The tetrode power transistor

Power transistor circuits are characterized by the fact that the collector current must swing over a wide range of values during any complete cycle of operation. One disadvantage of present-day alloyed junction power transistors is that the current gain decreases with increasing collector current. This causes distortion in linear applications and makes temperature stabilization in switching circuits more difficult. Power transistors having emitter areas large enough to handle currents in the amperes range can be made as tetrodes by use of an annular ring geometry. Experimental results show that the gain characteristics can be altered by applying a bias voltage or a portion of the signal voltage transversely across the base. The gain characteristic can be made flatter for improved fidelity in audio applications, or even reversed to give increasing gain with collector current for certain switching applications. Practical circuitry utilizing the improved gain characteristics of power tetrodes has been developed, and the annular geometry permits the fabrication of tetrodes using conventional alloying techniques.