An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application

Optimized AlGaN/AlN/GaN high electron mobility transistor (HEMT) with high mobility GaN channel layer structures were grown on 2-in. diameter semi-insulating 6H-SiC substrates by MOCVD. The 2-in. diameter GaN HEMT wafer exhibited a low average sheet resistance of 261.9 Omega/square, with the resistance un-uniformity as low as 2.23%. Atomic force microscopy measurements revealed a smooth AlGaN surface whose root-mean-square roughness is 0.281 nm for a scan area of 5 x 5 mu m. For the single-cell HEMTs device of 2.5-mm gate width fabricated using the materials, a maximum drain current density of 1.31 A/mm, an extrinsic transconductance of 450 mS/mm, a current gain cutoff frequency of 24 GHz and a maximum frequency of oscillation 54 GHz were achieved. The four-cell internally-matched GaN HEMTs device with 10-mm total gate width demonstrated a very high output power of 45.2 W at 8 GHz under the condition of continuous-wave (CW), with a power added efficiency of 32.0% and power gain of 6.2 dB. To our best knowledge, the achieved output power of internally-matched devices are the state-of-the-art result ever reported for X-band GaN-based HEMTs. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Determination of MgO/AlN heterojunction band offsets by x-ray photoelectron spectroscopy

MgO is a promising gate dielectric and surface passivation film for GaN/AlGaN transistors, but little is known of the band offsets in the MgO/AlN system. X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (Delta E-v) of MgO/AlN heterostructures. A value of Delta E-v=0.22 +/- 0.08 eV was obtained. Given the experimental band gap of 7.83 eV for MgO, a type-I heterojunction with a conduction band offset of similar to 1.45 eV is found. The accurate determination of the valence and conduction band offsets is important for use of III-N alloys based electronic devices.

Electric field driven quantum phase transition between band insulator and topological insulator

We demonstrate theoretically that electric field can drive a quantum phase transition between band insulator to topological insulator in CdTe/HgCdTe/CdTe quantum wells. The numerical results suggest that the electric field could be used as a switch to turn on or off the topological insulator phase, and temperature can affect significantly the phase diagram for different gate voltage and compositions. Our theoretical results provide us an efficient way to manipulate the quantum phase of HgTe quantum wells.

Formation of total-dose-radiation hardened materials by sequential oxygen and nitrogen implantation and multi-step annealing

Separation by implantation of oxygen and nitrogen (SIMON) silicon-on-insulator (SOI) materials were fabricated by sequential oxygen and nitrogen implantation with annealing after each implantation. Analyses of SIMS, XTEM and HRTEM were performed. The results show that superior buried insulating multi-layers were well formed and the possible mechanism is discussed. The remarkable total-dose irradiation tolerance of SIMON materials was confirmed by few shifts of drain leakage current-gate source voltage (I-V) curves of PMOS transistors fabricated on SIMON materials before and after irradiation.

Improved DC and RF performance of AlGaN/GaN HEMTs grown by MOCVD on sapphire substrates

High-quality AlGaN/GaN high electron mobility transistor (HEMT) structures were grown by metalorganic chemical vapor deposition (MOCVD) on 2-in. sapphire substrates. Two-dimensional electron gas (2DEG) mobility of 1410 cm(2)/Vs and concentration of 1.0X10(13) CM-2 are obtained at 295 K from the HEMT structures, whose average sheet resistance and sheet resistance uniformity are measured to be about 395 Omega/sq and 96.65% on 2-in. wafers, respectively. AlGaN/GaN HEMTs with 0.8 mu m gate length and 0.2 mm gate width were fabricated and characterized using the grown HEMT structures. Maximum current density of 0.9 A/ mm, peak extrinsic transconductance of 290 mS/mm, unity cutoff frequency (f(T)) of 20 GHz and maximum oscillation frequency (f(max) of 46 GHz are achieved. These results represent significant improvements over the previously fabricated devices with the same gate length, which are attributed to the improved performances of the MOCVD-grown HEMT structures. (c) 2005 Elsevier Ltd. All rights reserved.

