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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 6×104 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 fT and fmax are 25 GHz and 60 GHz, respectively. The larger value of fmax compared with fT might be attributed to low base resistance and low base-collector capacitance as expected from the collector-up structure.