Mosfet

The Metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of FET that consists of three layers: a metal top electrode (a conductor, called the gate), an oxide layer (working as an insulator separating the gate from the semiconductor layer), and a semiconductor layer (called the body). Its operation is based upon the modulation of the semiconductor conductivity by the electric field introduced in the body by the gate, the so-called field effect. This transistor was invented by Dawon Khang and Martin Atalla in 1960, at Bell Labs.

There are four contacts altogether: in addition to the gate and body contacts already described, there are two contacts atop the body at opposite sides of the gate called source and drain. Because the transistor is symmetrical, they can swap their functions. They do not permit current flow to the body in normal operation, as they form reverse biased diodes with the body. They do allow current between source and drain upon formation (by the gate) of a surface channel at the top surface of the body, next to the insulator. The channel conductivity depends upon the voltage difference between the gate and body (V_gb). The amount of current drawn in the channel depends upon the voltage drop across it, the drain to source voltage (V_ds). The channel strength also is affected by so-called "back gate bias", that is, by the body to source voltage (V_bs).

The modern MOSFET structure is shown in the figure (with a bow to artistic license). It is rather complex, as it is made to control a number of undesirable or parasitic effects that detract from its ideal behavior. Among these are unwanted capacitances between the gate and source and gate and drain, reduced by introduction of spacers at the sides of the gate. Another problem is high source-to-body and drain-to-body capacitance, which is reduced by raising the source and drain above the body, so the sidewall contact areas are reduced. These parasitic capacitances have to be charged and discharged between on and off conditions, and slow down device operation. Also undesired is penetration of the source and drain fields underneath the gate, which interferes with the channel formation. That effect is reduced by use of shallow junction extensions and screening halo implants, both designed to keep the source and drain fields close to their electrodes, and away from the gate.

For power applications a different geometry is used, notably asymmetric in drain and source geometry. Special attention is given to the drain, which is adapted to large drain voltages. The figure shows an example called the UMOSFET because of the U-shaped gate. This cross-section is is only one from a huge array of identical parallel fingers (the diagram is repeated to right and left with identical structures) all tied together in one device to handle large currents. As indicated by different colors in the figure, the source and drain are both n-type, and the body is p-type, or vice versa. The drain is specially doped to have a wide drift region, allowing reduction of the large drain voltage before it reaches the channel, and also allowing the current from the channel to spread out before reaching the drain contact, lowering the device resistance. The channel is present only when the gate voltage is large enough to turn the device "on", forming an inversion layer.