The feasibility of GaAlAs/GaAs heterojunction bipolar transistors as power
output transistors for 24-28 V cellular base station applications was studi
ed. A 1 W, 25 V test vehicle was fabricated, with predictable and uniform e
lectrical parameters. Maximum output power was 1.02 W with 9.1 dB gain at 2
GHz and 18 V supply voltage. Characterization at more than 22 V destroyed
all devices, even if emitter ballasting resistors were used for thermal bal
ance. Analysis indicated that the failures were caused by too wide emitters
in combination with the high supply voltage, causing current crowding and
large temperature spikes. Small (< 2-3 mu m) horizontal emitter dimensions
are required even with low base sheet resistivity, otherwise leading to des
tructive temperature non-uniformities, unless special arrangements to drast
ically reduce the thermal resistance can be applied.