Performance Of High Electron Mobility Transistors Research Articles

Modification/oxidation of GaAs surface in electrolytes for cell‐culture bio‐sensing devices

AbstractThe surface modification/oxidation of GaAs substrates in electrolytes has been investigated from the viewpoint of bio‐sensing devices using AlGaAs/GaAs high‐electron‐mobility‐transistor (HEMT) structures, where the cell culture directly on a bare gate‐surface is intended for a high sensitivity to cell‐viabilities such as adhesion or cell‐cell interaction, through nitric‐oxide concentration or charged‐peptide chemisorption. Although GaAs surface (gate surface of HEMT) was oxidized even in pure water under room light, we found that the oxide layer is not formed in the dark or removed with the existence of small amounts of NaH2PO4 (0.8 mM) in the solution. The photo‐excited oxidation and its etching by NaH2PO4 are discussed as well as the temporal change of the HEMT performance in solution. Cell adhesion on a bare GaAs surface was poor, but a surface modification by fibrionectin improved the cell adhesion, comparable to that on the conventional cell‐culture dishes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A surface trap model and its application to analysis of III‐nitride HEMT performance

AbstractA polarity‐dependent model of electron/hole traps on a free GaN:Si surface is suggested, accounting for both intrinsic and impurity‐induced surface states. The model parameters are fitted to provide the best agreement with available observations. The suggested approach is invoked to explain a recently observed non‐monotonic dependence of the surface barrier due to band bending upon the donor concentration in the GaN bulk. The model is used for analysis of DC performance of high‐electron mobility transistors (HEMTs) with different cap layer design. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.

Effect of Epitaxial Layer Crystal Quality on DC and RF Characteristics of AlGaN/GaN Short-Gate High-Electron-Mobility Transistors on Sapphire Substrates

The DC and RF characteristics of short-gate high-electron-mobility transistors (HEMTs) formed on four AlGaN/GaN wafers grown on sapphire substrates with different crystal quality have been investigated. Atomic force microscopy observation revealed many pits and trenches on the AlGaN surface, and the morphology of each sample was distinct. There were also differences in electron mobility and sheet carrier concentration. However, the gate length dependences of the measured transconductance and cut-off frequency were virtually the same. For a more detailed investigation, we subtracted the source resistance and AlGaN thickness contributions from measured DC and performed a delay time analysis for the RF characteristics. The results indicate that the intrinsic performance of HEMTs was independent of the surface morphology and that effective electron velocity ranged from 1.4 to 1.8×107 cm/s.

Kink-effect related noise in short-channel InAlAs/InGaAs high electron mobility transistors

We perform a microscopic analysis of the noise associated with the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs) by using a semiclassical two-dimensional (2-D) ensemble Monte Carlo simulator. The kink effect in HEMTs has its origin in the pile up of holes (generated by impact ionization) taking place under the source side of the gate, that leads to a reduction of the gate-induced channel depletion and results in a drain current enhancement. Our results indicate that the generation of holes by impact ionization and their further recombination lead to fluctuations in the charge of the hole pile up which provoke an important increase of the drain-current noise, even when the kink effect is hardly perceptible in the output characteristics. In addition, shot noise related to the hole gate leakage current is found at the gate terminal, which can further degrade the global noise performance of HEMTs.

Effect of surface passivation on performance of AlGaN/GaN/Si HEMTs

Performance of intentionally undoped and doped AlGaN/GaN/Si high electron mobility transistors (HEMTs) before and after passivation with SiO 2 and Si 3N 4 is investigated. Hall effect measurements show higher impact of Si 3N 4 than SiO 2 passivation on the carrier concentration increase in the channel. Improvements in DC performance of HEMTs after passivation with SiO 2 and Si 3N 4 correspond to the changes in sheet carrier concentration. Small signal microwave characterisation shows a decrease (from 18.6 to 9 GHz) and an increase (from 18.4 to 28.8 GHz) of the current gain cut off frequency after SiO 2 and Si 3N 4 passivation, respectively. Similar effect of passivation is found in microwave power changes––only about a half of the power is obtained after SiO 2 passivation but more than doubled power results from Si 3N 4 passivation, measured at 2 GHz. Higher density of interface states for SiO 2 than Si 3N 4 passivation is supposed to be responsible for these effects. However, for an optimal design of GaN-based power devices additional studies related to the interface between a passivation layer and GaN are needed.

