Degenerate Germanium. I. Tunnel, Excess, and Thermal Current in Tunnel Diodes

Abstract

The conduction processes in germanium tunnel diodes under forward bias are studied experimentally and compared with existing theories. Conductance is measured on an assortment of diodes with regrown $p-n$ junctions, doped with known amounts of arsenic and gallium, in the temperature range 4.2° to 300°K. Capacitance is measured as a function of voltage at room temperature. The studies deal in particular with the temperature and composition dependence of the thermal current, of the peak tunnel current and peak voltage, and of the exponential component of the excess current. The thermal current varies as $\exp \left(\frac{\mathrm{eV}}{\mathrm{kT}}\right)$. At low temperature the potential barrier controlling it has an unexpected independence of the doping, a fact not easily understood in terms of minority carrier injection. At 300°K injection does occur as shown by the capacitance measurements. The peak tunnel current and voltage are found to agree quantitatively with Kane's theory of direct band-to-band tunneling for all tunnel junctions, covering a doping range from 3×${10}^{18}$ to 3×${10}^{20}$/${\mathrm{cm}}^3$. The change of peak current with temperature is correlated with the variation of band-gap and Fermi level position. The excess current, which in some diodes has an exponential increase with bias for over half the band-gap voltage, is shown to be closely related to the peak current, in agreement with theory.