MOSFET (Meta-Oxide-Semiconductor Field-Effect Transistor)

Silicon and Germanium are two basic materials for semiconductor devices. When found it their pure crystalline form, they have the diamond-cubic perfectly regular lattice with four valence electrons per atom. This electrons interact with another electrons and are not free to move making these materials behave like insulators. When implanted with a small amount of certain impurities, both Germanium and Silicon can conduct electricity.

The impurities are of two basic types: donors and acceptors. The donors, created by implanting antimony, arsenic, phosphorus, etc. contribute excess electrons to the crystal, while the acceptors (gallium, indium, etc.) create electron deficiencies called holes. Under applied voltage, the holes behave as positive charges and move in the direction opposite to that of the electrons.

The semiconductors with excess electrons are called negative or n-type, while the acceptor-type semiconductors are called positive or p-type.

The Meta-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a four-terminal semiconductor device which is the basis for a large variety of digital integrated circuits (IC). The MOS transistor differs from the bipolar (junction) transistor (npn or pnp-type), which is used for signal amplification, in the following ways:

The current flow through MOSFET is controlled by an electric field rather then by a (base) current as in the bipolar transistor. This gives MOSFET a very high input resistance and thus requires very small input currents.
Unlike bipolar (junction) transistor, MOSFET is a unipolar transistor and relies on the majority carrier current only.
The MOS transistor is simpler to fabricate, but it is inherently slower than the bipolar transistor.

The MOSFET is, in fact, a voltage-controlled switch. It can amplify the signal as well, but is less efficient than the bipolar transistors.

The n-type MOSFET consists of a source and a drain made of conductive n-type semiconductor regions. A metal or poly-crystalline gate is separated from the semiconductor by the gate oxide layer. The next figure shows the n-type or n-channel MOSFET fabricated on a p-type Silicon substrate. n-type and p-type silicon is produced by implantation, as was discussed before.

When a positive voltage above the threshold level - the threshold voltage - is applied to the gate, it attracts electrons to the interface between the gate oxide and the substrate thus forming a narrow conductive channel, which connects the source and the drain. The transistor turns 'on' and current flows between source and drain. The flow of electrons through the channel (inversion layer) is controlled by the Gate voltage. There is no need in the current flow between the gate and the back contact to create the inversion layer, which thickness is on the order of 10 nm. When the gate is connected to the source or is negative with respect to the source, the resistance between the source and the drain is of the order of 10¹⁰ ohm making the channel disappear.

There are two types of MOSFET, which behavior is opposite to each other: n-type (shown above) and p-type, which uses p-type silicon for source and drain and s-type silicon as a substrate. Sometimes, these two types of MOS transistors are called NMOS and PMOS. The PMOS transistor operation is the same as for the NMOS transistor with the polarity of the gate voltage reversed.

When the gate voltage is higher than the threshold voltage, the source-drain connection acts like an open circuit, and conducts only when the gate voltage is more negative than the threshold voltage. Typical representations of the NMOS and PMOS transistors are shown in the next figure.