What Is MOSFET? MOSFET Structure, Types and Applications

Functioning as efficient voltage-controlled switches, MOSFETs enable everything from microchips to power conversion with unparalleled scalability, speed and energy efficiency. Learn the basics of MOSFET.

The Metal-Oxide Semiconductor Field Effect Transistor, better known as the MOSFET (pronounced 'moss-fett'), lies at the core of most modern electronic devices. Functioning as efficient voltage-controlled switches, MOSFETs enable everything from microchips to power conversion with unparalleled scalability, speed and energy efficiency. Their widespread use across industries makes understanding MOSFETs' principles as fundamental today as mastering electricity itself.

Structure of a MOSFET

A MOSFET is comprised of four main components packaged into a tiny yet mighty transistor. It features a Gate electrode separated from a semiconducting Channel region by an ultra-thin Insulating Layer of either silicon dioxide or a high-K metal oxide. The conductive Source and Drain terminals flank the Channel. Varying the voltage applied between the Gate and underlying Channel modulates the latter's ability to conduct current between the external Source and Drain connections.

Basic MOSFET Structure

From the Internet - Basic MOSFET Structure

● Gate - This terminal provides the control voltage that regulates whether the channel is allowed to conduct current or not. Applying a certain voltage potential to the gate compared to other terminals either forms or depletes the channel.

● Source - The source acts as the input terminal where the majority of charge carriers enter the channel region from the external circuit. In NMOS, the source is made of N+ doped semiconductor material.

● Drain - Located on the opposite side of the channel from the source. It serves as the output terminal where charge carriers exit the channel after conduction. Constructed using the same doping type (N+ or P+) as the source.

● Body/Substrate - This forms the underlying bulk semiconductor on which the other layers are built. Usually doped in opposite polarity (P-type or N-type) compared to source and drain. It acts as a sort of back gate for the conducting channel underneath the insulated gate oxide.

By independently contacting these four terminals, MOSFETs essentially behave as solid-state switches capable of easily modulating the flow of electrical charges based on the voltage signal applied between the gate and other terminals.

Careful semiconductor doping creates either N-type or P-type materials favoring negatively- or positively-charged charge carriers respectively. P-type utilizes electron vacancies called holes whereas N-type relies on loose valence electrons. Combining the different materials gives rise to distinct MOSFET behaviors.

Operating Principles

Depending on Gate voltage relative to the underlying Bulk/Substrate connection determines whether the majority of carriers accumulate at or near the Interface to form a low-resistance Channel - or invert their presence generating a conducting channel of opposite-type carriers.

For N-channel MOSFETs, applying a higher positive Gate voltage compared to both Source and Bulk produces accumulation or inversion at the Interface by electrostatically attracting electrons from the Bulk towards the Interface. This modulates the Channel's resistance permitting Drain-Source current to flow, switching the transistor ON.

Conversely, a negative or zero Gate voltage depletes the majority of carriers from the Interface, pinching off the Channel and switching the MOSFET fully OFF in its non-conducting state. Continuous electronic flow thus relies upon precise, rapid digital switching of the Gate voltage.

Types of MOSFETs

The basic design comes in two polarities optimized for specific complementary logic roles: P-channel MOSFETs (PMOS) and N-channel MOSFETs (NMOS).

P-channel MOSFET (PMOS)

As the name suggests, PMOS transistors utilize p-type semiconductor materials for their channels and source/drain regions. The bulk connection is made to the p-type substrate. When a positive voltage is applied to the gate relative to the source, it attracts electrons from the substrate to accumulate at the channel interface, enabling current flow. PMOS devices operate with gate-source voltage (VGS) and drain-source voltage (VDS) polarities reversed compared to NMOS.

PMOS transistors are commonly employed as load devices to complement NMOS switches in digital CMOS logic families. When fabricated onto p-type semiconductor wafers, the channel and source/drain regions are formed through localized p-type doping. With a p-channel connecting the source and drain, applying a positive gate bias attracts holes to accumulate at the channel interface and render it conducting.

PMOS devices also see analog uses exploiting their current handling capabilities. For example, they can serve as pass transistors selectively allowing current to flow into a node when their gates are enabled. By virtue of operating with reverse gate voltage polarity and current direction compared to NMOS, PMOS devices provide the indispensable complement allowing CMOS logic gates and circuits to approach theoretical zero static power consumption.

