Very Narrow Base Diode

Techniques have been developed to fabricate semiconductor diodes with rectifying junction to ohmic contact distances of the order of microns. The current-voltage relationship of such a diode is a function of the degree of imperfection of the ohmic contact. If it were possible to make ideal ohmic contacts, the diode would exhibit extremely poor rectification. The rectification ratio of germanium diodes for practical ohmic contacts, however, is of the order of 105 to 106. The current-voltage relationship, the small-signal frequency response, and the switching characteristics of the very narrow base diode are analyzed using the appropriate boundary condition at the ohmic contact. The alloy junction current-voltage characteristics follow very closely the (eqV/akT-1) relationship with values of a between 1.02 and the theoretical value 1.00. Because of the narrow base width, the series bulk resistance for typical designs is between 0.3 and 3 ohms. Thus the entire range of forward currents can be obtained at low forward voltages. The diode is a high-frequency device both for small-signal applications and for switching applications, although the ultimate high-frequency capability is reduced because of the imperfection of the ohmic contact. In switching applications, the reverse recovery time may be limited as much by junction capacitance as by hole storage. A method of fabrication is described and small-signal applications at uhf are discussed. A computer diode design that switches at speeds up to 5 mc is described. This diode has the advantageous combination of low forward-voltage drop and high-frequency capability.