MOSFET: Effective transverse field
Effective transverse field:
- In addition to this dependence of the channel mobility on gate bias or transverse electric field, there is also a strong dependence on drain bias or the longitudinal electric field.
- We know that the carrier drift velocity increases linearly with electric field (ohmic behavior) until the field reaches ξsat in other words, the mobility is constant up to ξsat.
- After this, the velocity saturates at vs, and it can no longer be described in terms of mobility.
These effects can be described as:
The maximum longitudinal electric field near the drain end of the channel is approximately given by the voltage drop along the pinch-off region (VD - VDsat.), divided by the length of the pinch-off region, ΔL:
From a two-dimensional solution of the Poisson equation near the drain end, one can show that the pinch-off region ΔL is approximately
where d is the gate oxide thickness and Xj is the source/drain junction depth. The factor of 3 is due to the ratio of the dielectric constant for Si to that of Si02.