The Metal Insulator Semiconductor FET (MISFET)
One of the most widely used electronic devices, particularly in digital integrated circuits, is the metal-insulator-semiconductor (MIS) transistor. In this device the channel current is controlled by a voltage applied at a gate electrode that is isolated from the channel by an insulator.
Basic Operation & fabrication:
- The basic MOS transistor is illustrated in Fig.a for the case of an enhancement-mode n-channel device formed on a p-type Si substrate.
- The n source and drain regions are diffused or implanted into a relatively lightly doped p-type substrate, and a thin oxide layer separates the conducting gate from the Si surface.
- No current flows from drain to source without a conducting n channel between them. This can be understood clearly by looking at the band diagram of the MOSFET in equilibrium along the channel (Fig.a).
- The Fermi level is flat in equilibrium. The conduction band is close to the Fermi level in the n source/drain, while the valence band is closer to the Fermi level in the p-type material.
- Hence, there is a potential barrier for an electron to go from the source to the drain, corresponding to the built-in potential of the back-to-back p-n junctions between the source and drain.
- When a positive voltage is applied to the gate relative to the substrate (which is connected to the source in this case), positive charges are in effect deposited on the gate metal.
- In response, negative charges are induced in the underlying Si, by the formation of a depletion region and a thin surface region containing mobile electrons.
- These induced electrons form the channel of the FET, and allow current to flow from drain to source.
- As fig (b) suggests, the effect of the gate voltage is to vary the conductance of this induced channel for low drain-to-source voltage, analogous to the JFET case.
- Since electrons are electrostatically induced in the p-type channel region, the channel becomes less p-type, and therefore the valence band moves down, farther away from the Fermi level.
- This obviously reduces the barrier for electrons between the source, the channel, and the drain. If the barrier is reduced sufficiently by applying a gate voltage in excess of what is known as the threshold voltage, VT, there is significant current flow from the source to the drain.
- Thus, one view of a MOSFET is that it is a gate-controlled potential barrier. It is very important to have high-quality, low-leakage p-n junctions in order to ensure a low off-state leakage in the MOSFET.
For a given value of VG there will be some drain voltage VD for which the current becomes saturated, after which it remains essentially constant.