🔥 0 ⚡ 0 XP
← Back to Path
1 / 4

TRANSFER CURVES

ID versus VGS, and the sideways shift

10⁻¹² 10⁻⁹ 10⁻⁶ 10⁻³ ΔVth fixed VGS VGS → log ID ↑ before after surface charge binds

Now hold VDS fixed and sweep the gate. Above threshold the current follows the square law. Below it, the current falls exponentially, which is a straight line on a log axis. When charge binds at the gate, the whole curve slides sideways by ΔVth.

Key idea A sensing event slides the curve sideways.

TRANSCONDUCTANCE

Gain, in one number

gm = ∂ID / ∂VGS

Transconductance is how much drain current changes per volt of gate change. In saturation gm = k·(VGS − Vth). With the numbers from before, that's 100 µA/V² × 10 × 0.5 V = 0.5 mS. A larger gm means a bigger current change for the same shift.

Higher gm turns a small voltage shift into a larger, easier-to-read current.

SUBTHRESHOLD

The most sensitive region

S ≥ 60 mV/decade

Below threshold the current changes exponentially. The subthreshold swing S = n·(kT/q)·ln 10 cannot be better than about 60 mV per decade at room temperature, since n ≥ 1. So every 60 mV of gate shift changes the current tenfold at best, which is why sensors love this region.

Small charge, small ΔVth, but a big change in current.

UNIT 7 STUDY COMPLETE

🔲

Ready for the Fab Challenge?

You've covered transconductance, subthreshold swing, and why sensing lives on the transfer curve.