AlN/Si‐interface is suppressed. Based on previous studies, this leads to the expectation that choosing MBE‐AlN may actually result in superior properties at the device level. Finally, the full HFET stack is grown by MOCVD on both types of AlN nucleation layers. Hall measurements on active devices yield comparable mobilities for both nucleation methods. Preliminary data suggest that MBE‐AlN does not have any detrimental impact the dynamic properties of the buffer. The vertical breakdown voltage measured at 1 μA mm−2 is increased by 30% and the leakage current measured at 600 V is reduced by two orders of magnitude.

TCAD Simulations and Small Signal Modeling of DMG AlGaN/GaN HFET

This article presents extraction of small signal model parameters and TCAD simulation of novel asymmetric field plated dual material gate AlGaN/GaN HFET first time. Small signal model is essential for design of LNA and microwave electronic circuit by using the proposed superior performance HFET structure. Superior performances of device are due to its dual material gate structure and field plate that can provide better electric field uniformity, suppression of short channel effects and improvement in carrier transport efficiency. In this article we used direct parameter extraction methodology in which S-parameters of device were measured using pinchoff cold FET biasing. The measured S-parameters are then transformed into Y-parameters to extract capacitive elements and then in to Z-parameters to extract series parasitic elements. Intrinsic parameters are extracted from Y-parameters after de-embedding all parasitic elements of devce. Microwave figure of merits and dc performance are also studied for proposed HFET. The important figure of merits of device reported in the paper include transconductance, drain conductance, current gain, transducer power gain, available power gain, maximum stable gain, maximum frequency of oscillation, cut-off frequency, stability factor and time delay. Reported results are validated with experimental and simulation results for consistency and accuracy.

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.

High Power GaN Field Effect Transistors on Si Substrates

In this paper, GaN-based power transistors for a switching application were reported. In order to realize low loss and high power devices, GaN HFET structure on a Si substrate is a significant configuration as well as one of the cost-effective solutions. Furthermore, attempts for normally-off GaN-FETs were examined. A hybrid MOS-HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

GaN Metal–Oxide–Semiconductor Field-Effect Transistor with Tetraethylorthosilicate SiO2 Gate Insulator on AlGaN/GaN Heterostructure

GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) with a tetraethylorthosilicate (TEOS) SiO2 insulator were developed and evaluated using an AlGaN/GaN HFET structure as the source and drain regions. Operation up to a gate voltage of 10 V was realized at a low gate leakage current. A new method of measuring the mobility of a MOSFET was developed to prevent the effect of hysteresis, in which a relay was used to switch between current measurement and capacitance measurement at the same gate voltage. The maximum field-effect mobility is approximately 45 cm2 V-1 s-1 at an interface state density of 1.02 ×1013 cm-2 eV-1.

Analytical performance evaluation of AlGaN/GaN metal insulator semiconductor heterostructure field effect transistor and its comparison with conventional HFETs for high power microwave applications

AbstractIn this work, a comprehensive analytical model for AlGaN/GaN MISHFET has been presented to evaluate the drain current characteristics, transconductance, and cut‐off frequency of the insulated device. The model takes into account polynomial dependence of sheet carrier density on position of quasi Fermi level to consider the quantum effects and to validate it from subthreshold to high conduction region. The effect of spontaneous and piezoelectric polarization at the AlGaN/GaN interface and parasitic source/drain resistances have also been incorporated in the analysis. Its advantages over conventional HFET structure are discussed in detail. For a MISHFET with quarter micron gate length, the cut‐off frequency is reported to be 52 GHz. The MISHFET shows remarkable 36% increase in drain saturation current. The model has a broad utility as it is equally applicable to HFETs as well. The present model is based on closed form expression and does not involve any fitting parameter. The results obtained are compared with experimental data and show excellent agreement, thereby proving the validity of the model. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 331–338, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23073

Control of surfaces and heterointerfaces of AlGaN/GaN system for sensor devices and their on-chip integration on nanostructures

