Physical mechanisms leading to deterioration of transistor life

Life tests on surface-barrier-type transistors have been conducted at various temperatures and power levels to identify and characterize the mechanisms which cause the transistor characteristics to deteriorate with time. Three mechanisms have been isolated: the formation of solution cavities in the base of the transistor, an increase in surface recombination velocity, and a decrease in surface resistance. In the normal surface-barrier transistor, the formation of solution cavities proceeds with an activation energy of about 20,000 cal. mole. This leads to an exponential dependence of life expectancy on temperature and dissipation. The formation of solution cavities is eliminated by the microalloy process, in which case the life expectancy is probably determined by the decrease in surface resistance or the increase in surface recombination velocity. The increase in surface recombination velocity causes a well-correlated decrease in current gain and grounded-base output impedance. The decrease in surface resistance produces an increase in the collector "saturation" current and may contribute to a decrease in output resistance. The formation of solution cavities brings about a decrease in punch-through voltage and grounded-emitter output impedance.