The transistor is a non-linear semiconductor three-terminal device. The transistor is considered to be one of the most important devices in the field of electronics. The transistor has transformed many aspects of man’s life. There are two main functions of transistors, to amplify input signals and to acts as solid-state switches. The transistor acts as a switch when operated either in saturation or cut-off region. Whereas it amplifies signals when used in the active region. It offers very high input resistance and very low output resistance.

Transistors are categorized into bipolar junction transistor and field effect transistor based on their construction.

Type of transistor:

  • BJT:  NPN and PNP,

Transistor symbol is represented as below.

transistor-symbol.jpgtransistor-symbol.jpgTransistor symbol

The most popular and commonly used transistors are BC547, 2N2222. Given below are a few common transistor packages:

transistor-packages.jpgtransistor-packages.jpgTransistor packages


The MOSFET (metal oxide semiconductor field-effect transistor) transistor is a semiconductor device that is different than bipolar junction transistor in terms of construction though the applications remain the same as switching and amplifying. It has four terminals such as drain, gate, source, and body. The body is shorted with a source terminal. The gate is insulated from the channel near an extremely thin layer of metal oxide. Due to which it offers very high resistance compared to BJT.

By controlling the gate voltage (VGS +ve/-ve) width of a channel along which charge carriers flow (electrons or holes) from source to drain can be controlled. The P-Channel MOSFET has a P-Channel region between the source and drain and for N-channel MOSFET has an N-channel region.

Advantages of MOSFET over BJT:

  • Very high input resistance
  • Low on-state resistance
  • Low power loss
  • High frequency of operations

transistor-operation.jpgtransistor-operation.jpgTransistor operation

Application of transistors (BJT/FET)

  • Amplification of analog signals
  • Used as switching devices in SMPS, microcontrollers, etc.
  • Oscillators
  • Over/under voltage protection
  • Modulation circuits & demodulation of signals
  • Power control in invertors and chargers (high-current power transistors)

Types of transistor packages

In terms of packaging BJT and MOSFET, transistors are available in through-hole (DIP) and SMD versions. e.g. DIP: TO-92, TO- 220 and  SMD: SOT23, SOT223, TO-252, D2PAK.

Transistor selection parameters

While selecting a transistor in any circuit, the user needs to take care of the following parameters:

  • Maximum collector current (Ic)
  • Max collector voltage (Vce)
  • VBE voltage
  • Saturation Vce (sat) voltage
  • Current gain, hfe/ß
  • Input resistance
  • Output resistance
  • Reverse breakdown voltage
  • Max reverse current
  • Power dissipation
  • Operating junction temperature
  • Size
  • Switching time/frequency

Manufacturers: Analog Devices, Rohm Semiconductor, Diodes Incorporated, On Semi, Texas Instrument, Panasonic, Infineon, Honeywell, etc.

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