We have examined proton irradiation damage in high-energy (1–10 MeV) and high-fluence (≳1013 cm−2) Si n+-p-p+ structure space solar cells. Radiation testing has revealed an anomalous increase in short-circuit current Isc followed by an abrupt decrease and cell failure, induced by high-fluence proton irradiation. We propose a model to explain these phenomena by expressing the change in carrier concentration p of the base region as a function of the proton fluence in addition to the well-known model where the short-circuit current is decreased by minority-carrier lifetime reduction after irradiation. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has two effects. First, broadening of the depletion layer increases both the generation–recombination current and also the contribution of the photocurrent generated in this region to the total photocurrent. Second, the resistivity of the base layer is increased, resulting in the abrupt decrease in the short circuit current and failure of the solar cells.
The conduction type of boron (B)-doped silicon (Si) changes from p type into n type by the 1×1017 cm−2 fluence irradiation (high-fluence irradiation) of 1 MeV electrons. The temperature dependence of the electron concentration n(T) obtained from Hall-effect measurements is reported. From the analysis of n(T), the density and energy level of the defects created by the high-fluence irradiation are determined to be 1.5×1014 cm−3 and EC−0.30 eV, respectively, where EC is the energy level at the bottom of the conduction band. Moreover, the compensated density is 9.5×1013 cm−3, which is in agreement with the density of B that acts as an acceptor, determined by Fourier-transform infrared spectroscopy.
We have carried out an investigation of n+–p–p+ silicon diodes after irradiation with 1 MeV electrons and 10 MeV protons and subsequently after annealing. The effects upon the material and device parameters of samples irradiated with different particles are compared by expressing the particle fluence in terms of an effective absorbed dose of 1 MeV electrons. Although the spectrum of defects (observed by deep-level transient spectroscopy) introduced by 1 MeV electrons and 10 MeV protons was slightly different, the total defect introduction rate per effective 1 MeV electron dose was similar, as was the effect upon the device parameters. After irradiation with high fluences of electrons or protons, the effective carrier concentration in the base of the diodes was reduced dramatically, an effect referred to as “carrier removal.” The effects of carrier removal upon the device parameters, in particular, the series resistance and saturation current, are discussed in detail. In addition, the relative importance of different radiation-induced defects is compared.
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