Equilibrium-to-nonequilibrium transition in MOS (surface oxide) tunnel diode

When the insulator barrier in a tunneling MIS diode is less than a certain critical thickness (approximately 30 Å for Al-SiO₂-Si at 300 °K), the semiconductor departs from thermal equilibrium and a new type of ``nonequilibrium'' current-voltage characteristic results. Such nonequilibrium diodes have been found to possses current multiplication properties and have been used as the basis of the tunnel surface oxide transistor (SOT) reported by Shewchun and Clarke. They have also been applied to direct energy conversion employing the electron-voltaic and photovoltaic effects. The transition of the MIS tunnel diode from the equilibrium to this nonequilibrium mode is investigated as a function of insulator thickness, metal work function, and temperature as well as a number of semiconductor parameters. A nomograph is presented to assist in determining when the MIS tunnel diode is in the nonequilibrium mode. Surface-state tunneling is seen to play an important role in determining part of the nonequilibrium current-voltage characteristic.