Space-charge effects upon unipolar conduction in semiconductor regions

Abstract

The effect of space charge upon unipolar drift currents in uniformly doped semiconductor regions is studied. The geometry is assumed one dimensional and diffusion is neglected. Both majority- and minority-carrier injection is considered. First, the carrier mobility is taken to be constant. The current and voltage are expressed as functions of the injection level and the normalized carrier transit time, not regarding the actual boundary condition (at the injecting electrode). A space-charge factor which indicates the nonuniformity of the electric field is also defined and computed. Then, various physical situations, such as the space-charge-limited current diode, the shallow-barrier contact diode, the Schottky emission diode, and the emitter-current-limited injection can be studied. Universal diagrams for majority- and minority-carrier injection, respectively, allow evaluation of the space-charge effects and calculation of the J-V dependence by taking into account the proper boundary condition. Finally, an arbitrary velocity-field dependence and the possibility of a negative conductance on the static characteristic is discussed.