50 GHz Si 1 − xGe x heterobipolar transistor: growth of the complete layer sequence by molecular beam epitaxy

Abstract

The six-layer structure of a Si 1 − x Ge x heterobipolar transistor (HBT) with flat doping levels and abrupt junctions is grown by a silicon molecular beam epitaxy (MBE) process in one run. The process, the MBE equipment used and analytical results (secondary-ion mass spectrometry) are described. The growth temperature is reduced from 650 °C at the collector to 325 °C at the emitter contact. A Si 1 − x Ge x base doping level of 5 × 10 19 cm −3 is obtained by coevaporation of silicon, germanium and boron from an electron beam evaporator, a BN effusion cell and a high temperature graphite cell respectively. The n-type doping of 10 16–10 20 cm −3 (collector, emitter and emitter contact) is obtained by applying three different antimony dopant incorporation methods. Test transistors fabricated from these structures exhibited low noise, good high frequency performance ( f T, f max > 50 GHz) and low base sheet resistance (less than 1 kΩ/□).