RF power amplification above 200GHz has become more critical as higher frequency bands are sought for radar and space applications. At 220GHz, a low-loss free-space propagation window is very alluring for potential systems, however, a single transistor cannot produce sufficient output power requisite for a wireless system. This work demonstrates solid-state power amplifier MMICs targeting up to 0.4W at 220GHz.

Power amplifier cells are designed using four 6&mgr;m-long common emitter and common base InP HBTs to form a cascode. Single InP HBTs with 250nm-wide emitters and have shown a peak fMAX of 700GHz and peak fT of 400GHz. The cascode cell shows above 15dB of small signal available gain at 220GHz. Non-inverted microstrip IC interconnects shield the InP substrate and provide a low-loss environment for impedance tuning and power combining. Multiple levels of power combining are performed on wafer to reach a high overall saturated output power at 220GHz. Experiments were designed to find the limits of transistor density, the optimal Class A load line, the optimal number of power combining levels and gain stages, the maximum physical size, and the maximum possible DC power consumption at the IC level.

In the first attempt, a 4-Cell SSPA was reported to have 48.8mW of saturated output power at 220GHz with a compressed gain of 4.5dB. On a second maskset, a 2-stage, 8-Cell SSPA had 90mW of saturated 220GHz output power at 8.2dB of compressedgain. A third set of designs demonstrates a 4x increase in output HBT periphery over the 8-Cell SSPAs in Tapeouts 1 and 2. In full-thickness RF power measurements, 180mW of saturated power was observed at 214GHz when air cooled. When heat removal by wafer thinning and mounting is complete, power levels may be close to the 400mW shown in simulation.