N-channel MOSFET (NMOS)

By far the most commonly used type, NMOS features n-type channels and source/drain junctions within a p-type substrate. Applying a positively biased gate voltage pulls electrons from the n-type channel to accumulate and form a conducting path between the source and drain. NMOS handles have become standardized as the default logic building block.

Almost universally adopted as the primary digital switch, NMOS transistors feature n-type source/drain junctions embedded within a p-type silicon substrate. With no gate voltage applied, the n-channel presents a high-resistance state as carriers are depleted near the silicon-silicon dioxide interface. However, positively biasing the gate attracts electrons to accumulate and form a conducting channel between the source and drain.

NMOS is also commonly employed as a common-source amplifier stage. By establishing current gain through a load resistor or PMOS pair, the gate-source voltage inputs can then control source voltages or currents amplified at the output drain terminal. Through refinement and scaling over decades, NMOS digital switching behavior has become thoroughly optimized and standardized as ubiquitous logic gates and circuits.

NMOS vs PMOS

Depletion-load MOSFET

These enhancement-mode devices have a small channel remaining even when the gate is unbiased. They see applications in non-critical bias circuits owing to minimal channel resistance.

Enhancement-mode MOSFET

Without a gate bias, enhancement MOSFETs have their channel fully "pinned off", displaying high resistance. Only once a threshold voltage is exceeded does a conducting channel form. Digital logic circuits universally employ enhancement-mode MOSFETs due to their reliable digital switching characteristics.

Dual-gate MOSFET

A specialized architecture featuring independent gate contacts for channel formation and electron flow modulation. Dual-gating affords additional control axes improving analog circuit functions such as power amplification. Independent-gate or depletion-load devices offer differentiation for tailored applications through channel pre-configuration abilities leveraging multiple electrodes. Varied structures fruitfully diversify electronic designs.

MOSFETs also differentiate as either Depletion or Enhancement mode based upon Channel type in their natural ungated state. Further sub-categories include CMOS (Complementary Metal-Oxide Semiconductor) combining both types, dual-gate and even floating-gate designs enabling non-volatile programmable behaviors for memory and logic applications.

Applications of MOSFET

MOSFET versatility underpins microchips' proliferation. Their scalable ON/OFF switching fulfills the roles of digital logic gates, transistors and circuits providing computing's foundation. Analog signal amplification and control also capitalize on the precision current modulation. Further domains include:

● Power electronics: MOSFETs regulate electricity between alternating and direct currents for charging stations, power tools, motor drives and more through efficient high-frequency switching conversion.

● Memory: Floating-gate capabilities realize non-volatile storage technologies like EPROM, EEPROM, flash memory chips inside computers, and removable media.

● Sensors: ISFET (Ion Sensitive Field Effect Transistor) chemical detection finds uses in biomedical analytics and environmental monitoring.

● Displays: LCD and OLED screen technologies closely integrate arrays of million pixel-scale MOSFETs driving individual color elements.

Truly their reach spans virtually all electronics by merit of unparalleled scalability, energy efficiency and fabrication commonality with standard integrated circuit manufacturing. MOSFETs perfectly exemplify the maxim 'form follows function' through adaptation to diverse workload demands.

Advantages over other Transistors

Early transistor technologies like BJT (Bipolar Junction Transistor) showed limitations surmounted by MOSFET refinements:

● Higher packing density: MOSFETs scale to far smaller dimensions enabled by simpler design relying on only one junction barrier rather than BJT's two.

● Lower power consumption: Absent base current needs render MOSFET switching far more energy efficient, and critical for battery longevity in handheld/embedded applications.

● Ease of manufacture: Co-locating billions of MOSFETs onto unitary silicon slices leverages economies of scale making integrated circuits affordable.

● CMOS design: Pairing complementary NMOS and PMOS transistors realizes optimum power efficiency through minimal power dissipation during switching transitions.

Difference Between MOSFET and BJT

MOSFET physical traits and fabrication commonalities with logic circuits thus revolutionized the electronics revolution establishing them as supreme building blocks. Further scaling potentials remain promissory into emerging nano-regimes.

In closing, the ubiquitous humble MOSFET silicon transistor epitomizes minimalist brilliance through elegantly converting voltage signals into the current flow. Their deterministic gate-controlled switching behaviors underpin digital information encoding across all computing and embedded technologies. Additional innovation sees MOSFETs continuously enable more advanced functions from artificial intelligence to renewable energy conversion. No single component has influenced modern living more than this basic field effect transistor - the cornerstone enabling our digital world.