Y.C. Pao

Effect of emitter-base spacing on the current gain of AlGaAs/GaAs heterojunction bipolar transistors

An investigation of the effect of surface recombination and emitter-base-contact spacing on the DC current-gain of AlGaAs/GaAs heterojunction bipolar transistor (HBT) using thin AlGaAs emitter structures is discussed. The selectively-etched, thin-AlGaAs-emitter layer has been used to prevent an exposed extrinsic base region, which has previously limited current gain because of high surface recombination. It is found that a factor of approximately 50 improvement in the current gain can be achieved by proper surface passivation and emitter-base-contact spacing.<<ETX>>

Resonant tunneling diodes for switching applications

Rise times for simple pulse‐forming circuits are presented. Switching times for present best devices are in the range of 5–15 ps. An equivalent circuit model for resonant tunneling diodes inclusive of space‐charge effects and transit time effects in the depletion region is presented. From these models it is shown that switching times are limited by the device RC time constants and are relatively unaffected by the resonant state lifetime or depletion layer transit times. Appropriate figures of merit for switching applications are the device capacitance and peak current density. Less emphasis should be placed on improving the peak‐to‐valley ratio. Optimally designed devices which maximize the current density should be capable of switching in under 5 ps.

Hyperabrupt‐doped GaAs nonlinear transmission line for picosecond shock‐wave generation

Voltage waveforms with 6 V amplitude and 1.6 ps fall time were generated by voltage shock‐wave formation on a hyperabrupt‐doped Schottky diode monolithic GaAs nonlinear transmission line.

Device quality growth and characterization of (110) GaAs grown by molecular beam epitaxy

Device quality (110)GaAs has been reproducibly grown by molecular beam epitaxy (MBE) for the first time. Angling of the substrate to expose stable, Ga‐rich ledges on the (110) surface has been shown to be the necessary condition for two‐dimensional growth. The layers exhibit a room‐temperature electron mobility of ∼5700 cm2/V s for NSi∼4×1015 and a strong exciton photoluminescence emission at 4 K. This breakthrough in MBE growth of III‐V compounds allows for fabrication of (110) GaAs devices which will take advantage of the unique properties of this orientation.

Characterization of surface faceting on (110)GaAs/GaAs grown by molecular beam epitaxy

Abstract Commonly observed faceting of (110) GaAs grown by molecular beam epitaxy has been systematically investigated. Faceted (110) films were examined with microscopic, electrical, and optical techniques. The geometry of the facets with respect to the GaAs crystal was determined and an atomic model of facet formation was derived from experimental results. The study ultimately led to the elimination of facet formation on (110) GaAs grown by molecular beam epitaxy.

Fabrication of 200-GHz f/sub max/ resonant-tunneling diodes for integration circuit and microwave applications

Room-temperature current densities of 1.3*10/sup 5/ A/cm/sup 2/ and peak-to-valley ratios of 2.5 have been achieved for resonant tunneling diodes (RTDs) in the GaAs/AlAs material system. The devices were fabricated in a microwave-compatible process using topside contacts and a semi-insulating substrate to allow device integration. Proton implantation creates a nonconducting surface compatible with high-frequency coplanar transmission lines and other passive microwave structures.<<ETX>>

Generation of 3.5-ps fall-time shock waves on a monolithic GaAs nonlinear transmission line

It is shown that nonlinear wave propagation on a monolithic GaAs nonlinear transmission line can form shock waves with fall time as short as 3.5 ps. An output fall time of 4.3 ps was measured for a single line driven at 15 GHz (20-ps fall time) while a cascade of two lines driven at 8 GHz (37-ps fall time) produced a 2-V wavefront with 3.5-ps fall time.<<ETX>>

Electron saturation velocity variation in InGaAs and GaAs channel MODFETs for gate length to 550 AA

MODFETs with InGaAs and GaAs channels have been fabricated with gate lengths between 2 mu m and 550 AA and have been DC characterized at room temperature. An effective electron saturation velocity was calculated for each device from the peak transconductance. The GaAs channel devices show a peak in the transconductance and saturation velocity at a gate length of approximately 0.15 mu m, while the transconductance and velocity of the InGaAs channel devices are nearly constant over the entire range of gate lengths.<<ETX>>

High output conductance of InAlAs/InGaAs/InP MODFET due to weak impact ionization in the InGaAs channel

InAlAs/InGaAs/InP MODFETs with short gatelength suffer from a high output conductance, G/sub 0/, and low breakdown voltage. In addition, the devices often show a kink effect in the I-V curve. The authors show by numerical calculations that weak impact ionization occurs in the In/sub .53/Ga/sub .47/As channel under relatively moderate fields. The high G/sub 0/ and the kink are mainly caused by mechanisms associated with weak impact ionization. The calculations also show that the low breakdown voltage of the device is caused by the drain-source breakdown in the InGaAs channel. The occurrence of the impact ionization process in the channel was experimentally verified by detection of photons from the device under moderate drain bias.<<ETX>>

Effect of hydrogen on undoped and lightly Si‐doped molecular beam epitaxial GaAs layers

This letter describes a series of experiments in which pure hydrogen gas (at up to 1.2×10−6 Torr partial pressure) was introduced during molecular beam epitaxial (MBE) growth of low‐doped and undoped GaAs at a substrate temperature of 580 °C. A major improvement in the electrical properties of the epitaxial layer has been observed. Electron mobilities at 77 K and deep level transient spectroscopy (DLTS) spectra are presented. A sharp increase in electron mobility and a dramatic reduction of M1 and M4 deep electron traps as shown by DLTS have been achieved with small amounts of H2 gas at 1.2×10−6 Torr partial pressure.