History and Development:
– Basic principle patented by Julius Edgar Lilienfeld in 1925.
– Proposed structure resembling MOS transistor by Bell scientists.
– Mohamed Atalla and Dawon Kahng demonstrated modern MOS transistor in the 1960s.
– Initially considered slower than bipolar transistors.
– Advantages include ease of fabrication and application in integrated circuits.
Composition and Structure:
– Semiconductor choice is usually silicon.
– Some use silicon-germanium (SiGe) in channels.
– Gallium arsenide not suitable for MOSFETs due to interface issues.
– High-κ dielectric replaces silicon dioxide for gate insulator.
– Gate separated from channel by thin insulating layer.
Operation and Functionality:
– MOS structure created by growing silicon dioxide on silicon substrate.
– Voltage across MOS structure modifies charge distribution in semiconductor.
– Positive gate-body voltage creates inversion layer.
– Threshold voltage determines conductivity modulation.
– Basis of n-type MOSFET involves p-type body and n-type regions.
Modes of Operation and Characteristics:
– MOSFET operates in three modes: cutoff, subthreshold, weak-inversion.
– The transistor is off in cutoff mode.
– Subthreshold current flows due to thermal energy effects.
– Drain voltage dependence varies with channel length.
– Operational modes are determined by gate-to-source bias.
Symbolism and Circuit Representation:
– Various symbols are used to represent MOSFETs in circuit diagrams.
– The basic design includes a line for the channel, with the source and drain departing at right angles.
– MOSFET Symbols: Comparison of enhancement-mode and depletion-mode symbols.
– Channel Connection: NMOS channel in P-well or P-substrate.
– Bulk Connection: Bulk connected to the source in discrete devices.
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The term metal–insulator–semiconductor field-effect transistor (MISFET) is almost synonymous with MOSFET. Another near-synonym is insulated-gate field-effect transistor (IGFET).
The basic principle of the field-effect transistor was first patented by Julius Edgar Lilienfeld in 1925.
The main advantage of a MOSFET is that it requires almost no input current to control the load current, when compared to bipolar junction transistors (BJTs). In an enhancement mode MOSFET, voltage applied to the gate terminal increases the conductivity of the device. In depletion mode transistors, voltage applied at the gate reduces the conductivity.
The "metal" in the name MOSFET is sometimes a misnomer, because the gate material can be a layer of polysilicon (polycrystalline silicon). Similarly, "oxide" in the name can also be a misnomer, as different dielectric materials are used with the aim of obtaining strong channels with smaller applied voltages.
The MOSFET is by far the most common transistor in digital circuits, as billions may be included in a memory chip or microprocessor. Since MOSFETs can be made with either p-type or n-type semiconductors, complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS logic.
