Hot Carrier Injection; Hot Electrons

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Hot Carrier Effects

The term 'hot carriers' refers to either holes or electrons (also referred to as 'hot electrons') that have gained very high kinetic energy after being accelerated by a strong electric field in areas of high field intensities within a semiconductor (especially MOS) device. Because of their high kinetic energy, hot carriers can get injected and trapped in areas of the device where they shouldn't be, forming a space charge that causes the device to degrade or become unstable. The term 'hot carrier effects', therefore, refers to device degradation or instability caused by hot carrier injection.

According to the 5th Edition Hitachi Semiconductor Device Reliability Handbook, there are four (4) commonly encountered hot carrier injection mechanisms. These are 1) the drain avalanche hot carrier injection; 2) the channel hot electron injection; 3) the substrate hot electron injection; and 4) the secondary generated hot electron injection.

The drain avalanche hot carrier (DAHC) injection is said to produce the worst device degradation under normal operating temperature range. This occurs when a high voltage applied at the drain under non-saturated conditions (VD>VG) results in very high electric fields near the drain, which accelerate channel carriers into the drain's depletion region. Studies have shown that the worst effects occur when VD = 2VG.

The acceleration of the channel carriers causes them to collide with Si lattice atoms, creating dislodged electron-hole pairs in the process. This phenomenon is known as impact ionization, with some of the displaced e-h pairs also gaining enough energy to overcome the electric potential barrier between the silicon substrate and the gate oxide.

Under the influence of drain-to-gate field, hot carriers that surmount the substrate-gate oxide barrier get injected into the gate oxide layer where they are sometimes trapped. This hot carrier injection process occurs mainly in a narrow injection zone at the drain end of the device where the lateral field is at its maximum.

Hot carriers can be trapped at the Si-SiO2 interface (hence referred to as 'interface states') or within the oxide itself, forming a space charge (volume charge) that increases over time as more charges are trapped. These trapped charges shift some of the characteristics of the device, such as its threshold voltage (Vth) and its conveyed conductance (gm).

Figure 1. DAHC injection involves impact ionization of carriers near

the drain area; source: Hitachi Semiconductor Reliability Handbook

Injected carriers that do not get trapped in the gate oxide become gate current. On the other hand, majority of the holes from the e-h pairs generated by impact ionization flow back to the substrate, comprising a large portion of the substrate's drift current. Excessive substrate current may therefore be an indication of hot carrier degradation. In gross cases, abnormally high substrate current can upset the balance of carrier flow and facilitate latch-up.

Channel hot electron (CHE) injection occurs when both the gate voltage and the drain voltage are significantly higher than the source voltage, with VG≈VD. Channel carriers that travel from the source to the drain are sometimes driven towards the gate oxide even before they reach the drain because of the high gate voltage.

Figure 2. CHE injection involves propelling of carriers in the

channel toward the oxide even before they reach the drain area;

source: Hitachi Semiconductor Reliability Handbook

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