Can you verify these statements?

[KingNothing]

1. In a MOSFET, increased channel width acts increases the gate capacitance.
2. In a MOSFET, increased channel length decreases gate capacitance.
3. In a MOSFET, an increase in temperature will cause an increase in the current flow.
4. In a BJT, an increase in temperature will cause a decrease in current flow.

 

[phyzguy]

Why do you think increasing the channel length of a MOSFET would decrease the gate capacitance?

 

[carlgrace]

1. In a MOSFET, increased channel width acts increases the gate capacitance.
2. In a MOSFET, increased channel length decreases gate capacitance.
3. In a MOSFET, an increase in temperature will cause an increase in the current flow.
4. In a BJT, an increase in temperature will cause a decrease in current flow.

1. Yes
2. No. Increasing channel length increases gate area just like increasing channel width.
3. Yes.
4. No. VBE declines with temp so current increases for a given bias point.

 

[KingNothing]

2. No. Increasing channel length increases gate area just like increasing channel width.
4. No. VBE declines with temp so current increases for a given bias point.

Thanks carl! Could you elaborate on what effect different channel lengths have?

 

[carlgrace]

Thanks carl! Could you elaborate on what effect different channel lengths have?

No prob. The gate capacitance is proportional to the product of the length and width. The area is what makes the gate cap. So, as gate far as capacitance is concerned, length has exactly the same effect as width. Different channel lengths affect the drain current and gm of the device (the key term is W/L).

 

[KingNothing]

So, for low-power high-frequency applications, one would set the W/L ratio as appropriate, then minimize them both. Correct?

Does connecting two transistors of different channel lengths produce any interesting effects?

 

[carlgrace]

So, for low-power high-frequency applications, one would set the W/L ratio as appropriate, then minimize them both. Correct?

Does connecting two transistors of different channel lengths produce any interesting effects?

That's true in general. Sometimes other considerations come into play, though, such as noise or output impedance, but generally you're right.

As for the connection of two devices, it depends on how they are connected. People vary channel lengths for different reasons all the time.