Influence of heavily doped contacts on photoconductive switch properties

To help analyze a photoconductive semiconductor switch where heavily doped regions about the metal electrodes assure low-resistance ohmic contacts, switch models based on analytical expressions are developed. These analytical expressions describe the positional dependence of the carrier profiles and the electric field, the photoconductive gain, and the turnoff transient. They apply for m/n⁺/i/n⁺/m, m/p⁺/i/p⁺/m, and m/p⁺/i/n⁺/m switches (where m denotes metal and the other symbols have standard meanings). The accuracy of these expressions is checked against contact-to-contact numerical solutions from a photoconductive device simulator. The accuracy is excellent. In contrast to previous studies of photoconductive switches, the analytical expressions and numerical solutions presented show that the limits on the photoconductive gain are more severe than previously set forth and that the current-voltage characteristics for constant illumination become sublinear at higher voltages, even though the carrier mobility remains independent of the applied electric field