# Gate current of MOSFET

#### voyak

Joined Oct 23, 2020
8
Hi guys I'm currently working on my MOSFET project.

I've first constructed the small-signal model of this circuit and changed each value of the device so that I can see the tendency.

While doing so, I've faced some tendency that I cannot understand.

The circuit diagram and AC sweep results are attached.

When I increase Width(W) of M19, the current generated by IAC(I12) seems to be sucked up to the gate of M19 on high frequency.

The first result is when the Width of M19 is 1u. The current generated by the IAC(1A) is red line. It decreases on high frequency but not considerably.

The second result is when the Width of M19 is 100u. The red line starts to decrease dramatically and the green line, the current on gate of M19, increases at the same time.

It seems like as the Width of M19 increase, it starts to 'suck up' the current on lower frequency. And it also changes the pole location(The pole location of curve of current through load cap, C19 in schematic).

I may guess that this is because of some capacitance between gate of M18 and M19 but I have no idea how should I understand and explain this.

There are some high-frequency model of MOSFET on google, but there aren't any model for this kind.

Can you give me some advice to understand/explain it?

#### crutschow

Joined Mar 14, 2008
32,934
The gate capacitance of a MOSFET is significant and is proportional to the area of the gate.
And the AC impedance of a capacitor is inversely proportional to the capacitance and frequency.

#### voyak

Joined Oct 23, 2020
8
The gate capacitance of a MOSFET is significant and is proportional to the area of the gate.
And the AC impedance of a capacitor is inversely proportional to the capacitance and frequency.

That's true but in my simulation, it says the current is leaked to the gate of M19. That is, the current flows from gate of M18 to gate of M19 increases.

The gate capacitance is the capacitance between gate-source or gate-drain in 'a' transistor as long as I know.

But in this situation, the node I concern is one gate to another gate.

So I think the gate capacitance is not enough to explain the leaking current between gate to gate in this case.

#### Alec_t

Joined Sep 17, 2013
13,797
So I think the gate capacitance is not enough to explain the leaking current between gate to gate in this case.
Both gates have capacitance and if those are not at the same voltage there will be a current flow between them.

#### jpanhalt

Joined Jan 18, 2008
11,087
Are M18 and M19 the same mosfet model?