A modulation doped transistor structure which can function as both a unipolar transistor (HFET) and a bipolar inversion channel FET (BICFET) is fabricated on the same MBE-grown wafer for the first time and demonstrated in both modes of operation. The parameters are the best yet reported. A I X 100 mu m(2) HFET shows a g. of 162 ms/mm and I-dsmax = 80 mA/mm with a threshold voltage of 0.5 V. BICFET gains of 20-40 are obtained with BVeco = 8 V and offset V-ec = 0.2 V. An analytical model is presented for both HFET and BICFET operation. The measured current gain is found to be dominated by the effects of current crowding into the source-emitter junction. The current gain, output resistance, breakdown voltage and collector offset voltage are accurately predicted and the effects of device heating are represented through the adjustable ideality factor. The current crowding mechanism indicates the potential for aggressive scaling of the BICFET into the submicron region. (C) 2009 Elsevier Ltd. All rights reserved.