Metal-Oxide Semiconductor FET (MOSFET) Information
Metal oxide semiconductor field-effect transistors (MOSFET) are commonly used in microprocessors and related technologies for amplifying or switching signals.
N- and p-type MOSFET symbols, showing the terminals described below.
Image credit: CircuitManiac
While MOSFETs technically have four terminals — source, gate, drain, and body (or S,G,D, and B) — the body terminal is typically connected to the source, effectively making them three terminal devices like other field-effect transistors (FET). As their name implies, the MOSFET design improves upon the basic FET design by adding a layer of silicon dioxide (SiO2) to the basic substrate; this layer is represented in orange in the image below.
A typical MOSFET. Image credit: DoITPoMS
MOSFETs, like all FETs, are similar to bipolar junction transistors (BJT) with a few major differences:
- FETs are voltage-controlled as opposed to current-controlled BJTs
- FETs have higher input impedance, while BJTs have higher gain
- FETs are less sensitive to variable temperatures and are better suited for IC use
- FETs are more sensitive to static than BJTs
- FETs consume less power than BJTs
- FETs are unipolar, meaning that only electrons produce the current. BJTs, as their name implies, are bipolar, meaning that electronics as well as holes produce the current.
Like all transistors, MOSFETs are commonly-used components that assist in forming the building blocks of all modern electronic devices and systems.
The video below provides a good general overview of transistors, specifically MOSFETs, and their use.
Video credit: Afrotechmods
Semiconductor current conduction is facilitated by either free electrons or "holes"; both terms together can be considered as types of charge carriers. The term "hole" is used to describe the theoretical lack of an electron where one could exist in an atomic structure. Both MOSFET types described below can be of p- or n-type (also known as p-channel and n-channel), but n-channel devices are the most prevalent.
MOSFETs can operate in two mode types: depletion mode (D-MOSFET) or enhancement mode (E-MOSFET).
A comparison of MOSFET types. Image credit: Prentice-Hall
In depletion mode, the negative gate-source voltage forces free electrons away from the gate, which forms a depletion layer that cuts into the channel, as shown below. In enhancement mode, the positive gate-source voltage attracts electrons from the substrate to the channel while driving holes away from the channel. This process results in a wider channel and results in a smaller steady current and larger drain current, compared to the larger current of depletion mode. MOSFETs can also operate in a zero bias state in which the gate is shorted to the source terminal, meaning that the transistor's drain current is equal to its steady state current. All three of these states, as well as a graph describing gate voltage's effect on the transistor's steady state current, are shown below.
Image credit: Prentice-Hall
Depletion MOSFETs have a gate channel — created by doping impurities into the p-type substrate — between the drain and the source terminals, both of which are connected to the n-type materials, which in turn lie on a p-type substrate. A D-MOSFET can operate in both depletion and enhancement modes, while an E-MOSFET, described below, can only operate in enhancement mode. As shown above, a D-MOSFET's primary difference from an E-type is that its drain and source terminals are connected by an n-type channel.
E-MOSFETs lack a built-in channel. Instead the drain and source terminals are created by doping the source and drain regions with n-type material. However, an n-type channel will form between the terminals if a positive voltage is applied between the gate and the source. E-MOSFETs are sometimes regarded as the most important type of MOSFET because of their ease of manufacture and outstanding switching and amplifying characteristics. P- and n-type E-MOSFETs can be combined to produce Complementary Metal Oxide Semiconductor (CMOS) devices.
When discriminating between different MOSFET products, two important specifications to consider include drain saturation current and gate-source cutoff voltage.
Drain saturation current (IDSS) is a measure of drain current saturation, a condition that occurs when the drain-source voltage equals the gate-source voltage. When a MOSFET's drain current reaches a maximum value it remains there despite any increases in the drain-source voltage; this extra voltage is accommodated by a depletion layer located at the drain end of the gate. This condition is known as drain current saturation, and is represented by IDSS as a maximum current value.
Gate-source cutoff voltage (VGS(Off)) represents the value of the gate-source voltage (VGS) which results in a drain current (ID) value of close to zero.