Query: What determines the conductivity in a FET?
Context: In a FET, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage (VGS) is increased, the drain–source current (IDS) increases exponentially for VGS below threshold, and then at a roughly quadratic rate (IGS ∝ (VGS − VT)2) (where VT is the threshold voltage at which drain current begins) in the "space-charge-limited" region above threshold.