Beta Irradiation of Silicon Junction Devices: Effects on Diffusion Length

The effects of electron irradiation on the minority carrier diffusion length in silicon devices has been studied by utilizing Pm-147 and Kr-85 beta sources. The beta spectra of Pm-147 and Kr-85 are characterized by maximum beta-particle energies of 230 and 670 kev, respectively. Short-circuit current for Li-doped p+n and n+p cells has been measured while the devices are continually irradiated. All devices have approximately 2 Ω-cm bases. The minority carrier diffusion length of the base region was deduced from the calculated betavoltaic collection efficiency. Finally, the diffusion damage coefficient has been calculated for each case. Kr-85 beta irradiated, Li-doped p+ n cells exhibit a much smaller damage rate for the first few months of testing than n+p cells. After decreasing from approximately 100 to 40 μm, the minority carrier diffusion length for the Li-doped cells began increasing until after 1.5 years of irradiation, it exceeded the initial value. Pm-147 beta-irradiated n+ p devices show only a small anmount of radiation effects. For example, typical values obtained for the damage coefficient are initially 10-13 compared to 10-11 for Kr-85 beta-irradiated n+p cells. Values of approximately 10-7 have been previously reported for n+p cells coupled to 0.5 Mev electrons.