Hot Carrier Effect (HCE)

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When carriers (electrons or holes) gain high kinetic energy due to the presence of high electric field within a semiconductor device. Hot electrons are more probable than hot holes since they have higher mobility to begin with. Hot carriers get injected/ trapped in certain areas and cause undesirable device behavior and/or degradation thereby giving rise to Hot Carrier Effects.

In short, Hot Carrier Effect –> Carriers get lodged into the gate oxide —> Vt Variation, Leakage Currents

There are a few types depending upon the location of the hot carrier impact.

1) Drain Avalanche Hot Carrier:

1. High voltages applied to the drain, cause high fields to be generated near the drain, causing channel carriers to be accelerated into the drain depletion region.
2. The accelerated carriers collide with the Si atoms in the lattice creating electron-hole pairs some of which may cause further impact ionization. This leads to some carriers lodge into the gate oxide. Over a period of time this leads to variations in Vt.
3. Injected carriers that do not get trapped in gate oxide make up the gate current. The electron-hole pairs that go into the substrate constitute the substrate current.

2) Channel Hot Electron Injection:

• When both gate and drain voltage are high, carriers accelerated toward the drain impinge on on the gate oxide before reaching the drain due to the high gate voltage

3) Substrate Hot Electron Injection:

• When substrate bias is high (|Vb| >> 0), the carriers in the substrate are accelerated towards the channel, gain kinetic energy due to the surface field and get lodged into the gate oxide

4) Secondary Generated Hot Electron Injection:

• This is similar to (1) where there is secondary electron-hole generation due to impact ionization, this combined with substrate bias, causes the secondary carriers to accelerate towards the surface and hit the gate oxide.

Why Hot Carrier Effect is important?

1. Device degradation lessens the lifetime of the device – we want a long lifetime.
2. Contributes to leakage current – we want low power devices
3. Due to scaling of dimensions but not much reduction is operating voltages, HCE is becoming more relevant