M. Abe

Short-channel effects in subquarter-micrometer-gate HEMTs: simulation and experiment

The authors studied the electrical properties of subquarter-micrometer-gate HEMTs (high electron mobility transistors) by Monte Carlo simulation and experiment. Simulation shows that subquarter-micrometer-gate HEMTs have extremely high performance, and the near-ballistic movement under the gate was confirmed. It is also shown that the aspect ratio of the channel could be used as a guide to determine the extent of short-channel effects. The transverse-domain formation inherent in short-channel HEMTs may also contribute to the smaller short-channel effect by limiting undesirable substrate current. Experimentally, only a negligible short-channel effect was observed when the gate length was reduced from 1.25 to 0.14 mu m. Thus it is not necessary to design and fabricate a special structure for HEMTs, as such a structure might have limited applications. >

Degradation of high‐radiance Ga1−xAlxAs LED’s

The slow degradation of single‐heterostructure Ga1−xAlxAs LED’s has been investigated. The samples which show the fast degradation are completely rejected by the selection of DLD‐free LED’s. Selection procedures and the results of accelerated aging tests for over 10u2009000 h are presented. The activation energy of the slow degradation is found to be 0.57 eV. Extrapolated room‐temperature half‐life in excess of 5×106 h is estimated. The degradation coefficients are not affected significantly by the operating current density (3.5–10 kA/cm2).

Reliability of high radiance InGaAsP/InP LED́s operating in the 1.2-1.3 µm wavelength

The reliability of high radiance InGaAsP/InP DH LEDs operating in the 1.2-1.3 mu m wavelength and the defect structures observed in this quaternary alloy have been presented. Threading dislocations and misfit dislocations do not act as strong nonradiative recombination centers, in contrast with the case in GaAs or GaAlAs optical devices. Dark-spot defects (DSDs) were sometimes generated in the emitting area during aging at elevated temperatures. These defects were analyzed microscopically using a transmission electron microscope and were identified as precipitates. To investigate the homogeneous degradation, accelerated aging at the ambient temperatures of 20, 60, 120, 170, 200, and 230°C has been carried out for over 15 000 h at the current density of 8 kA/cm2using LEDs without dark structures. The degradation rates were statistically calculated by assuming the normal distribution. The mean values of degradation rates and the values of standard deviation were determined at the temperatures above 170°C. The activation energy of homogeneous degradation was determined to be 1.0 eV and the extrapolated half-life in excess of 109h was estimated at the ambient temperature of 60°C.

TiW Silicide gate self-alignment technology for ultra-high-speed GaAs MESFET LSI/VLSI's

It has been found that TiW silicide film forms Schottky contacts on GaAs which are extremely stable even at temperatures of up to 850°C. Using this silicide for gate material, a novel self-alignment technique for GaAs MESFETs has been developed. A minimum propagation delay of 50 ps with 1.5-µ gate logic and successful fabrication of 1-kbit fixed address GaAs static memory cell arrays which are based on E/D type DCFLs indicate that TiW silicide gate self-alignment technology is a very promising candidate for achieving ultra-high-speed GaAs MESFET LSI/VLSIs.

New technology towards GaAs LSI/VLSI for computer applications

For future large-scale computer applications, new device technologies towards GaAs LSI/VLSI have been developed: self-aligned fully implanted planar GaAs MESFET technology and high electron mobility transistor (HEMT) technology by molecular beam epitaxy (MBE). The self-aligned GaAs MESFET logic with 1.5-µm gate length exhibits a minimum switching time of 50 ps and the lowest power-delay product of 14.5 fJ at room temperature. The enhancement/depletion (E/D) type direct coupled HEMT logic has achieved a switching time of 17.1 ps with 1.7-µm gate length at liquid nitrogen temperature and more recently a switching time of 12.8 ps with 1.1-µm gate HEMT logic, which exceeds the top speed of Josephson Junction logic and shows the highest speed of any device logic ever reported. Optimized system performances are also projected to system delay of 200 ps at 10-kilogate integration with GaAs MESFET VLSI, and 100 ps at 100-kilogate with HEMT VLSI. These values of system delay correspond to the computer performance of over 100 million instructions per second (MIPS).

