EFFECT OF BASE DOPANT SPECIES ON HETEROJUNCTION BIPOLAR-TRANSISTOR RELIABILITY

The electrical activation of carbon in GaAs grown by metal-organic molecular beam epitaxy has been shown to be excellent in as-grown material. At very high doping levels, however, the activation can be reduced substantially by annealing, even at rather moderate temperatures. This reduction in hole concentration combined with the reduction in gain observed with increasing dopant level in the base suggests a practical limit of less than 10(20) cm(-3) for base doping in GaAs/AlGaAs heterojunction bipolar transistors (HBTs). Using moderate base doping, p approximate to 7 x 10(19) cm(-3), carbon-doped devices do not exhibit short-term degradation at current densities less than or equal to 2.5 x 10(4) A cm(-2), even at operating temperatures up to 200 degrees C unlike Be-doped HBTs which typically degrade much more rapidly even at twice lower base doping levels. Operation of carbon-doped devices at higher current densities for longer periods of time, however, can result in current-induced decreases in d.c. gain which are correlated with the amount of hydrogen incorporated in the base layer during growth. During device operation, minority carrier injection induced debonding of hydrogen from neutral C-H complexes leads to an increase in effective base doping level and therefore to a decrease in gain. Post-growth in situ or ex situ annealing eliminates this effect by breaking up the C-H complexes. Properly designed HBTs are stable during operation at 125 degrees C even for very high collector current densities (10(5) A cm(-2)).