Effect of transistor geometry on the electrical characteristics of Si1-xGex heterojunction bipolar transistors at low temperatures

This paper investigates the effect of using an extrinsic implant on the behaviour of SiGe HBTs at low temperatures. The collector current characteristics of SiGe HBTs with different geometries are measured at temperatures from 77 to 300K. The temperature dependence of the collector current is different for devices with different geometries and this results from base profile broadening in the vicinity of the extrinsic base implant due to point defects. Process and device simulators are used to explain this effect. A method for eliminating this geometry dependence of the collector current is also described.