Interference and non-interference of sidebands in a quantum dot with oscillating levels

When a quantum dot is suffering an AC gate voltage, the sidebands turn up beside the static levels of the dot. We formularized the conductance and current when the effective coupling between levels in the quantum dot induced by the hybrid terms is included using a bi-unitary transform method, and we investigated the interference of the photon sidebands of deferent levels. The interference occurs if the same sidebands of deferent levels overlap, which is possible only when the static levels lie close to and overlap with each other. The overlap of different photon sidebands leads to a simple non-coherent superposition. (C) 2005 Elsevier Ltd. All rights reserved.

Space-charge-limited currents in GaN Schottky diodes

Unusual dark current voltage (I-V) characteristics were observed in GaN Schottky diodes. I-V characteristics of the GaN Schottky diodes were measured down to the magnitude of 10(-14) A. Although these Schottky diodes were clearly rectifying, their I-V characteristics were non-ideal which can be judged from the non-linearity in the semi-logarithmic plots. Careful analysis of the forward bias I-V characteristics on log-log scale indicates space-charge-limited current (SCLC) conduction dominates the current transport in these GaN Schottky diodes. The concentration of the deep trapping centers was estimated to be higher than 10(15) cm(-3). In the deep level transient spectra (DLTS) measurements for the GaN Schottky diodes, deep defect levels around 0.20 eV below the bottom of the conduction band were identified, which may act as the trapping centers. The concentration of the deep centers obtained from the DLTS data is about 5 x 10(15) cm(-3). SCLC measurements may be used to probe the properties of deep levels in wide bandgap GaN-AlGaN compound semiconductors, as is the case with insulators in the presence of trapping centers. (c) 2005 Elsevier Ltd. All rights reserved.

Device for charge- and spin-pumped current generation with temperature-induced enhancement

We have proposed a device, a superconducting-lead/quantum-dot/normal-lead system with an ac voltage applied on the gate of the quantum dot induced by a microwave, based on the one-parameter pump mechanism. It can generate a pure charge- or spin-pumped current. The direction of the charge current can be reversed by pushing the levels across the Fermi energy. A spin current arises when a magnetic field is applied on the quantum dot to split the two degenerate levels, and it can be reversed by reversing the applied magnetic field. The increase of temperature enhances these currents in certain parameter intervals and decreases them in other intervals. We can explain this interesting phenomenon in terms of the shrinkage of the superconducting gap and the concepts of photon-sideband and photon-assisted processes.

Electron mobility related to scattering caused by the strain variation of AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance- voltage curves of Ni Schottky contacts with different areas on strained AlGaN/ GaN heterostructures and the current- voltage characteristics for the AlGaN/ GaN heterostructure field- effect transistors at low drain- source voltage, we found that the two- dimensional electron gas (2DEG) electron mobility increased as the Ni Schottky contact area increased. When the gate bias increased from negative to positive, the 2DEG electron mobility for the samples increased monotonically except for the sample with the largest Ni Schottky contact area. A new scattering mechanism is proposed, which is based on the polarization Coulomb field scattering related to the strain variation of the AlGaN barrier layer. (C) 2007 American Institute of Physics.

Quantum mechanical effects in nanometer field effect transistors

The atomistic pseudopotential quantum mechanical calculations for million atom nanosized metal-oxide-semiconductor field-effect transistors (MOSFETs) are presented. When compared with semiclassical Thomas-Fermi simulation results, there are significant differences in I-V curve, electron threshold voltage, and gate capacitance. In many aspects, the quantum mechanical effects exacerbate the problems encountered during device minimization, and it also presents different mechanisms in controlling the behaviors of a nanometer device than the classical one. (c) 2007 American Institute of Physics.

Structure optimization of field-plate AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) on 6H-SiC with varying field-plate length and gate-drain spacing were fabricated and analyzed. The classical small signal FET model and the well-known ColdFET method were used to extract the small signal parameters of the devices. Though the devices with field plates exhibited lower better f(T) characteristic, they did demonstrate better f(max), MSG and power density performances than the conventional devices without field plate. Besides, no independence of DC characteristic on field-plate length was observed. With the increase of the field-plate length and the gate-drain spacing, the characteristic of f(T) and f(max), degraded due to the large parasitic effects. Loadpull method was used to measure the microwave power performance of the devices. Under the condition of continuous wave at 5.4 GHz, an output power density of 4.69 W/mm was obtained for device with field-plate length of 0.5 mu m and gate-drain length of 2 mu m. (c) 2006 Elsevier Ltd. All rights reserved.

