  We propose an on-demand single photon source for quantum cryptography using a
metal-insulator-semiconductor quantum dot capacitor structure. The main
component in the semiconductor is a p-doped quantum well, and the cylindrical
gate under consideration is only nanometers in diameter. As in conventional
metal-insulator-semiconductor capacitors, our system can also be biased into
the inversion regime. However, due to the small gate area, at the onset of
inversion there are only a few electrons residing in a quantum dot. In
addition, because of the strong size quantization and large Coulomb energy, the
number of electrons can be precisely controlled by the gate voltage. After
holding just one electron in the inversion layer, the capacitor is quickly
biased back to the flat band condition, and the subsequent radiative
recombination across the bandgap results in single photon emission. We present
numerical simulation results of a semiconductor heterojunction and discuss the
merits of this single photon source.
