HOME BLOG GITHUB
MOSFET Transistors DRAM Cell DRAM Cell MOSFET Transistors MOSFET Transistors DRAM Cell->MOSFET Transistors Capacitors Capacitors DRAM Cell->Capacitors Electric Force Electric Force MOSFET Transistors->Electric Force SRAM Cell SRAM Cell SRAM Cell->MOSFET Transistors Logic Circuits Logic Circuits Logic Circuits->MOSFET Transistors NOT Gate NOT Gate Logic Circuits->NOT Gate AND Gate AND Gate Logic Circuits->AND Gate OR Gate OR Gate Logic Circuits->OR Gate Fundamental Forces Fundamental Forces Electric Force->Fundamental Forces Electron Charge Electron Charge Electric Force->Electron Charge

May 27, 2021

MOSFET Transistors

An electronic component that acts as a switch: turn on or off the connection between Source pin and Drain pin by changing the voltage on its Gate pin.

Transistors

The transistor is the most important electronic component for electronics and computers. It forms the basis of almost all logic operations on data and is also used for storing data (for example, in SRAM memories).

A transistor is basically a 3 terminal device that acts as an electronic switch.

Example of a push-switch represented by its schematics.

Like a switch, it has an input (left terminal on the above schematics), output (right terminal) and a control terminal (the push mechanism on top). In one position (when open) no electric current can flow from input to output terminals and in the other position a current can freely flow from input to output.

In a transistor, the push part of the switch is actually determined by the voltage (i.e. charge) applied on the control terminal.

MOSFET transistors

The typical implementation of a transistor for computer and logic operations is the so-called MOSFET transistor (Metal Oxide Semiconductor Field Effect Transistor). These are using the properties of semiconductor materials to implement the switch behavior by stacking 3 different materials:

  • A semiconductor (typically silicon)
  • An oxide (typically Silicon oxide for large technologies)
  • A metal on top

This stack of materials gives part of the name to the transistor (the MOS part).

Physical representation of a MOSFET transistor.

The three terminals of a MOSFET transistor are usually called the Drain and Source (for input/output terminals) and the Gate terminal, that acts as the switch control. When a given voltage (which value is given by transistor intrisic characteristics) is applied on the gate, the Source/Drain connection becomes like a wire. The fourth connection on the image above (Body) is not necessary in most of the cases and is usually wired to the source. The n+ and p corresponds to the doping type of the semiconductor, which will determine the type of the transistor (see below MOSFET types).

MOSFET types

There exists two main types of MOSFET transistors that we call n-type and p-type MOSFETs. Without going into the details of why we call them like this, the main difference is that a n-type MOSFET will let the current flow from Drain to Source only when a positive voltage is applied on its gate. The p-type will do so only when a negative voltage is applied on its gate (with respect to the source terminal voltage).

In electronics we usually represent devices with symbols. Here are the symbols for the nMOSFET and pMOSFET transistors.

The p and n letters corresponds to the doping of the semiconductor, such as Silicon, used for these devices. Implanting atoms that have more electrons (n type) or less electrons (p type) than Silicon allows to modify the electrical properties of the semiconductor. See the Wikipedia article on semiconductor doping and the Wikipedia article on MOSFETs for more information.