High-Energy Radiation Damage in Silicon Transistors

An experimental investigation of electron and gamma ray damage in silicon transistors is presented. At low values of fluence (Φe < 1014 electrons/cm2), loss in common-emitter dc current gain of medium frequency n-p-n planar transistors at collector currents of one to 10 milliamperes is attributed to changes in the surface recombination velocity. Displacement-induced recombination centers in the base region cause a reduction in gain when Φe is greater than 1014 electrons/cm2. A technique of saturating the surface damage with low energy electrons (E = 125 kev) so as to permit a separation of surface and bulk damage is demonstrated. The minority-carrier lifetime-damage constant, Kτ, has been estimated from the separated bulk-damage curve. It agrees with the value determined from electron irradiation of a low frequency (fαb = 1.25 Mc/sec) mesa n-p-n transistor which is shown to suffer degradation in gain only from bulk recombination current losses within the base region. Surface damage from both electrons and gamma rays is annealed at 250°C or by injecting emitter currents of 200 milliamperes which generate a high internal temperature. In contrast to this behavior, electron irradiation of p-n-p transistors caused loss in gain which is attributed to bulk damage. Damage constants, Kτ, determined from the data show that p-n-p transistors suffer bulk radiation damage about five times greater than n-p-n transistors.