19 - Practical high-frequency HBTs

Summary

The test of all knowledge is experiment. Experiment is the sole judge of scientific ‘truth’.

The Feynman Lectures on Physics, Richard Feynman

Introduction

In the previous chapter, we laid the foundation for quantitative understanding of the performance of heterojunction bipolar transistors (HBTs) with a particular emphasis on f_T and f_max. In particular, the connection of these quantities to microscopic physics such as carrier transport was described, as was their connection to higher-level parametric descriptions of the transistor, as embodied in an equivalent circuit.

In this chapter, we will discuss the practical constraints imposed by real material systems, epitaxial growth techniques and fabrication processes. The effect of these constraints on f_T and f_max will be studied using the results of Chapter 18. The overall goal of the chapter is to develop physical insight into three areas: (i) the key elements which distinguish material technologies that are suitable for HBTs, and the relationship between the choice of material system and device performance; (ii) the interplay of process development issues and device performance; (iii) other factors than transport, doping behavior, and device geometry (such as reliability) that further constrain device performance. In particular, we will cover the history and evolution of material systems, from AlGaAs/GaAs to InP/GaAsSb, which have been used for HBTs, and we will describe a representative fabrication sequence. Armed with this background knowledge, we will apply some of the theoretical results of Chapter 18 to a state-of-the-art production HBT, and examine the prospects for improving its performance. We will also look at some examples of other problems that arise when a device is scaled and operated for maximum f_T and f_max.