A study of L-band GaAlAs/GaAs HBTs for high-voltage RF-power

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.