Depletion and enhancement modes

Depletion-type FETs under typical voltages: JFET, poly-silicon MOSFET, double-gate MOSFET, metal-gate MOSFET, MESFET.
  Depletion
  Electrons
  Holes
  Metal
  Insulator
Top: source, bottom: drain, left: gate, right: bulk. Voltages that lead to channel formation are not shown.

In field-effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an on state or an off state at zero gate–source voltage.

Enhancement-mode MOSFETs (metal–oxide–semiconductor FETs) are the common switching elements in most integrated circuits. These devices are off at zero gate–source voltage. NMOS can be turned on by pulling the gate voltage higher than the source voltage, PMOS can be turned on by pulling the gate voltage lower than the source voltage. In most circuits, this means pulling an enhancement-mode MOSFET's gate voltage towards its drain voltage turns it on.

In a depletion-mode MOSFET, the device is normally on at zero gate–source voltage. Such devices are used as load "resistors" in logic circuits (in depletion-load NMOS logic, for example). For N-type depletion-load devices, the threshold voltage might be about −3 V, so it could be turned off by pulling the gate 3 V negative (the drain, by comparison, is more positive than the source in NMOS). In PMOS, the polarities are reversed.

The mode can be determined by the sign of the threshold voltage (gate voltage relative to source voltage at the point where an inversion layer just forms in the channel): for an N-type FET, enhancement-mode devices have positive thresholds, and depletion-mode devices have negative thresholds; for a P-type FET, enhancement-mode have negative, and depletion-mode have positive.

Key voltages (with +3 V or −3 V threshold voltage)
NMOS PMOS
Enhancement-mode Vd > Vs (typ)
on: VgVs + 3V
off: VgVs
Vd < Vs (typ)
on: VgVs − 3V
off: VgVs
Depletion-mode Vd > Vs (typ)
on: VgVs
off: VgVs − 3V
Vd < Vs (typ)
on: VgVs
off: VgVs + 3V

Junction field-effect transistors (JFETs) are depletion-mode, since the gate junction would forward bias if the gate were taken more than a little from source toward drain voltage. Such devices are used in gallium arsenide and germanium chips, where it is difficult to make an oxide insulator.