Phenomenon - Fet operation:

For either improvement- or depletion-mode devices, at drain-to-source voltages very much less than gate-to-source voltages, changing the gate voltage will change the channel resistance, drain current will be proportional to drain voltage (referenced to source voltage). In this type of mode the FET operates such as a variable resistor and the FET is said to be operating in a linear modeor ohmic mode.

If drain-to-source voltage is gets increased, this creates a important asymmetrical change in the shape of the channel because of a gradient of voltage potential from source to drain. The shape of the inversion region turns "pinched-off" near the drain end of the channel. If drain-to-source voltage is raised further, the pinch-off point of the channel starts to move away from the drain in the direction of the source. The FET is supposed to be in saturation mode; a number of authors refer to it as active mode, for a better analogy along with bipolar transistor operating regions. The saturation mode, or the region in between the ohmic and saturation, is employed while amplification is required. The in-between region is occasionally considered to be part of the ohmic or linear region, even in which drain current is not approximately linear with drain voltage.

Although the conductive channel made by gate-to-source voltage no longer connects source to drain throughout saturation mode, from flowing the carriers are not blocked. Considering once again an n-channel device, a depletion region available in the p-type body that surrounding the conductive channel and drain and source regions. The electrons that have the channel are free to move out of the channel by the depletion region if attracted to the drain through drain-to-source voltage. The carriers' depletion region is free and comprises a resistance identical to silicon. Any type of rise of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance because of the depletion region proportionally to the applied drain-to-source voltage. This proportional change results in the drain-to-source current to remain comparatively fixed independent of changes to the drain-to-source voltage and quite different the linear mode operation. So in saturation mode, the FET acts like a constant-current source rather than like a resistor and can be used most efficiently as a voltage amplifier. In this type of case, the gate-to-source voltage ascertains the level of constant current by the channel.