Trends in development of bipolar power transistors

Monolithic integrated circuit technology was applied to bipolar power transistors which included Darlington transistors with preamplification. They are produced with two stabilizing resistors and sometimes also with two diodes for higher switching speed and for overvoltage protection respectively. Darlington transistors have higher current gain than plain silicon transistors of the same size so that they are indispensable for inverters. The main trend and corresponding changes in manufacturing methods are toward higher current and voltage ratings for direct connection without step-down transformers. Low voltage transistors with thin low resistivity epitaxial layers have the optimum combination of performance characteristics, while most transistors with rating up to 50 A and 400 V have n-n+ epitaxial structures. Triple diffusion is still more successful than the epitaxial process in producing high current high voltage transistor structures and is the predominant technology. This is attained by increasing the chip size or allotting up to half the chip area to the emitter, at a sacrifice in switching speed because of the larger thickness of active layers and attendant longer lifetime of minority carriers in them. Consequently, bipolar lower transistors for voltages higher than 1000 V are not likely to be produced soon.