Abstract For successful construction of sensor devices and their future on-chip integration on nanostructures, this paper discusses the present status of understanding and control of surfaces and heterointerfaces of the AlGaN/GaN material system by reviewing a series of works recently carried out by the authors’ group. Leakage currents in Schottky contacts are explained by the authors’ thin surface barrier (TSB) model. An important role is played by oxygen shallow donors in leakage in AlGaN Schottky diodes. A large leakage reduction has been achieved by a novel surface control process for oxygen gettering. An unprecedented high sensitivity has been obtained in AlGaN/GaN Schottky diode hydrogen sensor by applying the surface control process. Liquid-phase AlGaN/GaN sensors having an open-gate HFET structure show a very good pH sensing capability as well as a good sensing capability of polar liquids. Finally, the selective MBE growth of AlGaN/GaN nanowire network is discussed as a basic hardware structure for the on-chip integration of sensors, paying attention to the heterointerface control.

Epitaxial Growth of ZrO2 on GaN by MOMBE for High Dielectric Material Applications

ABSTRACTEpitaxial growth of ZrO2 has been achieved on MOCVD grown GaN template by oxides MBE using a reactive H2O2 oxygen source. Using the low temperature buffer followed by high temperature in-situ annealing and high-temperature growth, monoclinic, (100)-oriented ZrO2 thin film with decent structural quality and smooth surface morphology was achieved. The full width at half maximum of ZrO2 (100) rocking curve is 0.4 arc degree and the rms roughness for a 5μm by 5 μm AFM scan is 4.1Å. The use of epitaxially grown ZrO2 in the AlGaN/GaN HFET structure as a gate dielectric layer have resulted in the increase of the saturation-current density and pinch-off voltage as well as showed near symmetrical gate-drain I-V behavior.

Influence of annealed ohmic contact metals on polarisation of AlGaN barrier layer

The influence of annealed ohmic contact metals on the polarisation of the AlGaN barrier layer has been investigated by the Schottky contacts on the AlGaN/GaN HFET structure. The analysed result shows that annealed ohmic contact metals weaken the polarisation of the AlGaN barrier layer. When ohmic contact metals are close to Schottky contact metals, the weakened polarisation decreases the 2DEG sheet carrier concentration in the channel.

First AlGaN/GaN MOSFET with photoanodic gate dielectric

We present the first GaN based MOSFET with wet chemical processed gate oxide. The oxide was grown photoelectrochemically (PEC) in KOH based aqueous solutions and was determined to be Al x Ga 2− x O 3. This process offers low surface damage. The gate contact for our created PEC-MOSHFET (metal oxide semiconductor heterostructure field effect transistor) was fabricated by e-beam evaporation of tungsten on the Al x Ga 2− x O 3 layer, followed by a lithographic step and wet etch by H 2O 2. Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance ( g m) are 64 mS mm −1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm 2 V −1 s −1. We achieve a maximum drain current I Dmax of 540 mA mm −1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO 2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages V th of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure V th is about −9 V and for the SiO 2-MOSHFETs it varies between −11.5 and −14 V depending on the wet chemical pre-treatment. Leakage currents depend on device isolation and on gate currents, which are lowest for the SiO 2-MOSHFETs (∼2 pA) and several orders of magnitude bigger for the HFET (∼4 μA). Gate currents for the PEC-MOSHFET depend on the oxide growth and vary between microamperes and a few picoamperes.

Computational and experimental studies on strain-induced effects in a InGaAs/GaAs HFET structure using C-Vprofiling

We analyse strain-induced effects on the capacitance-voltage (C-V) profile for an MBE-grownGaAs/In0.1Ga0.9As/GaAs HFET structure. Thecalculations of C-V profile were made using a newsmall-signal approach developed for the quantum well (QW)structures. The self-consistent numerical simulations andresults of measurements show that strain causes significantchanges in the electron distribution of the QW structure andits C-V profile.

Limitations in MBE-grown GaN and AlGaN/GaN due to dislocations and lateral inhomogeneities

We have studied inhomogeneities in AlGaN/GaN heterostructures due to dislocations and limitations in the growth process. The electron occupancy in acceptor-like states along dislocations in wurtzite GaN were modelled. The reduction in free electron concentration and mobility was calculated as a function doping concentration, dislocation concentration and compensation. We also studied the mobility and PL-signal in heterostructures. Since the MBE-growth takes place on insulating substrates, in a charged nitrogen plasma, effects due to substrate size may be expected. We report Hall effect measurements on Al 0.2Ga 0.8N/GaN HFET structure grown on 2′′ sapphire and 1 4 and 1 8 pieces. The mobility was a factor of 2, larger on 2′′ substrates compared to the pieces. Also photoluminescence variation that depend on substrate size and shape were presented.

