Hin-Fai Chau

Thermal properties and thermal instabilities of InP-based heterojunction bipolar transistors

We investigate the physical parameters which are critical to the understanding of the thermal phenomena in InP-based heterojunction bipolar transistors. These parameters include thermal resistance, thermal-electric feedback coefficient, current gain, and base-collector leakage current. We examine the thermal instability behavior in multi-finger HBTs, and observe for the first time the collapse of current gain in InP-based HBTs. Based on both measurement and modeling results, we establish the reasons why the collapse is rarely observed in InP HBTs, in a sharp contrast to AlGaAs/GaAs HBTs. We compare the similarities and differences on how InP-based HBT, GaAs-based HBT, and Si-based bipolar transistors react once the thermal instability condition is met. Finally, we describe the issues involved in the design of InP HBTs.

Characterization of bias-stressed carbon-doped GaAs/AlGaAs power heterojunction bipolar transistors

We report on the performance of carbon-doped heterojunction bipolar transistors (HBTs) bias stressed at elevated temperatures. We have determined that in devices without a thin passivating layer of AlGaAs covering the extrinsic base, a tunneling-recombination current that increases in magnitude with the duration of the stress is generated. This current is seen in both the collector and the base at cryogenic temperatures. The variation of this current with temperature is primarily due to carrier freeze-out in the AlGaAs emitter. We hypothesize that this conduction mechanism is related to the generation of midgap traps in the base layer as a result of electron-hole recombination events.<<ETX>>

High-speed InP/InGaAs heterojunction bipolar transistors

Very-high-performance common-emitter InP/InGaAs single heterojunction bipolar transistors (HBTs) grown by metalorganic molecular beam epitaxy (MOMBE) are reported. They exhibit a maximum oscillation frequency (f/sub T/) of 180 GHz at a current density of 1*10/sup 5/ A/cm/sup 2/. this corresponds to an (R/sub B/C/sub BC/)/sub eff/=f/sub T//(8 pi f/sup 2//sub max/) delay time of 0.12 ps, which is the smallest value every reported for common-emitter InP/InGaAs HBTs. The devices have 11 mu m/sup 2/ total emitter area and exhibit current gain values up to 100 at zero base-collector bias voltage. The breakdown voltage of these devices is high with measured BV/sub CEO/ and BV/sub CEO/ of 8 and 17 V, respectively.<<ETX>>

High-speed InGaP/GaAs HBTs using a simple collector undercut technique to reduce base-collector capacitance

High-speed InGaP/GaAs HBTs were fabricated using a simple collector undercut (CU) technique to physically remove the collector material underneath the extrinsic base region by selective etching for reducing base-collector capacitance (C/sub BC/). The best HBTs achieved a f/sub T/ of 80 GHz and a f/sub max/ (MSG/MAG) of 171 GHz. To our knowledge, this is the highest f/sub max/ (MSG/MAG) ever reported for the InGaP/GaAs HBTs. Compared to the HBTs without CUs, the CU HBTs showed a factor of 1.38 times improvement in the highest achievable f/sub max/ (MSG/MAG) due to the significant reduction of the C/sub BC/.

Laterally etched undercut (LEU) technique to reduce base-collector capacitances in heterojunction bipolar transistors

We report a novel fabrication process aimed at reducing the parasitic junction capacitance of AlGaAs-GaAs heterojunction bipolar transistors. The process, named as the Laterally Etched Undercut (LEU) process, physically removes the extrinsic base-collector junction area and results in a cantilever structure. The DC, small-signal, and large-signal performances of the LEU devices are compared to those obtained from the conventional devices.

Low thermal impedance MMIC technology

A technology has been developed that reduces the thermal resistance of monolithic microwave integrated circuits (MMICs). Novel processing techniques are used to fabricate thin-film capacitors and microstrip lines on the back side of the chip. The front side of the chip is metallized to serve as the ground-plane; the completed chip is assembled inverted so that the active devices are next to the heat sink, but the chip otherwise is a drop-in replacement for conventional MMICs. With very conservative deembedding of circuit losses, an AlGaAs/GaAs heterojunction bipolar transistor (HBT) fabricated in this technology achieved record performance at 20 GHz: over 1.2 W output power with 53% power-added efficiency while operating at 12.7 V.

InP-based HBTs and their perspective for microwave applications

Significant progress has been made over the past few years in both the technology and microwave performance of InP-based heterojunction bipolar transistors (HBTs). Emphasis has, however, been placed mainly on transistor performance. Other critical issues have largely been ignored, including the influence of InGaAs on device thermal resistance, the role of base-collector leakage current, and the phenomenon of thermal instability. This paper first briefly reviews recent microwave results achieved to date and then investigates the aforementioned critical issues that affect their use in microwave applications.

28V High-Linearity and Rugged InGaP/GaAs Power HBT

This paper reports on the improvement of a previously developed InGaP/GaAs HBT for 24-28V linear power operation. The improvements achieved were: application of dynamic bias circuit which improves the ACLR under WCDMA modulation; modification of device technology improving ruggedness to sustain 10:1 VSWR at 30V collector bias under P1dB driving conditions and over 6 dB of gain compression; maintenance of lifetime and reliability simultaneously. Building blocks of HBT were strung together for higher power and good scaling of performance was achieved supporting the validity of the layout approach and the thermal design. Devices delivering P1dB equiv 8W under CW conditions provided ACLR equiv -50 dBc at 8.5 dB back-off and 16% efficiency for WCDMA signal (PARequiv8.7 dB) at 2.14 GHz. Lifetime test over 3000 hours was repeated for 28V bias and 0.05mA/mum2 current density at 315 degree C junction temperature. Therefore, the InGaP/GaAs HBT technology is mature now for the high linearity power amplification

The use of organometallic group-V sources for the metalorganic molecular beam epitaxy growth of In0.48Ga0.52P/GaAs and In0.53Ga0.47As/InP heterojunction bipolar device structures

This paper describes the use of tertiarylbutylarsine (TBA) and tertiarylbutylphosphine (TBP) as replacements for arsine and phosphine in MOMBE/CBE for the production of device structures with state-of-the-art performance. The growth system used for this work is based on the use of elemental group-III and dopant sources, and embryos thermal crackers for the low pressure precraking of TBA and TBP. Device structures fabricated in the In 0.48 Ga 0.52 P/GaAs material system include single- and double-heterojunction bipolar transistor (SHBTs and DHBTs)

High-power high-efficiency X-band AlGaAs/GaAs heterojunction bipolar transistors with undercut collectors

We report on the power performance of X-band AlGaAs/GaAs heterojunction bipolar transistors with undercut collectors for reduced base-collector capacitance. A 10×(2.8×50) μm2 HBT unit cell exhibited 2.09 W continuous wave (CW) output power (4.18 W/mm power density), 62.2% power-added efficiency, and 7.13 dB associated gain at 10 GHz at a collector bias voltage of 10 V. When tuned for maximum efficiency, the same transistor delivered a CW output power of 1.36 W, a power-added efficiency of 74.2%, and an associated gain of 7.32 dB at the same frequency and collector bias voltage. To our knowledge, this is the first demonstration of high-power (>1.3 W), high-efficiency (>74%) AlGaAs/GaAs HBTs using a simple collector undercut technique without the need for significant modifications of baseline HBT process.