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What are Transistors?

MOSFET, Darlington and Bipolar Transistors

A transistor is used to amplify and switch electronic signals and electrical power. They are used in a variety of circuits and they come in many different shapes. You can use a transistor as a switch or you can use a transistor as an amplifier.

MOSFET Transistors

Metal-oxide-semiconductor field-effect transistor is a type of transistor commonly found in digital and analog circuits to amplify or switch electronic signals. The three-terminal device has a source (S), gate (G) and drain (D) and is available in both P-channel (PMOS) and N-channel (NMOS). MOSFETs may also be known as insulated-gate field-effect transistors (IGFET) or metal-insulator-semiconductor field-effect transistor (MISFET). Although similar to a junction field-effect transistor (JFET), on a MOSFET, the gate input is electrically insulated from the main circuit carrying channel.

A MOSFET is available in two basic forms, depletion type and enhancement type. Depletion type is like a "Normally Closed" switch and requires a Gate-Source voltage to turn the device off. Enhancement type is like a "Normally Open" switch and requires a Gate-Source voltage to turn the device on. Typical package types are TO-252, TO-251, TO-247, TO-220, TO-92, SO-8, SPT-23 and SOT-223.

MOSFET Construction and Type
MOSFET Construction and Type

BU Series: Amplifier applications (Includes BUZ series – BUZ77 and BUZ78)
IRF Series: Amplifier applications (Includes IRFP and IRFZ series – IRF540 and IRFP350)

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Darlington Transistors

Darlington transistors are two standard NPN or PNP bipolar transistors that are connected together. The emitter of one transistor is connected to the base of the other to produce a more sensitive transistor with a much larger current gain.

Darlington transistor pairs can be made from two individually connected bipolar transistors or a one single device commercially made in a single package with the standard: base, emitter and collector connecting leads and are available in a wide variety of case styles and voltage (and current) ratings in both NPN and PNP versions.

Also known as a "Darlington pair" or "super-alpha circuit", the transistors are connected together so that the emitter current of the first transistor TR1 becomes the base current of the second transistor TR2.

Basic Darlington Connection
Basic Darlington Connection

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Bipolar Transistors

The Bipolar transistor, more commonly known as the bipolar junction transistor (BJT), is a type of transistor that relies on the contact of two types of semiconductors for its operation. They consist of three terminals: collector, base and emitter, which can be arranged as amplifiers or switches. Depending on their polarity, they can be either NPN or PNP.

Bipolar transistors work as current-controlled current regulators. They restrict the amount of current passed according to a smaller, controlling current. They are called bipolar because the controlled current must go through two types of semiconductor material: P and N.

Bipolar Construction
Bipolar Construction

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Other Types of Transistors

General Purpose Transistors
General Purpose SMD Transistors
Transistor Arrays