I came across instances where NMOS was used as a capacitor in analog circuits. This is done by shorting the drain and source. The drain/source acts as one terminal of the capacitor while gate acts as the other terminal, as shown below. Here are some observations about its working:
1) This acts like a capacitor if drain/source terminal is connected to ground. If drain/source is connected to VDD, its as good as open (0 capacitance) 2) To get a good capacitance value when drain/source is grounded, we have to use a large length and small width for the NMOS
Now, I am trying to reason out these observations.
1) I guess that when drain/source is grounded, the gate oxide acts like the capacitor with n-doped regions providing the charges. But what happens when drain/source is set to VDD. Why can't it still act like a capacitor between gate and the body?
2) How to reason out the need for large length and small width? I guess it is to reduce the effect of overlap caps between the gate and drain/source. But I am not able to arrive at a good explanation.