A Complete Guide to MOSFET Transistors

There are two primary types of transistors. The first is the Bipolar Junction Transistor (BJT), and the second is the Field Effect Transistor (FET). MOSFETs are a type of FET. BJTs are usually used for electrical currents of under one amp, while MOSFETs are typically used for higher-current applications.

Users can choose between MOSFETs featuring depletion or enhancement modes. The depletion mode operates in a similar way to the closed switch, with current flowing when the on voltage is applied. If a negative voltage is applied, then the current will stop. On the flip side, enhancement mode MOSFETs are the most commonly used type for modern-day applications.

The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a commonly used semiconductor in digital and analogue circuits and is also a useful power device. As the original compact transistor, it is suitable for a wide variety of electrical applications.

It is a core component of the integrated circuit, and it can be designed and fabricated in a single chip due to its compact size. It features four terminals, these being the Source (S), Gate (G), Drain (D), and Body (B). The body is typically connected to the source terminal so that the MOSFET functions as a field-effect transistor.

The Impact of MOSFETs on Technology

It has been argued that many of the 21st century’s technological developments wouldn’t have been possible without the MOSFET. It is more widely used than the BJT as it requires minimal current for load-current control. The level of conductivity can be increased from the ‘normally off’ state when the MOSFET is set to enhancement mode. Voltage transmitted via the gate can minimise conductivity from the ‘normally on’ state. The process of miniaturising MOSFETs is relatively simple, and they can be effectively scaled down for compact applications.

Key Advantages of MOSFETs

There are multiple advantages of MOSFETs, and they primarily include the following:

• Rapid Switching: They excel at fast switching, which is especially vital for processing digital signals swiftly and efficiently.
• Energy Efficiency: MOSFETs stand out for their minimal power consumption, aligning with the global push towards energy-efficient technologies.
• High-Density Integration: Their capability to support high-density integration makes them perfect candidates for large-scale integrated circuits (ICs).

MOSFET Transistors vs Bipolar Junction Transistors (BJTs)

Bipolar Junction Transistors (BJTs) are semiconductor devices that amplify or switch electrical currents through the interaction of electrons and holes across two junctions, making them integral components in various electronic circuits. When compared to BJTs, MOSFETs, being voltage-controlled devices, stand out in scenarios that demand efficient power management and the seamless integration of digital circuits.

MOSFETs have an edge in efficiency, typically drawing less power in their off-state due to their high input impedance. In comparison, BJTs exhibit a continuous draw of power even when idle, a consequence of the requisite base current. This distinction extends to switching speed as well; MOSFETs boast quicker switching capabilities, a vital attribute for high-frequency applications. This performance comes at the cost of requiring a higher drive voltage, usually between 10-15V for full enhancement, whereas BJTs can function with lower base-emitter voltages.

In addition, due to their voltage-controlled nature, MOSFETs reduce the complexity of the required drive circuitry. BJTs, on the other hand, demand current-driven bases, adding a layer of complexity to their operation.

MOSFETs are more commonly used than BJTs in applications in the current age simply due to them being voltage-controlled devices, possessing higher speed, and being more effective in use.

PMOS Logic

As previously mentioned, the integration of a MOSFET allows for high levels of circuit efficiency when compared with BJTs. P-channel MOSFETs can be used with PMOS logic to implement digital circuitry and logic gates.

NMOS Logic

NMOS logic is similar to PMOS logic, with the exception that N-channel MOSFETs are applied to logic gates and related digital circuitry. As a general rule, N-channel MOSFETs can be smaller than P-channel MOSFETs, which makes them more attractive in certain situations. However, NMOS logic constantly consumes power, whereas PMOS logic does not.

CMOS Logic

Complementary metal-oxide-semiconductor (CMOS) logic is a technology used to produce integrated circuits. Such circuits are featured across a range of electrical components and are known to generate electrical power. Both P and N-channel MOSFETs are used in conjunction with connected gates and drains in order to reduce power consumption and excess heat generation.

Depletion Mode MOSFET Devices

Depletion mode MOSFET devices are among the less common types of MOSFETs. They have a low channel resistance, with the channel being considered as ‘ON’. When set to the no-power state, these switches will be conducted in accordance with their design. The channel resistance will be linear, with low distortion across the signal amplitude range.

MISFETs

All MOSFETs are MISFETs (Metal Insulator Semiconductor Field Effect Transistors), but not all MISFETs are MOSFETs. The gate dialectic insulator featured in this type of component is silicon dioxide in a MOSFET. However, alternative materials can be used. The gate dialectic is positioned underneath the gate electrode and above the MISFET channel.

Floating-Gate MOSFETs (FGMOS)

The floating-gate MOSFET features an electronically isolated gate. This has the effect of generating a floating node in DC together with a range of secondary gate inputs situated above the floating gate. Among various other uses, the FGMOS is typically used as a floating-gate memory cell.

Power MOSFETs

This type of MOSFETs has a vertical structure, allowing for simultaneous high blocking voltage and high current. The transistor's voltage rating depends on the N-epitaxial layer's doping and thickness, while the channel width determines the current rating. The device's area directly affects the current it can handle. Power MOSFETs offer low gate drive requirements, fast switching speed, and the ability to be paralleled.

DMOS

These double-diffused metal oxide semiconductors come in lateral and vertical varieties. The majority of power MOSFETs are constructed using this kind of technology.

MOS Capacitors

This type of capacitator has the MOSFET structure, with the MOS capacitator being flanked by dual P-N junctions. It is typically used as a memory chip storage capacitator and support for the charge-coupled device (CCD) in image sensor technology.

