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The concept of threshold voltage is the onset of strong inversion of a MOSFET or MOS Capacitor. Assuming that the substrate voltage pins to 2ψb after the threshold onset, which is a good approximation specially for long channel devices, where ψb is the potential difference between the edges of the intrinsic and fermi levels of the substarte.
The value of Vth (Threshold voltage) is given by:
Vth=Vfb+2ψb+γ(2ψb)^1/2, where γ is a constant, Vfb is the flat-band voltage.
In other words, Vth is propotional to the doping cocentration, the more concentration the more the threshold voltage. The less the doping the less the threshold voltage.
For short devices, we have many other phenomena, the pinning of the substrate voltage occurs at a voltage more than 2ψb. But its the same concept.