Metal-Oxide-Semiconductor Structure

A traditional metal-oxide-semiconductor abbreviated as MOS structure is acquired by growing a layer of silicon dioxide (SiO₂) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is typically used). Since the silicon dioxide is a dielectric material, its structure is equal to a planar capacitor, along with one of the electrodes replaced by a semiconductor.

While a voltage is applied across a MOS structure, it changes the distribution of charges in the semiconductor. If we refer a P-type semiconductor (along with N_A the density of acceptors, p the density of holes; p = N_A in neutral bulk), a positive (+ive) voltage, V_GB, from gate to body (see figure) forms a depletion layer by forcing the positively charged holes away from the gate-insulator or semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ion. If V_GB is sufficiently high, a high concentration of negative charge carriers forms in an inversion layer situated in a thin layer next to the interface in between the semiconductor and the insulator. Not like the MOSFET, in which the inversion layer electrons are supplied fast from the source or drain electrodes, in the MOS capacitor they are generated much more slowly by thermal generation by carrier generation and recombination centers in the depletion region.

                                      Figure: MOSFET structure and channel formation

Usually, the gate voltage at which the volume density of electrons in the inversion layer is similar as the volume density of holes in the body is called the threshold voltage. This structure along with p-type body is the basis of the N-type MOSFET that needs the addition of an N-type source and drain regions.