Direct observation of secondary ionization current in n-channel MOSFETs

Abstract

Hot-electron current in an n-channel MOSFET at high drain bias and on or off gate bias generates secondary currents amounting to a fraction of the primary (drain) current, in a surrounding n-well and the underlying p-substrate for a triple well device. Secondary current can also be collected in the source for the gate-off condition. The relative magnitude of these secondary currents is studied depending on MOSFET length and channel doping concentration. For a MOSFET with a heavy doping concentration in the channel near the drain junction, it is shown that a significant part of the secondary current is made of electrons originating from the space charge region of the drain, that escape the drain field and diffuse through the neutral substrate toward the source and surrounding n-well. These electrons are likely the product of secondary impact ionization by holes generated in the space-charge region of the drain. Similar secondary current is collected in the n-well surrounding a MOSFET in Fowler-Nordheim (FN) tunneling from the inversion layer to the gate. In this case, too, secondary electrons generated by holes returning from the oxide into the MOS channel appear to escape into the p-type body and flow to the surrounding n-well. These findings contradict the belief that secondary currents associated with hot carrier phenomena are only induced by hot carrier generated light, or secondary bipolar injection. Secondary ionization as a source of secondary current was rejected for older MOS devices. This paper shows experimentally and theoretically that secondary ionization and the ensuing hot carrier escape have become significant in today's deep submicron devices.