Paul J. Rudeck

Effects of Source Diffusion on SILC and Cycling-Induced Charge Loss in Source-Bias Erase Flash Cells

A recent report reveals that in source-bias erase flash cells, light source doping can cause room temperature erratic charge loss after program/erase cycling. In this paper, we present tunnel oxide hole trapping and stress induced leakage current (SILC) measurements under source-bias erase stress conditions, in cell structures with different source doping profiles. Data suggests the deep depletion in cell source during erase causes hole trapping in tunnel oxide above the source diffusion, which is responsible for the room temperature charge loss after P/E cycling for light doping source

Direct observation of secondary ionization current in n-channel MOSFETs

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 todays deep submicron devices.