The operation of power lateral pnp transistors in gamma radiation field was
examined by detection of the minimum dropout voltage on heavily loaded
low-dropout voltage regulators LM2940CT5, clearly demonstrating their low
radiation hardness, with unacceptably low values of output voltage and
collector-emitter voltage volatility. In conjunction with previous results on
base current and forward emitter current gain of serial transistors, it was
possible to determine the positive influence of high load current on a slight
improvement of voltage regulator LM2940CT5 radiation hardness. The
high-current flow through the wide emitter aluminum contact of the serial
transistor above the isolation oxide caused intensive annealing of the
positive oxide-trapped charge, leading to decrease of the lateral pnp
transistor's current gain, but also a more intensive recovery of the
small-signal npn transistors in the control circuit. The high current density
in the base area of the lateral pnp transistor immediately below the
isolation oxide decreased the concentration of negative interface traps.
Consequently, the positive influence of the reduced concentration of the
oxide-trapped charge on the negative feedback reaction circuit, together with
the favourable effect of reduced interface traps concentration, exceeded
negative influence of the annealed oxide-trapped charge on the serial pnp
transistor's forward emitter current gain.
Power lateral pnp transistor operating with high current density in irradiated voltage regulator
