Process development and device modelling of gallium arsenide heterojunction bipolar transistors

Abstract

This thesis discusses the processing and analysis of high speed semiconductor devices with emphasis on GaAs-based heterojunction bipolar transistors. The heterojunction transistor process is developed as an essential part of this thesis. Device physics is first reviewed in depth to construct a solid basis for physical one dimensional simulation of heterojunction bipolar junction transistors. Theory is then applied to a simulation platform in a way which facilitates device design and evaluation at practical level. The simulation platform was used in designing epitaxial layers for a transistor structure with heavily doped base layer and current gain target at 50. The developed transistor process relies on wet chemical isolation etching, and takes into account the restrictions that arise from the academic perspective of the processing environment. The process development goal was educational robustness.

The development effort for HBT process is explained in detail, and processing steps are illustrated with scanning electron microscope images. The most critical processing steps were for defining isolation depths. Isolation is based on slow citric acid wet chemical etching monitored with a high precision profilometer. Active devices form isolated hills or "mesas" on the semi-insulating substrate. Because of the rather tall etched structures the lithography is of planarizing type. The process includes a unique double layer planarising lithography for AZ 5214E resist, developed within the framework of this thesis. The lithography is doubly functional such that it also allows two resist layers to be patterned separately on top of each other, which is utilised in defining shallow air bridges in the transistor structures.

The most important measurement results are explained. Degradation of transistor performance after excessive heating or current stress is also demonstrated, and a method for processing devices with minimal amount of heating is introduced as a means to tackle the problem. Measured collector characteristics of various types of HBTs are given. Best DC characteristics were achieved with a transistor structure including non-alloyed contacts and Schottky diode collector. This thesis focused on process development and DC analysis of the transistor. Frequency characteristics were measured only for completeness. It is shown that even the non-optimized process was capable of producing transistors with power gain cut off frequency exceeding 1 GHz.