I would like the detailed answer please 2 What are the prope

I would like the detailed answer, please.

2. What are the properties of a good switch (i.e. MOSFET)? How can we design a fast/high speed MOSFET device? (hint: how shall we define channel length? Width, mobility etc.?) (Points 10).

Solution

The N-channel e-Mosfet operate using a positive voltage and has an extremely high input resistance making it possible to interface with nearly any logic gate.

The operation of the enhancement-mode MOSFET, or e-MOSFET, can best be described using its i-v characteristics curves shown below. When the input voltage, ( V_IN ) to the gate of the transistor is zero, the MOSFET conducts virtually no current and the output voltage ( V_OUT ) is equal to the supply voltage V_DD. So the MOSFET is “OFF” operating within its “cut-off” region.

Mosfet characteristics curves

The minimum ON-state gate voltage required to ensure that the MOSFET remains “ON” when carrying the selected drain current can be determined from the v-i transfer curves above. When V_IN is HIGH or equal to V_DD, the MOSFET Q-point moves to point A along the load line. The drain current I_D increases to its maximum value due to a reduction in the channel resistance. I_D becomes a constant value independent of V_DD, and is dependent only on V_GS. Therefore, the transistor behaves like a closed switch but the channel ON-resistance does not reduce fully to zero due to its R_DS(on) value, but gets very small.

Channel length

This in MOSFET is caused by the increase in depletion layer width at the drain as the drain voltage is increased. This leads to a shorter channel length and increased drain current.

Mobility

The surface mobility is important and corresponding to the values quoted.