Subthreshold slope in a MOSFET

Subthreshold region of MOSFET operation refers to the weak inversion regime where off-current is not ideally zero. Under weak inversion, current flow is dominated by the diffusion mechanism due to the lateral gradient of carrier concentration. Further note that there is no lateral electric field in the channel region and hence the channel surface potential is almost constant along the length of the channel. Drain current which now represents a leakage current during off-state is exponentially proportional to the gate-source voltage, V_{GS}.

I_{D}=I_0 exp(V_{GS}/nV_{T})

Subthreshold swing is the change/swing in gate voltage \del V_{GS} in order to the change the drain current by a factor of 10 or simply by one decade. This is normally written as

S = ln (10) n V_T = ln (10) n (kT/q)

where subthreshold factor, n can be written as

n = 1 + (C_B/C_{ox})

The lowest possible value of n can be 1 provided that C_{ox} is infinity with a perfect gate insulator.

Thus the value of S can be

S = 60 mV/decade

Then the subthreshold slope is basically the reciprocal of the subthreshold swing, S and that is 1/S.