Photo-ionization spectroscopy of traps in AlGaN/GaN high-electron mobility transistors

Four different layer structures are used to study deep-level traps in AlGaN/GaN high-electron mobility transistors (HEMTs) by photo-ionization spectroscopy. The structures grown on sapphire substrates by metal-organic chemical vapor deposition show nearly identical Hall data. However, the direct current (DC) performance of HEMTs with identical geometry is found to differ strongly. In all structures investigated, two distinct defect levels, namely, at 2.84–2.94 eV and 3.24–3.28 eV, were found from the fits of the photo-ionization cross-sectional data. Additionally, different trap concentrations can be deduced. These are in good correlation with the different transconductance and drain current measured. It is assumed that the defect levels observed are related to the AlGaN surface.

Kilovolt AlGaN/GaN HEMTs as Switching Devices

SiC devices have been the focus for several years as potential high voltage switches working at high temperature with very low on-resistance. Another competitive candidate for this application is the GaN HEMT (high electron mobility transistor). GaN has slightly wider bandgap, higher electric strength, and higher saturated velocity than SiC. The most prominent is that by utilizing the AlGaN/GaN heterojunction, the GaN HEMT has much higher charge density (up to 2 × 10 13 cm -2 ) and mobility (up to 2200 cm 2 /Vs) in the channel compared to SiC MOSFET or JFET, yielding much lower on-resistance than SiC devices. GaN grown on SiC substrate also takes the advantage of high thermal conductivity of SiC. A GaN HEMT with a breakdown voltage of 1050 V was fabricated with a specific on-resistance of 3.4 mΩ cm 2 and current density of 4 kA cm -2 . State-of-art power device figure of merit of V 2 BR /R on = 3.24 × 10 8 [V 2 Ω -1 cm -2 ] was achieved on this device. Projected performance of GaN HEMTs is also discussed and compared with SiC devices in this paper.

Effect of growth termination conditions on the performance of AlGaN/GaN high electron mobility transistors

The effect of the growth conditions of the top 2.5 nm thick AlGaN cap layer and wafer cool down conditions on AlGaN/GaN high electron mobility transistor performance was investigated. The AlGaN/GaN heterostructures were deposited on sapphire by metalorganic chemical vapor deposition and consisted of a 3 μm thick semi-insulating GaN layer and an 18 nm thick Al0.33Ga0.67N layer, the top 2.5 nm of which was deposited under various conditions. The power performance of the devices severely degraded for all samples where the Al content of the top 2.5 nm of AlGaN was increased and/or the ammonia flow during growth of the top layer was decreased. A modest improvement in the output power density was observed when the growth conditions of the cap layer were identical of those of the rest of the AlGaN layer, but when the wafer was cooled down in pure nitrogen.

Nonlinear electronic transport and device performance of HEMTs

We assess the impact of nonlinear electronic transport and, in particular, of real space transfer (RST) on device performance for advanced III/V high electron mobility transistors (HEMTs) using the device simulator MINIMOS-NT. In this context, we discuss DC and RF performance issues for pseudomorphic AlGaAs-InGaAs-GaAs HEMTs that are especially relevant for gate-lengths of about 150 nm. All results are compared to and found to be consistent with experimental data for devices processed in two different foundries.

Impact of two-step-recessed gate structureon RF performance of InP-based HEMTs

The RF performance of InP-based lattice-matched high electron mobility transistors (HEMTs) is improved by using a two-step-recess process. 0.07 µm gate HEMTs show a cutoff frequency (fT) of 300 GHz, a value previously achievable only with a gate length of 0.05 µm in the conventional gate structure, and a maximum frequency of oscillation fmax of 400 GHz. The high fT indicates that the effective gate length is successfully suppressed by the two-step-recessed gate structure. Moreover, owing to the selective etching property, this gate recess process provides high uniformity of the threshold voltage and the cutoff frequency of the HEMTs on a 3 in wafer.

Single and Double δ-Doped Al0.25Ga0.75As/In0.25Ga0.75As Pseudomorphic Heterostructures Grown by Molecular-Beam Epitaxy

Single and double δ-doped Al0.25Ga0.75As/In0.25Ga0.75As pseudomorphic heterostructures grown by molecular-beam epitaxy are examined to improve the performance of high-electron-mobility transistors (HEMTs). Double δ-doped layers, which are inserted in uniformly-doped AlGaAs carrier supply layers, can reduce the distance between the surface and the channel down to 19 nm without degrading the electrical properties. The mobility and sheet electron concentration at room temperature are 6050 cm2/(V·s) and 2.32×1012 cm-2, respectively ( 16270 cm2/(V·s) and 2.30×1012 cm-2 at 77 K). Shubnikov-de Haas oscillations confirm that the parallel conduction in the carrier supply layer is negligible. This work indicates that δ-doping in uniformly-doped carrier supply layers can be a key factor in improving the performance of HEMTs.

