Space-charge limited current and capacitance in double-junction diodes.

An analytical theory is given for the space-charge limited current in nin or pip diodes. The exact I-V characteristic is obtained in a parametric form. In the limit of high currents or for a large width of the intrinsic (i) base region the characteristics reduce to a Mott-Gurney form. In the low-current limit a linear I-V characteristic is obtained. The space-charge barrier presents a conceptually different case from a conventional barrier current in that the position of the barrier MOVES depending on the biasing condition. It is this motion, which is responsible for the linear regime - which extends over a substantial range (~ 10 kt/e) of the applied voltage. The distribution of the electrostatic potential and of the quasi-Fermi level in the base, as well as the position of the potential maximum (the virtual cathode), are shown for different current levels. The differential capacitance of the double-junction diode is calculated and shown to be strongly dependent on the applied bias.