Cryogenic operation of power bipolar transistors

The results of detailed measurements, simulations and modeling on 500 V, 4 Amps NPN BJTs are reported in the 300-77 K temperature range. For these devices, as the operating temperature is reduced from 300 to 77 K, the current gain has been found to decrease by more than an order of magnitude; the on-state collector-emitter and base-emitter voltages increase by 40 and 80% respectively; although the collector-base breakdown decreases by about 20%, the collector-emitter breakdown increases by about 20%, and the storage and fall times reduce by 10× and 6×, respectively. Through numerical simulations it is shown that the emitter current crowding is much more severe at 77 K than at 300 K. Using verified analytical models and established optimization techniques, it is shown that a 77 K optimally designed BJT has a lower emitter, base and collector dopings and a larger emitter area than a similarly rated 300 K optimized device