Contact Issues of GaN Technology

In this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.

High-temperature performance of GaAs-based HFET structure containing LT-AlGaAs and LT-GaAs

Using low-temperature grown layers a GaAs-based HFET structure was developed, which demonstrates for the first time high performance at high temperatures up to 540/spl deg/C, where the gate diode shunts through. The device was designed for operation in the hot electron regime using an LT-AlGaAs passivation layer. Thus, the open channel current density and gain bandwidth product are exceptionally stable (I/sub D500/spl deg/C//I/sub DR.T./=0.9; f/sup T200/spl deg/C/f/sub TR.T./=0.9). The f/sub max/ cutoff frequency is the most temperature sensitive parameter {(f/sub max//f/sub T/)/sub R.T./=3.9 and (f/sub max//f/sub T/)/sub 200/spl deg/C/=2.8} due to the thermal activation of the buffer layer leakage, which is kept extremely small using LT-GaAs.

Contact Issues of GaN Technology

AbstractIn this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.

InGaAsP channel HFET's on InP for OEIC applications

A model for the calculation of the input noise of a high impedance photoreceiver is proposed, taking into account the contributions of low-frequency characteristics of the FET. Simulations based on this approach show that excess gate leakage current and low-frequency excess noise, usually observed in InGaAs channel FET's, strongly penalize the photoreceiver sensitivity for low to medium data rates. New InGaAsP channel HFET's have been developed and fabricated to solve those problems, dc measurements on 1/spl times/100 /spl mu/m/sup 2/ gate HFET's show good I/sub ds/-V/sub ds/ characteristics with associated gate leakage currents lower than 200 nA. Promising ft of 18 GHz and f/sub max/ of 40 GHz have been recorded on 0.5/spl times/200 /spl mu/m/sup 2/ gate transistors. Low-frequency gate and channel noise measurements demonstrate the suitability of InGaAsP channel HFET structure and technology for low noise applications. A hybrid pin-HFET high impedance photoreceiver has been assembled with a 1/spl times/150 /spl mu/m/sup 2/ gate transistor. A very close agreement is found between photoreceiver input noise predicted by our model and experimental results. Record sensitivities of 34.8 dBm at 622 Mbit/s and -28.7 dBm at 2.5 Gbit/s are inferred from noise measurements, confirming the strong potential of InGaAsP channel HFET's for the fabrication of high sensitivity photoreceivers operating at moderate data rates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Anisotype-gate self-aligned p-channel heterostructure field-effect transistors

A new self-aligned p-channel HFET structure was evaluated for application to complementary HFET circuits. The AlGaAs/InGaAs HFET structure uses an anisotype graded n/sup +/ InGaAs/GaAs semiconductor gate to enhance the barrier height of the FET, resulting in a significant reduction in gate leakage current at low voltages. With AlGaAs composition of x=0.3, and a thin AlAs spacer of 60 AA, leakage current was reduced by a factor of about 1000 at gate voltage of 1 V, when compared to AlGaAs/InGaAs HIGFET of aluminum content x=0.75. The anisotype PFET maintains high device transconductance, typically 50 mS/mm for 1.3*10 mu m PFETs, high reverse breakdown voltages 9-10 V, and low capacitance. Microwave S-parameter characterization resulted in F/sub t/ of 5 GHz for a 1*50 mu m PFET.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sidegating in a GaAs MBE-grown HFET structure

The sidegating effect in a MBE-grown HFET GaAs structure is considerable and persists to large distances. On the other hand, the leakage current is very low. A model that takes into account the unusual features of sidegating in this structure and is consistent with other studies of similar structures is proposed. On the basis of the model, a change in the isolation process was implemented. This resulted in a large reduction of the sidegating effect and confirmed the main features of the model. >