MOSFET
Which generally describes the relationship between a MOSFET's gate length (the length of the gate electrode on the mask or after patterning) and its effective channel length?
- AThe effective channel length is shorter than the gate length by the amount the source and drain extend laterally under the gate
- BThe effective channel length is always longer than the gate length
- CThe effective channel length always equals the gate length exactly
Show answer and explanation
Answer: A. The effective channel length is shorter than the gate length by the amount the source and drain extend laterally under the gate
Source and drain dopants spread slightly sideways under the gate electrode during implantation and annealing. The channel length through which current actually flows (the effective channel length) is therefore shorter than the gate electrode by the overlap on both sides. The overlap regions also form gate-to-source/drain overlap capacitance.
- A. Correct. It is shorter by the lateral extension (overlap).
- B. Dopants extend under the gate, so the effective length gets shorter, not longer.
- C. Because of lateral diffusion, they generally do not match.