Darrell G. Hill

Current gain collapse in microwave multifinger heterojunction bipolar transistors operated at very high power densities

The rapid development of heterojunction bipolar transistor (HBT) technologies has led to the demonstration of high power single-chip microwave amplifiers. Because HBTs are operated at high power densities, the ultimate limits on the performance of HBTs are imposed by thermal considerations. The authors address a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phenomenon, referred to as the collapse (of current gain), occurs when suddenly one finger of the HBT draws most of the collector current, leading to an abrupt decrease of current gain. A quantitative model and the condition separating the normal operation region and the collapse are presented. Critical difference of the collapse in the constant l/sub b/ and constant V/sub be/ modes of operation is discussed for the common-emitter l-V characteristics. The collapse in the common-base l-V characteristics and its relationship with avalanche breakdown are also described. A solution to eliminate the collapse is experimentally verified. >

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>>

Novel HBT with reduced thermal impedance

Heterojunction bipolar transistors have been fabricated using a novel process in which the majority of the front side of the chip is metallized to serve as the groundplane. The completed chip is assembled inverted so that the emitters are next to the heat sink; base and collector are contacted using through-wafer vias and microstrip lines on the back side of the chip. These devices show a 50% reduction in thermal impedance compared to conventionally fabricated devices and have achieved power densities of 10 W/mm of emitter length. Such devices are expected to have substantially lower emitter inductance as well, which may lead to improved gain at higher frequencies. >

Hydrogen-related burn-in in GaAs/AlGaAs HBTs and implications for reliability

We report a burn-in effect in carbon-doped GaAs/AlGaAs HBTs that results in an increase in dc current gain. The burn-in is the result of the annihilation of hydrogen-related recombination centers due to electron injection into the base. This burn-in effect needs to be taken into account in long-term bias stress testing of HBTs. Unrealistic values of mean time to failure and activation energy may be calculated otherwise.

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.

A 0.5-W complementary AlGaAs-GaAs HBT push-pull amplifier at 10 GHz

X-band high-efficiency complementary push-pull amplifiers using P-n-p and N-p-n AlGaAs-GaAs HBTs were demonstrated. One of the amplifiers achieved an output power of 500 mW with 6-dB gain and 41.8% power-added efficiency at 10 GHz. High efficiency was achieved by Class-B push-pull operation, which also results in second harmonic cancellation.<<ETX>>

Two Selective Etching Solutions for GaAs on InGaAs and GaAs / AlGaAs on InGaAs

Two solutions for selectively etching on and on are presented. A solution of is shown to have a etch rate more than 50 times higher than the etch rate of . A solution of etches both and selectively relative to , with a selectivity of 8 or more for.

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.

X-band power AlGaAsInGaAs P-n-p HBTs

AlGaAs/InGaAs P-n-p heterojunction bipolar transistors (HBTs) were fabricated using carbon-doped material grown by nonarsine metal-organic vapor-phase epitaxy (MOVPE). F/sub max/ of 39 GHz and f/sub t/ of 18 GHz were obtained. Operated in common-base mode, a P-n-p HBT achieved 0.5-W output power with 8-dB gain at 10 GHz; saturated output power was 0.69 W. Results are presented for devices with emitter lengths from 120 to 600 mu m.<<ETX>>

28 V low thermal impedance HBT with 20 W CW output power

AlGaAs/GaAs heterojunction bipolar transistors have been fabricated which exhibit record output power for GaAs flip-chip technology, and record operating voltage for GaAs microwave power devices. Transistors with 2 mm emitter length readily achieve 20 W CW output power at 2 GHz when biased at 28 V, with typical power-added efficiencies of 62% (typical collector efficiencies of 70%). Maximum CW output power of 25 W has been obtained, corresponding to a power density of 12.5 W/mm.