Heterojunction Bipolar Transistor Technology for High-Speed Integrated Circuits

Abstract

A bipolar structure provides many potential benefits for picosecond electronic circuits: 1) current flow is vertical, through epitaxial layers whose total thickness is only a few tenths of a micron, decreasing carrier transit times; 2) large amounts of current can be carried by modest size transistors, enabling rapid charging of interconnect capacitances; and 3) threshold voltages are tied to built-in potentials of p-n junctions, which are easy to reproduce device-to-device, enabling accurate logic voltages to be established. Added advantages result from using III–V heterojunctions rather than Si [1]: a) high electron mobility; b) pronounced transient electron velocity overshoot effects; c) ability to incorporate quasi-electric fields into the devices by semiconductor composition variations; and d) ability to insure adequate emitter efficiency with wide gap emitters, and therefore produce devices with heavily doped, low resistance base regions together with lightly doped, low capacitance emitter regions. As a result of these advantages, HBTs offer prospects for switching time delays in the sub 10 ps regime, with a robust, manufacturable fabrication process.