# MOSFET saturation current

#### smrams28

Joined Mar 29, 2013
13
Hi. Im a first year engineering student and was having trouble on a problem. I am probably overthinking it.

Q)The n-channel MOSFET threshold voltage is +2 V. The gate voltage is +4V. The drain voltage is +5V. Is the MOSFET in the current saturation mode?

I think VGS-VT = VDS for it to be in saturation but this one is not. So is it not in saturation?

#### tshuck

Joined Oct 18, 2012
3,534
I think VGS-VT = VDS for it to be in saturation but this one is not. So is it not in saturation?
Nope. Saturation is:
$$provided\ that\ \ V_{GS} \ > \ V_{TH},\ \ then\ V_{DS}\ >\ V_{GS} - V_{TH}$$

See here.

• smrams28 and screen1988

#### smrams28

Joined Mar 29, 2013
13

#### Ron H

Joined Apr 14, 2005
7,014
Nope. Saturation is:
$$provided\ that\ \ V_{GS} \ > \ V_{TH},\ \ then\ V_{DS}\ >\ V_{GS} - V_{TH}$$

See here.
What?
4V>2V
5V>(4V-2V)

That looks like saturation to me. • screen1988

#### tshuck

Joined Oct 18, 2012
3,534
I think VGS-VT = VDS for it to be in saturation
Nope. Saturation is:
$$provided\ that\ \ V_{GS} \ > \ V_{TH},\ \ then\ V_{DS}\ >\ V_{GS} - V_{TH}$$
What?
4V>2V
5V>(4V-2V)

That looks like saturation to me. Wasn't saying that it wasn't in saturation, but that the equation for whether or not it is is wrong. Your red line is the equation in the OP.

• screen1988

#### Ron H

Joined Apr 14, 2005
7,014
Wasn't saying that it wasn't in saturation, but that the equation for whether or not it is is wrong. Your red line is the equation in the OP.
OK, I see. The OP thought the expression was an equation, rather than an inequality.
I saw that, but thought it was just sloppy writing by the OP. • screen1988

#### WBahn

Joined Mar 31, 2012
26,398
Notice that the OP gave the threshold voltage, the gate voltage, and the drain voltage. They didn't give any indication of the source voltage. Perhaps they mean the other voltages as voltages relative to the source. Perhaps the source is tied to ground. But perhaps niether of these is the case.

• screen1988