V-band high-efficiency high-power AlInAs/GaInAs/InP HEMT's

The authors report on the state-of-the-art power performance of InP-based HEMTs (high electron mobility transistors) at 59 GHz. Using a 448- mu m-wide HEMT with a gate length of 0.15 mu m, an output power of 155 mW with a 4.9-dB gain and a power-added efficiency of 30.1% were obtained. By power-combining two of these HEMTs, an output power of 288 mW with 3.6-dB gain and a power-added efficiency of 20.4% were achieved. This is the highest output power reported with such a high efficiency for InP-based HEMTs, and is comparable to the best results reported for AlGaAs/InGaAs on GaAs pseudomorphic HEMTs at this frequency. >

Effects of the MBE growth temperature on Si-doped AlxGa1-xAs (x=0, 0.26) and HEMT

The effects of the growth temperature on photoluminescence (PL) as well as the net donor concentration have been studied concerning heavily Si-doped GaAs and Al0.26Ga0.74As grown by molecular beam epitaxy (MBE). Attention is especially paid to the PL band related to donor-vacancy complexes (SiIII-VIII), i.e. the so-called self-activated (SA) centres. The SA luminescence intensity normalized by the band-to-band emission intensity, which can be regarded as being a characteristic of the SA centre concentration, is found to be minimum at a growth temperature of 650 degrees C in both GaAs and Al0.26Ga0.74As; the normalized intensity in Al0.26Ga0.74As, however, is always about 40 times stronger than that in GaAs grown at the same temperature. While the net donor concentration in GaAs is independent of the growth temperature, that in Al0.26Ga0.74As is found to be maximum at 650 degrees C. Furthermore, a high-electron-mobility transistor (HEMT) fabricated on an MBE-grown N-Al0.26Ga0.74As/GaAs structure exhibits maximum performance when the structure is grown at that optimum temperature of 650 degrees C. The authors propose a model in which the growth temperature dependence of the vacancy (VIII) concentration, which is supposed to have the minimum at 650 degrees C, is the cause of the change in the HEMT performance, as well as that in the net donor concentration in Al0.26Ga0.74As. The growth-temperature dependence of the vacancy concentration is explained in terms of a thermodynamical mechanism in the low-temperature range (<650 degrees C), and by a kinetic mechanism in the high-temperature range (>650 degrees C).

Fabrication of electroplated T gates with 60 nm gate length for pseudomorphic high electron mobility transistor devices

We have fabricated and electrically characterized mm-wave high electron mobility transistors (HEMTs) on pseudomorphic heterostructure GaAs/InGaAs samples. The T- and Γ-shaped gates were produced using electroplating and a conventional single resist-layer lift-off process. High frequency measurements of the same device before and after plating demonstrate the reduction in gate resistance. Direct current and high frequency properties of the HEMTs depend strongly on gate length and gate recess depth. Characterization of parallel conducting layers, low field mobility, and sheet carrier concentration depth profiles were obtained with gated Hall measurements. Best HEMT performance was obtained at a gate length of 60 nm, giving an extrinsic (intrinsic) transconductance of 620 mS/mm (840 mS/mm) and a cutoff frequency ft of 135 GHz.

An investigation of the effects of doping profile variations on AlGaAs/GaAs high electron mobility transistor performance

We present results from a theoretical study of the influence of doping profile variations on the performance of delta-doped AlGaAs/GaAs high electron mobility transistors (HEMTs). An ensemble Monte Carlo simulation coupled with a self-consistent solution of the two-dimensional Poisson equation is used to investigate HEMTs which employ both single and double delta-doped profiles with varying doping configurations. The calculated results reveal that single delta-doped HEMTs designed with identical threshold voltages exhibit improved device behavior when thinner delta-doped layers with more heavily doped concentrations are utilized. For double delta-doped HEMTs with an identical total doping in the AlGaAs layer, improved threshold voltage control is obtained as the spacing between two delta-doped layers increases. However, this increase in spacing also causes a degradation in transconductance, cut-off frequency, and switching time. As gate bias increases, the dependence of device performance (or degradation) on the spacing between doping planes becomes less pronounced due to the upward shift in threshold or ‘‘onset’’ of parallel conduction in the AlGaAs layer.

