Hot carrier injection

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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.

Advances in semiconductor manufacturing techniques and ever increasing demand for faster and more complex Integrated Circuits (ICs) have driven the associated Metal Oxide Semiconductor Field Effect Transistor (MOSFET) sizes close to their physical limits. On the other hand, it has not been possible to scale the supply voltage used to operate these ICs proportionately due to factors such as compatibility with previous generation circuits, noise margin, power and delay requirements, and non-scaling of threshold voltage, subthreshold slope, and parasitic capacitance. While the consequent increase in internal electric fields in aggressively scaled MOSFETs comes with the additional benefit of increased carrier velocities, and hence increased switching speed, it also presents a major reliability problem for the long term operation of these devices. As devices are scaled the benefits of higher electric fields saturate while the associated reliability problems get worse. The presence of large electric fields in MOSFETs implies the presence of high energy carriers, referred to as “hot-carriers”, in such devices. The carriers that have sufficiently high energies and momenta can get injected from the semiconductor into the surrounding dielectric films such as the gate and sidewall oxides as well as the buried oxide in the case of Silicon-On-Insulator (SOI) MOSFETs (Fig. 1). The presence of mobile carriers in the oxides triggers various physical processes that can drastically change the device characteristics during normal operation over prolonged periods of time eventually causing the circuit to fail. Such degradation in device and circuit behavior due to injection of energetic carriers from the silicon substrate into the surrounding dielectrics is be referred to as “hot-carrier degradation”. It is clear that the presence of large electric fields has major influence on the long term operation of modern ICs. These Hot-Carrier (HC) related device instabilities have become a major reliability concern in modern Metal Oxide Semiconductor (MOS) transistors and are expected to get worse in future generation of devices. The study of the fundamental physical processes that result in device parameter variation due to HC injection is essential to provide guidelines for avoiding such HC-induced device degradation has been the subject of numerous studies over the past several decades. The effect of carrier heating has been observed in a variety of applications and device structures. In fact, certain carrier heating processes have been utilized as the basis of operation of circuits such as Electrically Erasable Programmable Random Access Memory (EEPROM) cells. As soon as the potential detrimental influence of HC injection on the circuit reliability was recognized, several fabrication strategies were devised to reduce it without compromising the circuit performance.

[edit] Causes

There are several mechanisms which can cause hot carrier injection.

1. 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.

2. 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.