• Dhanush kumar S

MOSFET Transistors

Updated: Dec 1, 2020

Metal Oxide Semiconductor Field Effect Transistor is a type of insulated-gate field-effect-transistor fabricated by controlled oxidation of semiconductors such as silicon, germanium. These #transistors are majorly employed in the manufacturing of chips using #VLSI technology. This transistor has been miniaturized over the period of time and used in a wide range of applications in the electronics industry.


It enables us to design high-density ICs such as memory chips and microprocessors and is known as the workhorse of the electronics industry. In this article, let us discuss in detail about the configurations of #MOSFET, advantages, and its applications.


MOSFET TRANSISTOR


The main building block of the VLSI concept is nothing but MOS transistors. The MOSFET( Metal Oxide Semiconductor Field Effect transistors) is the most commonly used transistor in both analog and digital #circuits. It was invented by Mohamed M. Atalla and Dawon Kahng in 1959. The first commercial MOS integrated circuit was introduced into the market by General Microelectronics in 1964.


It is fabricated by the controlled oxidation of the semiconductor (silicon). The device consists of four terminals namely drain[D], source[s], gate[G], bulk[B]. The electrical conductivity of the device is determined by the voltage of the gate. And the device is made up of three layers, a poly-silicon layer (gate terminal), an oxide layer (gate oxide), a single crystal semiconductor layer (substrate). The polysilicon gate is employed here for its high-temperature stability.


Working of MOSFET


The working of MOSFET is dependent mainly on the MOS capacitor. The voltage applied to the gate terminal acts as a conducting channel between the source and drain. When the power supply has been given, the gate attracts the carriers to the substrate interface and closes the circuit, thereby allowing the flow of #current through it.


When we apply a positive gate voltage the holes present under the oxide layer are pushed down to the substrate with a repulsive force. The depletion layer is populated by negatively charged acceptor atoms and eventually, the channel is formed. The applied positive gate voltage also attracts the electrons from the source and drain regions into the channel. The channel which is formed can allow the electric current to pass through it.


Types of MOSFET


The MOS transistors are classified based on the type of dopant used in the terminals. There are several types of MOS transistors existing in the market, but most preferable 3 kinds are used in VLSI technology. They are N-MOS, P-MOS, and C-MOS transistors. Both the N-MOS and P-MOS transistors can be operated in enhancement mode as well as depletion mode. The N-MOS, P-MOS, and the C-MOS transistors are explained in a separate article and the link is given in the see also section. Do check.



Enhancement mode:

  • The Enhancement-mode MOSFETs are the common switching elements in most integrated circuits.

  • This mode of MOSFETs is equivalent to the "Normally Open" switch and requires a gate-source voltage to switch 'ON' the device.

  • In enhancement mode, there is no conduction or existing channel at zero voltage. A channel is established when the gate voltage is more than the source voltage.

  • The oxide layer is known as the inversion layer. The channel is formed between the drain and source in the opposite type ( N or P-type) to the substrate.

  • The conductivity of the channel due to electrons or holes is depending on the N-type or P-type channel respectively.



Depletion mode:

  • The depletion-mode MOSFETs are known as "Switched On" devices because these transistors are generally in the closed state when there is no bias voltage at the gate terminal.

  • If the applied gate voltage is more negative, then the width of the channel gets decreased and the MOSFET may enter into the cut-off region.

  • If the voltage gets more positive, then the width of the channel increases and the transistor gets ON.

  • The depletion-mode MOSFET is rarely used in the electronic circuits, as enhancement mode is far better than depletion mode in several aspects.



Advantages of MOSFET

  • MOSFETs provide greater efficiency while operating even in low voltages.

  • It also consumes less power and plays an important role in the concept of energy-saving.

  • Absence of gate current results in high input impedance, which results in high switching speed.

  • They have high drain resistance due to the lower resistance of the channel.

  • MOSFETs can be manufactured easily and also at a low cost when compared to JFETs.

Disadvantages of MOSFET

  • The MOSFET may become unstable at the condition when the overload voltage passes through it.

  • These are vulnerable to permanent damage because of the presence of a thin oxide layer. The damage can be caused by electrostatic charges.

Applications of MOSFET

  • MOSFET amplifiers are extensively employed in radio frequency applications.

  • DC motors can be regulated by power MOSFETs.

  • It acts as a passive element like resistors, capacitors, and inductors.

  • The high switching speed of MOSFETs is very much useful in designing the chopper circuits.

  • Depletion mode MOSFETs are employed in manufacturing the linear voltage regulator circuits.

  • MOSFETs can be used as the constant current source to design current monitoring circuits in conjunction with op-amps.

  • Variable frequency drives, voltage-ramp generator circuits, and voltage-controlled potentiometers also employ MOSFETs in their design.


See also:

  1. What is VLSI technology?

  2. Design flow of VLSI technology.

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