Heterojunction bipolar transistors (HBT) based on indium phosphide (InP) material have the potential to realize both analog and digital electronic systems operating beyond 100 GHz, and so this transistor technology could form the electronic platform for applications in the millimetre-wave and terahertz bands of the electromagnetic spectrum. In these devices, however, parasitic elements dominate device behaviour at the minimum design rule, which at present is about 0.25 mm emitter width. Reducing parasitic components of the base-collector junction (mainly the capacitance) is the theme of several advanced fabrication processes, including transferred-substrate and undercut collector HBTs. For emitter widths below 0.25 mm, however, the emitter contact resistance (as well as thermal resistance) starts to dominate device behaviour limiting performance.

We are investigating two approaches for the realization of ultra-broadband InP HBTs (> 500GHz bandwidth). In the first approach, the collector-up configuration in which the base-emitter junction outside of the collector is re-grown is being considered. The challenge here is to remove the parasitic base-emitter junction which would otherwise severely degrade the gain. In the other approach, a new device geometry which is realizable using the transferred-substrate technique is under consideration. In both approaches, the emitter contact is much wider than the active device region (transistor), junction capacitances extrinsic to the transistor are eliminated and a Schottky collector is employed, and so there are no (obvious) limitations to transistor scaling.

For further information contact DR. Edward Wasige – e.wasige@elec.gla.ac.uk