Operation of MOSFET

A narrow, surface inversion layer of electrons forms at large enough positive gate voltages. The horizontal dashed line indicates the Fermi level, the energy of levels (should they exist) that are half-occupied at the selected temperature.

The control of the channel by the gate is similar to the formation of an inversion layer in the MOS capacitor, which is only a two-terminal device (gate and body contacts). The case of a p-type semiconductor body in which mobile holes are introduced in the valence band by introducing acceptor impurities is described below. The acceptors suck electrons out of the valence band, becoming fixed negative ions, and leaving electron vacancies in the valence band that behave as positively charge mobile holes.

The formation of this layer is understood by examining the behavior of the energy band edges under an applied field. The left-hand panels of the figure depict the lowest energy level of the conduction band of energies and the highest energy level of the valence band of energies (separated by the forbidden gap with no available energy levels) as a function of depth into the semiconductor body. In the figure's top panel, an applied voltage bends these band edges (left). This bending causes the valence band to become filled with electrons, so no holes are present. On the upper right, the figure shows the charge inducing this bending (Q) is balanced by a layer of negative acceptor-ion charge –Q_A in this depletion region where there are no holes. This depletion region of negative acceptor ions widens until neutrality is reached Q−Q_A = 0. In the bottom panel, a larger applied voltage further depletes holes from the surface but the conduction band becomes low enough in energy to populate with electrons near the surface, forming an inversion layer. The charge inducing the inversion layer is now balanced both by the inversion layer charge –Q_n and by the depletion layer charge –Q_A, so now Q–Q_n−Q_A = 0. Once the applied voltage is large enough to begin formation of the inversion layer, the charge balance is dominated by this layer, and the depletion region no longer expands significantly.

The electrons in the surface channel are mobile and form a conducting surface layer atop the insulating layer of fixed, immobile acceptor ions in the depletion region. The source and drain contacts on the body surface become connected by this conducting surface layer, so the formation of the inversion layer allows current to flow from the source to the drain. By contrast, when the conducting surface layer is not present, no conduction occurs, even when the surface layer is not depleted and holes are present. The contacts cannot conduct using holes because they are n-type semiconductor regions, and form pn-diode junctions with the body.

Thus, the source and drain are not electrically connected for voltages between zero and the threshold voltage for inversion layer formation. But once threshold voltage is exceeded, connection is established. The MOSFET constitutes an electrically controlled switch. The quantitative current-voltage behavior of the modern MOSFET is described by complex computer models.

Two device types: n- and p-channel

The device above uses a p-type body and an electron inversion layer, or n-channel. In CMOS circuits these n-channel MOSFETs are combined with p-channel MOSFETs that use an n-type body and a hole inversion layer. These CMOS circuits consume low power as only the active devices in a sub-circuit (either the n-channel or the p-channel devices) are operational at a given time, and the complementary devices are "off".