# questions on ON Resistance

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#### airboss

##### Member level 3
Hi,

I have a few questions about ON resistance.

1) "MOSFET can serve as a switch; it can be on when it carries zero current." This statement somehow doesn't make sense to me. Ron is the equivalent resistance across D and S so if I_DS is zero, doesn't it mean infinite resistance, or an open ckt?

Is that statement measurable? I mean, how can I set up a test bench for the above statement and measure the ON resistance?

2) I need to figure out the ON resistance of an NFET with gate voltage and a passing voltage (Vref) given. So my coworker tells me, put a DC source (=Vref) at S and an AC source (AC magnitude =1, AC phase =0, DC voltage Vref) at D. Run the AC simulation and the ON resistance is the reciprocal of the drain current at DC.

Before running simulation, I don't expect to see any current at DC because I think as DC, VD=VS. However, after the AC sim, the AC voltage drop across D and S at 0Hz is 1V and there IS current flowing.

So my question is, intuitively, what should I expect to see the VDS at 0Hz?

#### timof

A switch can be in one of two states - ON (low resistance) or OFF (high resistance).

When the gate voltage on a MOSFET is zero, its drain-source resistance is high, and its drain-source current is low - MOSFET in the OFF state.

When gate voltage is high, the resistance of the MOSFET (On-resistance, Rdson) is low, and, if there is non-zero drain-source voltage, there is a high drain current. Thus, MOSFET is in ON state.

Zero source-drain current tells nothing about resistance. Resistance can have any value if voltage is also zero. If voltage is non-zero for zero current - then the resistance is high.

#### jecyhale

you can get the Ron wave by sweep the Vgs.

As a switch, when it is on, the current is zero. but as transmission gate, when it is on, the Ron does not change

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