Pt-based gate enhancement-mode InAlAs/InGaAs HEMTs for large-scale integration

The barrier heights of several n-InAlAs/metal Schottky contacts are discussed. A thin-Pt/Ti/Pt Au multilayer gate is proposed for InP-based InAlAs/InGaAs HEMTs. Its Schottky barrier height was measured as 0.83 eV, and it shows good threshold voltage stability. Performance measurements of an enhancement-mode HEMT fabricated using a 1.1- mu m-long multilayer gate indicate a threshold voltage of 0.05 V and a transconductance of 540 mS/mm.<<ETX>>

Performance of a quarter-micrometer-gate ballistic electron HEMT

The electrical properties of a quarter-micrometer-gate HEMT have been studied by simulation and experiment. An IDSof 11 mA/50 µm, a gmof 500 mS/mm, and an fTof 110 GHz have been predicted by two-dimensional Monte Carlo simulation for certain conditions. The reasons underlying the high performance are discussed in terms of the electron dynamics in the device. A record room-temperature propagation delay time of 9.2 ps/gate at a power dissipation of 4.2 mW/ gate with the maximum transconductance of 400 mS/mm was obtained experimentally for a 0.28-µm-gate HEMT. Only a negligible short-channel effect was observed for reducing the gate length from 1.4 to 0.28 µm.

Microwave power double-heterojunction HEMT's

The RF and dc characteristics of microwave power double-heterojunction HEMts (DH-HEMTs) with low doping density have been studied. Small-signal RF measurements indicated that the cutoff frequency and the maximum frequency of oscillation in DH-HEMTs with 0.8-1 µm gate length and 1.2 mm gate periphery are typically 11- 16 GHz and 36-41 GHz, respectively. However, the cutoff frequency in DH-HEMTs degrades strongly with increasing drain bias voltage. This may be caused by both effects of increasing effective transit length of electrons and decreasing average electron velocity, due to Gunn domain formation. In large-signal microwave measurement, the DH-HEMT (2.4 mm gate periphery) delivered a maximum output power of 1.05 W with 2.8 dB gain and 0.58 W with 1.6 dB gain at 20 and 30 GHz, respectively. These are the highest output powers yet reported for HEMT devices. For the dc characteristics, the onset of two-terminal gate breakdown voltage is found to correlate with the drain current Idssand recessed length, and three-terminal source-drain breakdown characteristics near pinchoff are limited by the gate-drain breakdown. A simple model on gate breakdown voltage in HEMT is also presented.

High-efficiency long-lived GaAlAs LED's for fiber-optical communications

A new fiber-mounted GaAlAs LED has been proposed and developed to satisfy efficient performance and high-reliability requirements in Practical fiber-optical communication systems. By utilizing the lens effect of a spherical-ended fiber, the LED-to-fiber coupling is greatly improved and a coupling efficiency of 8.5 percent is obtained. At a forward current of 100 mA, the output power of 305 µW is obtained from 0.14-numerical-aperture 85- µm-core step-index fiber of 3-cm length. The power in the cladding part of the fiber is almost completely removed by the epoxy. Long-term operating-life tests of LEDs described here are in progress. After about 8000 h of operation, no power reduction has occurred. The accelerated aging at elevated temperatures has also been performed and an activation energy of 0.5 eV has been determined for DLD-free LEDs. A half-life in excess of 106h is estimated for room-temperature operation at a constant-current density of 10 kA/cm2.

Recent advances in ultrahigh-speed HEMT LSI technology

The current status of, and recent advances in, high electron mobility transistor (HEMT) technology for high-performance submicrometer VLSI are presented with a focus on materials, self-aligned device fabrication, and HEMT LSI implementation. The HEMT is a very promising device for ultrahigh-speed LSI/VLSI applications because of the high-mobility GaAs/AlGaAs heterojunction structure. The authors project an optimized chip delay of 40 ps at 10 K-gate VLSI at room temperature. >