InGaP/GaAs heterojunction bipolar transistor ballasted with extended ledge resistor

Abstract

This thesis deals with two topics in the field of compound semiconductor device technology. One is InGaP/GaAs heterojuction bipolar transistor which is expected as an attractive alternative to the more conventional AlGaAs/GaAs HBT. The other is the applications of heterojunction bipolar transistors for high power devices. HBT′s based on the InGaP/GaAs material system have received considerable interest as an alternative devices of AlGaAs/GaAs HBT′s. So, in this thesis we have developed a standard process for InGaP/GaAs HBT′s using self-aligned base metallization and polyimide planarization/passivation process. HCl-based solution was used to selectively remove InGaP layer and self-aligned metallization formed separation of 1500Å between emitter and base contact metal. Four times polyimide coating was done to obtain a planarized surface. The fabricated $2\times10\mum^2$ emitter design device showed DC current gain of 20, offset voltage of 150 mV, $f_\tau$ of 42 GHz and $f_{max}$ of 64 GHz. We investigated thermal characteristics of HBT′s and its influence to device performance. Firstly, we addressed the thermal characteristics of 1-finger device and constructed the large signal DC model including thermal effects. From this model, we could estimate thermal characteristics of multi-finger devices. After that, several thermal designs, which can suppress the thermal runaway phenomenon, were presented and analyzed by using previously developed model. Above mentioned common solutions to the thermal problems were the use of large Δ$E_v$ material, hetero-structure emitter, thermal shunt technique, cascode configuration, and ballast resistor. Then, a novel base ballast resistor was proposed, fabricated, and measured. Proposed device has a structure in which the extended passivation ledge can work as a ballast resistor. The small signal parameters of fabricated device were extracted and ballast resistance of 35 Ω was extracted. Also, the temperature dependency of ba...