  Voltage current characteristics of a PN-junction diode are intrinsically
nonlinear in nature. It is shown in this paper that a mathematical form of
nonlinearity of a PN-junction diode resembles the nonlinear response of
electric polarization of a dielectric medium to the electric field.
Nonlinearity of a PN-junction can be expressed in a series of successively
increasing orders of the nonlinearity. For a PN-junction diode, higher order
nonlinear terms become significant as a voltage across the diode is increased.
In this paper, a gradual emergence of a nonlinear regime with the amplitude of
a sinusoidal voltage is presented. Higher order harmonics are produced by
utilizing the nonlinearity of a single PN-junction diode. An experimental
realization of a frequency comb with the highest frequency up to the twentieth
harmonics is also presented. In addition, in the same circuit by making the
nonlinearity significant up to the second order, an experiment on generation of
the sum and difference of frequencies is realized.
