Large signal RF behaviour of low supply voltage (<3.5 V) bipolar junction transistors

Test devices with collector doping and thickness variations have been made to investigate conventional double poly bipolar junction transistors (BJTs) for use in RF power amplifiers at supply voltages down to 2 V. Measurements and simulations have been performed on power efficiency, gain and linearity. It has been found that efficiencies saturate at about 62-65% (at 1.8 GHz) for thin collectors. From comparison with harmonic balance simulations, this effect is explained by observing that efficiency is limited by hard saturation (which is the same for all devices presented in this paper) rather than by the current and gain limiting "quasi-saturation" effect which dominates in devices with thicker epilayers. Two-tone intermodulation measurements indicate that BJTs are suitable for use in handsets based on e.g. Enhanced Datarate for GSM Evolution (EDGE).