# interview question needed to help out

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#### ethan

##### Member level 4
Dear all,

The question is

"What is the physical meaning of gm when MOSFET operating in saturation?"

I answered "Transcoductance gm reflects the change in the drain current divided by the change in the gate-source voltage. It represents the sensitivity of the device."

But it seems not completed from the interview feedback.

Any help on this? from intuitively? Appreciate!

#### mickey_melomane

##### Member level 2
The gm in a saturation mode implies that my transfer conductance is not a function of the "change" in drain current divided by "change" in gate source voltage, but the "direct" function of the actual DC drain current flowing in it... i.e. a small signal parameter is a function of the large signal parameter... gjm=sqrt(2*beta*drain current)

#### seen25

##### Newbie level 5
dear mickey , actually what you say is equal to the ethan's answer, and both of you are correct,
B: Beta
MOS in SAT:

(1) Id = B/2 * (Vgs-Vt)^2

(2) gm= del Id /del Vgs

using 1 in 2 gives

gm= 2*B/2 * (Vgs-Vt) (Eq 3)

from (1), Vgs= sqrt(2Id/B) + Vt (Eq 4)

using Eq 4 in Eq 3 again gives,

gm= B * sqrt(2*Id/B) = sqrt (2*B*Id)

#### thunderr

##### Banned
it means that the transistor is current source ie.the current is constant with changing voltage

#### n1cm0c

##### Full Member level 2
What did they expect as an answer ? That gm represents the modulation of carrier density in the transistor channel by the potential difference across the gate oxide, minus the flatband voltage and trapped charge?

Your answer seems essentially correct to me. Are you sure that they are not testing your reaction to being contradicted when you believe yourself correct?
Like, if you become mad and throw the chair by the window?

#### wallycharlo

##### Newbie level 6
perhaps they were just looking for the easy description ... ie in saturation:

gm = 2*ID/Vod

ID: Drain current
Vod : overdrive (equivalent of Vgs-Vth in older technologies)

##### Full Member level 4
Or maybe they wanted to hear that gm is the slope of the ID vs Vgs curve