AlGaN/AlN/GaN/SiC HEMT structure with high mobility GaN thin layer as channel grown by MOCVD

Enhancement of the electrical properties in an AlGaN/GaN high electron mobility transistor (HEMT) structures was demonstrated by employing the combination of a high mobility GaN channel layer and an AlN interlayer. The structures were grown on 50 mm semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD). The room temperature (RT) two-dimensional electron gas (2DEG) mobility was as high as 2215 cm(2)/V s, with a 2DEG concentration of 1.044 x 10(13)cm(-2). The 50 mm HEMT wafer exhibited a low average sheet resistance of 251.0 Omega/square, with a resistance uniformity of 2.02%. The 0.35 Pin gate length HEMT devices based on this material structure, exhibited a maximum drain current density of 1300 mA/mm, a maximum extrinsic transconductance of 314 mS/mm, a current gain cut-off frequency of 28 GHz and a maximum oscillation frequency of 60 GHz. The maximum output power density of 4.10 W/mm was achieved at 8 GHz, with a power gain of 6.13 dB and a power added efficiency (PAE) of 33.6%. (c) 2006 Elsevier B.V. All rights reserved.

Nonlinear Rashba model and spin relaxation in quantum wells

We find that the Rashba spin splitting is intrinsically a nonlinear function of the momentum, and the linear Rashba model may overestimate it significantly, especially in narrow-gap semiconductors. A nonlinear Rashba model is proposed, which is in good agreement with the numerical results from the eight-band k center dot p theory. Using this model, we find pronounced suppression of the D'yakonov-Perel' spin relaxation rate at large electron densities, and a nonmonotonic dependence of the resonance peak position of the electron spin lifetime on the electron density in [111]-oriented quantum wells, both in qualitative disagreement with the predictions of the linear Rashba model.

Self-consistent analysis of double-delta-doped InAlAs/InGaAs/InP HEMTs

We have carried out a theoretical study of double-delta-doped InAlAs/InGaAs/InP high electron mobility transistor (HEMT) by means of the finite differential method. The electronic states in the quantum well of the HEMT are calculated self-consistently. Instead of boundary conditions, initial conditions are used to solve the Poisson equation. The concentration of two-dimensional electron gas (2DEG) and its distribution in the HEMT have been obtained. By changing the doping density of upper and lower impurity layers we find that the 2DEG concentration confined in the channel is greatly affected by these two doping layers. But the electrons depleted by the Schottky contact are hardly affected by the lower impurity layer. It is only related to the doping density of upper impurity layer. This means that we can deal with the doping concentrations of the two impurity layers and optimize them separately. Considering the sheet concentration and the mobility of the electrons in the channel, the optimized doping densities are found to be 5 x 10(12) and 3 x 10(12) cm(-2) for the upper and lower impurity layers, respectively, in the double-delta-doped InAlAs/InGaAs/InP HEMTs.

A bistable, self-latching inverter by the monolithic integration of resonant tunnelling diode and high electron mobility transistor

This paper reports that the structures of AlGaAs/InGaAs high electron mobility transistor (HEMT) and AlAs/GaAs resonant tunnelling diode (RTD) are epitaxially grown by molecular beam epitaxy ( MBE) in turn on a GaAs substrate. An Al0.24Ga0.76As chair barrier layer, which is grown adjacent to the top AlAs barrier, helps to reduce the valley current of RTD. The peak-to-valley current ratio of fabricated RTD is 4.8 and the transconductance for the 1-mu m gate HEMT is 125mS/mm. A static inverter which consists of two RTDs and a HEMT is designed and fabricated. Unlike a conventional CMOS inverter, the novel inverter exhibits self-latching property.