The effects of 3 MeV proton and cobalt 60 gamma irradiations on light emission in silicon emitter-base bipolar junctions

Bipolar junction transistors ST 2N2219A (npn) and 2N2905A (pnp) were exposed at room temperature to 3 MeV protons at fluences of 5×10⁸, 4.6×10⁹, 5×10¹⁰, 4.9×10¹¹, 5×10¹² and 5×10¹³ p/cm², and to gamma irradiation at doses of 8.2, 16.4 and 24.6 Mrad (Si). The electroluminescence (EL) spectrum generated by direct and indirect polarisation of the emitter-base junction was recorded in the spectral range 1.0 to 2.5 eV. The intensity of the EL signal decreases as a function of fluence in direct mode operation. This is attributed in part to the creation of radiation induced mid-gap defects which shorten the non-radiative lifetime. The intensity of the EL signal is independent of irradiation in reverse bias operation. It is concluded that light emission in reverse bias does not involve transitions to the ground state (valence band), nor to the first exited state (conduction band). This conclusion was confirmed with temperature dependent measurements in non-irradiated junctions. For forward bias operation, the emitted light is sensitive to junction temperature. The light intensity is independent of temperature in the case of reverse bias operation. The results are more consistent with the intraconduction bands model of light emission in reverse biased p-n junctions