{"id":2344,"date":"2024-01-30T16:58:48","date_gmt":"2024-01-30T16:58:48","guid":{"rendered":"https:\/\/partstack.com\/blog\/?p=2344"},"modified":"2024-01-30T17:03:52","modified_gmt":"2024-01-30T17:03:52","slug":"what-is-mosfet-its-working-principle","status":"publish","type":"post","link":"https:\/\/partstack.com\/blog\/what-is-mosfet-its-working-principle\/","title":{"rendered":"MOSFET Working Principle: Types and Applications"},"content":{"rendered":"

Introducing MOSFET<\/h2>\n

The metal-oxide-semiconductor field-effect transistor (MOSFET)<\/strong> has become the most commonly used transistor in integrated circuits and electronic devices. MOSFETs have almost completely replaced bipolar junction transistors (BJTs) in digital circuits due to several advantageous characteristics. We will explore the working principles, structure, types, and applications of MOSFETs at an introductory level.<\/p>\n

MOSFET Structure and Composition<\/strong><\/h2>\n

A MOSFET<\/strong> is a special type of field-effect transistor<\/a> where a gate electrode is placed over a semiconductor substrate that has an undetectable layer of insulating material like silicon dioxide. This forms a metal-insulator-semiconductor layered structure that gives rise to the name \u201cmetal-oxide-semiconductor.\u201d The four terminals of a MOSFET device are called source (S)<\/strong>, gate (G)<\/strong>, drain (D)<\/strong>, and body\/substrate (B)<\/strong>. The semiconductor substrate on which the layers are fabricated is typically high-purity silicon, doped appropriately, to form n-channel or p-channel MOSFETs. Impurity atoms like phosphorus or boron are precisely introduced in a process called doping to alter the number of free charge carriers.<\/p>\n

\"Basic<\/p>\n

Fig 1: Basic Construction of MOSFET<\/p>\n

Operating Principle<\/strong> of MOSFET <\/strong><\/h2>\n

The MOSFET is a transistor<\/a> designed to be used as a voltage-controlled switching device<\/strong><\/em>. The core functionality of this integrated circuit relies on forming an inversion channel that conducts current between the source and drain terminals in the underlying layer of semiconductor<\/a> material. The formation of this conducting channel is controlled by an electrically isolated gate electrode, which sits on top of a thin insulating dielectric oxide layer residing over the substrate. By applying a voltage to this gate, an electric field develops across the insulating dielectric layer, which in turn influences the concentration and flow of carriers within the silicon underneath.<\/p>\n

Specifically, in an n-channel, enhancement-mode type of MOSFET, applying a positive gate voltage that surpasses a defined threshold voltage causes electrons to be drawn toward the oxide-semiconductor interface. Here they accumulate to form an n-type, electron-rich region known as an inversion layer or n-channel. The source and drain terminals have n-type semiconductor deposits on opposite sides of the created channel.<\/p>\n

Therefore, when the n-channel is induced, it allows current flow between the electron-rich n-type source and drain regions, turning the device ON like a closed switch. The gate itself draws negligible control current, while the channel modulation occurs between the drain and the source. Removing the gate voltage causes the inversion layer to disappear and switches the device into a non-conducting OFF state with very small current. This voltage-activated formation of the inversion channel makes the MOSFET behave as an electrically operated switch. The gate voltage amplitude controls whether the switch is ON, allowing full conduction between sources and drain terminals or switched OFF.<\/p>\n

Types of MOSFET<\/strong><\/h2>\n

The following are the main categories of MOSFET and a brief description of their work.<\/p>\n

    \n
  1. \n

    Enhancement-type MOSFET<\/strong><\/h3>\n<\/li>\n<\/ol>\n

    The enhancement type MOSFET (E-MOSFET) operates solely in enhancement mode, as shown in Figure 4(b), without any depletion mode capabilities. Unlike the depletion type MOSFET, the E-MOSFET does not have a built-in channel for electric current to flow between the drain and source terminals. Instead, the E-MOSFET requires a positive voltage applied to the gate terminal above a minimum threshold voltage to induce a conducting channel in the substrate region next to the insulating oxide layer separating the gate. Applying a gate voltage above this threshold causes an inversion layer in the p-type semiconductor substrate, allowing electron carriers to flow and create a current channel.<\/p>\n

    Increasing the positive gate voltage further pulls more free electrons into this induced channel, increasing the conductivity through the E-MOSFET. When the gate voltage falls back below the threshold, the induced channel disappears, preventing any current flow through the transistor. Schematic symbols used for n-channel or p-channel E-MOSFETs contain broken lines for the channel, indicating no permanent physical channel. An inward arrow on the gate represents an n-channel E-MOSFET using electron carriers, while an outward arrow represents a p-channel version using hole carriers. Some E-MOSFET designs also provide a separate electrical connection to the transistor substrate.<\/p>\n