Hot carrier injection

Hot carrier injection is the phenomenon in solid state devices or semiconductors where either an electron or a "hole" gains sufficient kinetic energy to overcome a potential barrier, becoming a "hot carrier", and then migrates to a different area of the device. The term usually refers to the effect in a MOSFET where a carrier is injected from the silicon substrate, to the gate dielectric. To become 'hot', and enter the conduction band of the dielectric, an electron must gain a kinetic energy of 3.3eV (for an SiO₂ dielectric). For holes the valence band offset dictates they must have a kinetic energy of 4.6eV. Often, this results in the carrier's no longer being in its originally designed position and as such, hot carrier injection represents a degradation phenomenon in the device. Hot carriers can degrade the gate dielectric causing electron and hole traps to form which increase the leakage current and cause shifts in the threshold voltage and ultimately, the device will become unstable and fail. However, flash memory exploits the principle of hot carrier injection by deliberately injecting a carrier and having it reside at the floating gate where in memory terms it represents a '1' until such time as the memory is erased, and the carrier is removed from the gate.

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Causes

There are several mechanisms which can cause hot carrier injection. Since carriers are accelerated by the strength of the electric field, designs which use too high a voltage coupled with a small dielectric thickness will create a stronger field across the layer and increase the presence of hot carriers. Since a carrier gains kinetic energy in the electric field only while it has "room to run", and this "room" is essentially the mean-free path of the carrier, and since mean-free path in turn decreases with increasing temperature, low operating temperatures can be a problem. This is the opposite of most wear-out phenomena in solid state electronics.