Impact of buffer layer design on the performance of AlInAs-GaInAs HEMTs

The authors report a study of the impact of buffer layer design on the characteristics of Al/sub 0.48/In/sub 0.52/As-Ga/sub 0.47/In/sub 0.53/As HEMTs (high-electron-mobility transistors). The aim of the study is to understand and correct the problem of high output conductance observed in devices with a high transconductance. Devices with 1.0- mu m gate lengths were fabricated using modulation-doped Al/sub 0.48/In/sub 0.52/As-Ga/sub 0.47/In/sub 0.53/As epitaxial layers which had sheet charge densities between 3*10/sup 12/ and 3.5*10/sup 12/ cm/sup -2/ and mobilities at 300 K between 9000 and 10000 cm V/sup -1/ S/sup -1/. The different buffer layer designs used were: (1) a standard undoped Al/sub 0.48/In/sub 0.52/As buffer 250-nm-thick; (2) an Al/sub 0.48/In/sub 0.52/As buffer with a 20-AA thick highly doped p-type region 50 AA below the channel; (3) a Ga/sub 0.47/In/sub 0.53/As buffer with a 20-AA-thick highly doped p-type region below the channel; and (4) a low-temperature AlInAs buffer layer. The device with the low-temperature AlInAs had the best output characteristics, signifying that it was the best mode of confining electrons in the channel.

DC and microwave characteristics of sub-0.1- mu m gate-length planar-doped pseudomorphic HEMTs

Analytical modeling of these very-short-channel HEMTs (high-electron-mobility transistors) using the charge-control model is given. The calculations performed using this model indicate a very high electron velocity in the device channel (3.2+or-0.2*10/sup 7/ cm/s) and clearly demonstrate the advantages of the planar-doped devices as compared to the conventional uniformly doped HEMTs. Devices with different air-bridged geometries have been fabricated to study the effect of the gate resistance on the sub-0.1- mu m HEMT performance. With reduced gate resistance in the air-bridge-drain device, noise figures as low as 0.7 and 1.9 dB were measured at 18 and 60 GHz, respectively. Maximum available gains as high as 13.0 dB at 60 GHz and 9.2 dB at 92 GHz, corresponding to an f/sub max/ of 270 GHz, have also been measured in the device. Using the planar-doped pseudomorphic structure with a high gate aspect-ratio design, a noise figure of less than 2.0 dB at 94 GHz is projected based on expected further reduction in the parasitic gate and source resistances.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ensemble Monte Carlo simulation of a 0.35- mu m pseudomorphic HEMT

Theoretical analysis and precise comparison to experiment of the performance of a 0.35- mu m pseudomorphic Al/sub 0.15/Ga/sub 0.85/As/In/sub 0.15/Ga/sub 0.85/As high-electron-mobility transistor (HEMT) are presented. The calculations are made using an ensemble Monte Carlo simulation with the unique inclusion of real space transfer as well as the full details of the two-dimensional electron gas, velocity overshoot, and ballistic transport, and the effects of the two-dimensional electric field profile. The calculated current-voltage characteristic is compared to recent experimental measurements showing excellent agreement to within approximately 10% over a full range of gate and drain biases. It is found that near the source, the two-dimensional system dominates the transport physics, while near the pinch-off point, the effects of real space transfer become apparent. It is further determined that the high-speed performance of the pseudomorphic HEMT stems predominantly from the high electron confinement within the two-dimensional system, and the high electron mobility and confinement within the gamma valley in the bulk InGaAs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microwave performance of AlInAs-GaInAs HEMTs with 0.2- and 0.1- mu m gate length

The millimeter-wave performance is reported for Al/sub 0.48/In/sub 0.52/As-Ga/sub 0.47/In/sub 0.53/As high-electron-mobility transistors (HEMTs) with 0.2- mu m and 0.1- mu m-long gates on material grown by molecular-beam epitaxy on semi-insulating InP substrates. Devices of 50- mu m width exhibited extrinsic transconductances of 800 and 1080 mS/mm, respectively. External f/sub T/ (maximum frequency of oscillation) of 120 and 135 GHz, respectively, were measured. A maximum f/sub T/ of 170 GHz was obtained from a 0.1*200- mu m/sup 2/ device. A minimum noise figure of 0.8 dB and associated gain of 8.7 dB were obtained from a single-stage amplifier at frequencies near 63 GHz. >