# LtSPICE transconductance

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#### Decaf14

Joined Jul 14, 2018
1
Can anyone explain what the Kp MOSFET parameter is in LtSPICE? I am getting tripped up with default value and units. I have read the help page, and understand it should be in units of A/V^2, and it is the transconductance parameter. This is not helping me, because default values for standard industry MOSFETS seem unreasonable based on the known equation for MOSFET drain current and the standard values as per LtSPICE's help page.

I'm looking at the standard MOSFET equation (w.o channel width modulation): Id = 0.5 * k' * (W/L) * (Vgs - Vto)^2

For the equation, the units of k' are A/V^2. I expect the value of k' to be somewhere on the order of 1e-6.

LtSPICE's help page states that Kp should be around 2e-5 (the default value) which is close to what I expect in the equation. However, on nearly every MOSFET I can choose from the NMOS library, Kp is in the order of 10, not 10e-6. According to some tutorials, Kp is chosen as the forward transconductance from the datasheet, which has units of S or 1/ohm. This is not the same as A/V^2. I'm very confused by this discrepancy.

I'm performing a learning exercise where I'm supplying the MOSFET with constant current and sweeping the gate voltage. I'm calculating voltage drop across a diode from drain to source. I simply want to calculate the expected drain current for a given gate-source voltage. I'm struggling to determine what to input to the equation for k' to obtain theoretical drain current.

Any help would be greatly appreciated.

#### Bordodynov

Joined May 20, 2015
3,059
There is some confusion about the issue. For a specific transistor, the value of Beta = W / L * K is important. But sometimes they are by default 100 microns each. Then, keep the Beta value, take the Kequivalent = W / L * K, (W / L = 1).

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