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# Why BJT has higher Gm than CMOS?

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#### lionel_lei

##### Newbie level 3
Conceptually, why BJT has higher transconductance than CMOS? No equation please. Can anyone give an intuitive explanation? Thanks

-lionel

The transconductance of bipolar transistor is Gm=Ic/VT=Ic/26mv, VT is thermal voltage.
The transconductance of MOS transistor is Gm=2Id/(Vgs-Vt)=2Id/100mv, assumed (Vgs-Vt)min=100mv when the nmos transistor is in the saturation region.
From the above two equation, it can be seen "when the Ic=Id=1mA, the Gm(bioplar)=0.038A/V and the Gm(NMOS)=0.02A/V", so the bipolar has higher Gm than CMOS.

### lionel_lei

Points: 2
for bjt, Ic~Vbe exponential
for cmos, Id~Vgs square

### lionel_lei

Points: 2
Let me start with the difinition of transconductance, the variation of the output current with respect to the input voltage.

CMOS conducts the current through its majority carriers and thus is quite sensitive to the input voltage. In the mean time, BJTs conduct the electricity using both electrons and holes. So if the input voltage changes, first some of the moved charges should return back to their previous position and it's only then that we will see the effect of the input change.

All conductors with this property exhibit the behaviour. As an example thyristors also have this intrinsic property. (The time needed for the carriers to return back to the appropriate position is called holding time)

Hope that this helps

Cheers

### lionel_lei

Points: 2
cmos always according the square law in saturation.
but bipolar always with the e power law.

### lionel_lei

Points: 2
well basically it is a architectural differnece between the two of them

Points: 2