MOSFET

Overview

nMOS and pMOS transistors are the two main types of MOS devices used in microprocessors. nMOS transistors turn on with a high gate voltage, while pMOS transistors turn on with a low gate voltage. Together, they form complementary circuits like cMOS logic gates.

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.

MOSFETs are broadly classified into two main types based on their channel material and operation:

N-channel MOSFET (nMOS): Uses electrons as the primary charge carriers.

P-channel MOSFET (pMOS): Uses holes as the primary charge carriers.

The main advantage of a MOSFET is that it requires almost no input current to control the load current, when compared with bipolar transistors (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.

NMOS Transistor

NMOS is a type of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor); the terms do not refer to two different kinds of devices in a distinguishing way. The key difference is that a MOSFET is a general category of transistors, while an NMOS is a specific N-channel variety within that category. MOSFETs are broadly classified into two main types based on their channel material and operation: N-channel MOSFET (NMOS): Uses electrons as the primary charge carriers.

Oxidation

In the context of transistors, particularly MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), oxidation is the crucial process used to grow a layer of silicon dioxide on the silicon substrate. This oxide layer serves as a vital electrical insulator that separates the gate electrode from the semiconductor channel. The "O" in "MOSFET" specifically stands for "Oxide," highlighting its fundamental importance to the device's function.

Purpose and Role of Oxidation

The primary functions of the oxide layer formed by oxidation are:

Gate Dielectric: The silicon dioxide layer acts as a high-quality dielectric material (an electrical insulator). This insulation is essential because it prevents current leakage between the gate and the channel beneath it.

Voltage Control: The insulating layer allows the electric field from the voltage applied to the gate to effectively control the conductivity of the channel without physical contact. By insulating the gate, the oxide layer enables the gate voltage to modulate the flow of current between the source and the drain.

Layer Separation: Oxidation helps create insulating barriers between different components on the integrated circuit, ensuring that various regions of the transistor and surrounding components operate independently

Surface Passivation: The process of growing an oxide layer also helps to passivate the silicon surface, reducing the number of defects and traps that could otherwise interfere with the transistor's performance.

Links of Interest

CMOS - Grokipedia

MOSFET - Grokipedia

MOSFET - Wikipedia

NMOS Transistors and PMOS Transistors Explained