Band-Edge Steepness Obtained From Esaki/Backward Diode Current-Voltage Characteristics

While science has good knowledge of semiconductor bandgaps, there is not much information regarding the steepness of the band edges. We find that a plot of absolute conductance, I/V versus voltage V, in an Esaki diode or a backward diode will reveal a best limit for the band tails, defined by the tunneling joint density of states of the two band edges. This joint density of states will give information about the prospective subthreshold swing voltage that could be expected in a tunneling field-effect transistor. To date, published current-voltage characteristics indicate that the joint band-tail density of states is not steep enough to achieve <60 mV/decade. Heavy doping inhomogeneity, among other inhomogeneities, results in a gradual density of states extending into the bandgap. The steepest measured tunnel diodes have a tunneling joint density of states >90 mV/decade.