| 8.2 Mobility Degradation |
| Mobility is important because the current in MOSFET depends upon mobility of charge carriers(holes and electrons). |
| We can describe this mobility degradation by two effects : |
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(i) Lateral Field Effect: In case of short channels, as the lateral field is increased, the channel mobility becomes field-dependent and eventually velocity saturation occurs (which was referred to in the previous lecture). This results in current saturation.. |
| (ii) Vertical Field Effect: As the vertical electric field also increases on shrinking the channel lengths, it results in scattering of carriers near the surface. Hence the surface mobility reduces (Also explained by the mobility dependence equation given below). |
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| Thus for short channels, we can see (in the figure 8.2) the mobility degradation which occurs due to velocity saturation and scattering of carriers. |
| Figure 8.2: Mobilty degradation graph |
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| 8.3 Subthreshold Current |
An effect that is exacerbated by short channel designs is the subthreshold current which arises from the fact that some electrons are induced in the channel even before strong inversion is established. For the low electron concentration (typically of subthreshold regime), we expect diffusion current (propotional to carrier gradients) to dominate over drift currents (propotional to carrier concentrations). For very short channel lengths, such carrier diffusion from source to drain can make it impossible to turn off the device below threshold. The subthreshold current is made worse by the DIBL effect(will be explained in later sections) which increases the injection of electrons from the source.
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