TFT

The thin-film transistor (TFT) is a unique type of MOSFET. Creating this variety involves the deposition of thin semiconductor films combined with a dialectic layer and metallic contacts over a supporting substrate. A range of semiconductor materials may be used, with silicone being a common variety. They can be made to be completely transparent and are used in the manufacture of video display panels.

Bipolar-MOS Transistors

The BiCMOS is an integrated circuit featuring BJT and CMOS transistors on a single chip. The insulated-gate bipolar transistor (IGBT) has similar functionality to the MOSFET and bipolar junction transistor (BJT).

MOS Sensors

A range of MOS sensors has been developed to accurately measure physical, chemical, biological, and environmental variables. Examples include the open-gate FET (OGFET), ion-sensitive field-effect transistor (ISFET), gas sensor FET, charge flow transistor (CFT), and enzyme-modified FET. Commonsensors used for digital imaging include the couple-charged device (CCD) and active-pixel sensor (CMOS sensor).

Multi-Gate Field Effect Transistors

The dual-gate MOSFET has a tetrode configuration, with the level of current being controlled by the two gates. It is typically used for small-signal devices in radio frequency applications that call for the reduction of gain loss associated with the Miller effect. This effect occurs when separate transistors are replaced in a cascade configuration.

RHBD

It is quite common for the enclosed-layout transistor (ELT) to be used to create a radiation-hardened-by-design (RHBD) device. The gate of the MOSFET typically surrounds the drain, which is situated close to the centre of the ELT. The source of the MOSFET surrounds the gate in this instance. The H-gate is another type of MOSFET that ensures minimal radiation leakage.

MOS Integrated Circuits

The MOSFET is the most popular type of transistor and is essential for the electrical operation of integrated circuit (IC) chips. They do not require the same series of steps as bipolar transistors for P-N junction isolation on a chip. However, they do allow for relatively easy separation.

CMOS Circuits

Complementary metal-oxide-semiconductor (CMOS) technology is widely used in the production of IC chips, offering high noise immunity, minimal static power consumption, and the integration of high-density chip logic functions.

Analogue Switches

MOSFETs offer advantages in analogue circuits, including reliability, zero gate current, high output impedance, and the ability to adjust circuit characteristics and performance through MOSFET size adjustments.

Power Electronics

MOSFETs are used across a broad range of power electronics. They are integrated for reverse battery protection, switching power between alternate sources, and the powering down of unrequired loads. Their key features include a small footprint, high current.

MOS Memory

The development of the MOSFET allowed for the convenient use of MOS transistors for memory cell storage. MOS technology is one of the key components of DRAM (dynamic-access random memory). It offers higher levels of performance, consumes minimal power, and is relatively affordable when compared with magnetic core memory.

MOSFET Sensors

MOSFET sensors, otherwise referred to as MOS sensors, are commonly used for measuring physical, chemical, biological, and environmental parameters, as well as in microelectromechanical systems (MEMS) and image-sensing applications.

Quantum Physics

The quantum field-effect transistor (QFET) and quantum-well field effect transistor (QWFET) are both types of MOSFET that make use of quantum tunnelling to enhance transistor speed by eliminating electron conduction areas, relying on rapid thermal processing (RTP) and fine layer building materials.

It is essential that you select the appropriate N-channel or P-channel MOSFET for your purposes. Each of these MOSFETs functions as an electrical switch. The regular power application features a MOSFET, which is connected to the ground, with the lead hooked up to the rail voltage. This is considered to be a low-side switch and requires the fitting of an N-channel MOSFET due to the levels of voltage that must be transmitted in order to switch the device on and off. The P-channel variety should be used when there is a high-side switch connected to the bus and a load resistor connected to the ground.

The MOSFET voltage must be taken into account. This should exceed both the rail and bus voltage to ensure sufficient protection and prevent MOSFET failure. The current rating should be of the maximum capacity for the load in all instances. Other factors that should be taken into consideration include the technology impact, thermal requirements, and switching performance. The gate parameters must also correspond with the driver circuitry. In short, the appropriate selection will depend on the application of the MOSFET.

Once you select the types of MOSFET transistors for use, it is important to follow proper handling and usage practices when working with them. The following are some general guidelines you can adhere to:

• Static Precautions: Always handle MOSFETs in a static-free environment or use anti-static wrist straps to prevent damage.
• Gate Protection: Use a resistor (typically 1kΩ to 10kΩ) between the gate and source to protect against unwanted gate-source voltages that could turn the MOSFET on unintentionally.
• Heat Sinking: For high-power applications, ensure adequate heat sinking to prevent overheating. MOSFETs can handle high currents, but they also generate heat that must be efficiently dissipated.
• Gate Drive Voltage: Ensure the gate drive voltage is sufficient to enhance the MOSFET fully but does not exceed the maximum gate-source voltage rating to avoid damaging the device.
• Parasitic Inductance: Minimise parasitic inductance in the circuit by keeping lead lengths short, especially in high-frequency applications, to prevent oscillations.
• Testing: Before integration into a circuit, test MOSFETs using a multimeter in diode mode to check for basic functionality and ensure they are not shorted or open.
• Circuit Simulation: Use circuit simulation software to model the MOSFET within the circuit before physical implementation. This can help identify potential issues early on.
• Datasheet Consultation: Always refer to the MOSFET's datasheet for specific characteristics like threshold voltage, drain current, and power dissipation to ensure compatibility with your application.
• Protection Circuits: Implement protection circuits where necessary, such as overvoltage, overcurrent, and thermal shutdown, to prolong the lifespan of the MOSFET.