Small-sized collector-up Ge/GaAs heterojunction bipolar transistors with high gain, low base resistance, and high fmax

Small-sized collector-up Ge/GaAs HBT's are successfully fabricated and their operation at a high collector current density and at a high frequency is realized for the first time. The current gain of these devices reaches a peak value as large as 200 at a current density 6×10
⁴
Acm/sup /spl minus/2/, and no degradation in the current gain is observed as the collector width is decreased down to 2 μm. The capability of lower voltage operation is also shown. Intrinsic and extrinsic base resistances are as low as 180 /spl Omega///spl square/ and 90 /spl Omega///spl square/, respectively. The calibrated values of f
_T
and f
_max
are 25 GHz and 60 GHz, respectively. The larger value of f
_max
compared with f
_T
might be attributed to low base resistance and low base-collector capacitance as expected from the collector-up structure.