Carbon doping of GaAs for heterojunction bipolar transistors: a comparison between MBE and CBE

This paper presents a comparative study of GaAs carbon doping by molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE), using a heated graphite filament and trimethylgallium (TMG), respectively, as sources of carbon. With a growth rate twice higher, CBE allowed us to obtain higher hole concentrations (3 X 10(20) cm-3) than MBE (8 X 10(19) cm-3). The most striking feature was the difference in minority carrier lifetimes, which were up to two orders of magnitude lower in MBE samples. Heterojunction bipolar transistor structures were grown by the two techniques, using a carbon-doped base. Poor performances were obtained on MBE structures. But devices grown by CBE, with a base doping level of 5 X 10(19) cm-3 and a base thickness of 50 nm, exhibited DC current gain as high as 194, with an excellent uniformity of the base sheet resistance.