Electron irradiation effect on minority carrier lifetime and other electrical characteristics in silicon power devices

Abstract

Irradiation of silicon power rectifiers with electrons of 12 MeV energy has been carried out. Minority carrier lifetime τ, forward voltage V_F, reverse recovered charge Q_RR, reverse recovery time t_RR for the diodes, circuit commutated turn-off time t_q, and on-state voltage V_T for the thyristors are measured as a function of dose. Power diodes and thyristors obtained from 〈111〉 neutron transutation or phosphorus doped float-zone silicon slices having 120Ωcm and 65Ωcm starting resistivity respectively and Ga-diffused are irradiated at room temperature. A linear relationship between carrier lifetime of irradiated diodes and electron dose is found and the calculated damage coefficients are k_τ = 1.1x10^-8 cm²/s and 7.2x10^-9 cm²/s for the low-level and high-level lifetimes respectively at 25°C. For irradiated thyristors the linear relationship between turn-off time and dose yields k_tq = 3x10^-9 cm²/s at 125°C. Electron irradiation also affects the resistivity of the starting n-type silicon, increasing it of ≈ 15Ωcm for radiation doses > 1×10⁴ Gy. A dose rate effect on the electrical characteristics of the devices using pulses of different duration is analyzed. Annealing studies are carried out at 150 °C, 200°C and 360°C to assess the stability of the defects produced by the electron bombardment by monitoring the variation of the electrical characteristics of the irradiated devices in the temperature range of interest. DLTS measurements performed on electron irradiated power rectifiers have revealed a complex defect pattern. The E₁ defect level (E_c-0.17 ev) is the principal recombination center that controls lifetime following room temperature irradiation. The energy levels and capture cross sections of these irradiation induced-defects are reported. This study confirms that lifetime control in silicon power devices is feasible by high energy electrons. The major advantages of this technique over metallic diffusion or ⁶⁰Co γ-irradiation methods are: better quality, lower processing cost and higher device yields. Annealing after irradiation is important to ensure long-term device stability.