High voltage heterojunction bipolar transistors

GaAs HBTs offer exciting opportunities for high voltage, high power operation for applications where Si devices are either unavailable or fall short of performance goals. Invented a long time ago, the HBT has realized its promise in the last few years, mainly due to significant advances in the materials technologies. AlGaAs/GaAs based HBTs were the workhorse of the scientific community for many years but InGaP/GaAs, due to its more favorable band alignment has recently generated more interest. High-voltage operation in HBTs is obtained almost entirely through suitable design of the base-collector junction. Unlike silicon, HBTs allow the use of a wide band-gap collector as an additional degree of freedom when designing a high-voltage device. High voltage operation in the HBTs is made possible by the simple design of the base-collector pn junction and limited only by material and processing concerns. While design considerations such as the doping and thickness of collector layer can be generally obtained from the data available for the high-voltage silicon semiconductor devices, use of a highly doped base-layer poses a number of practical problems in terminating the high-voltage HBT. In this paper we will discuss design issues for high voltage GaAs HBTs. Design of collector doping and thickness using empirical expressions will be discussed. Practical issues relating to the isolation of devices and the effect of the isolation techniques on the high voltage operation of the HBTs will be examined.