Active component – Transistors

Transistors:  It is the active component used as amplifier or switch. It transfers the resistance from one part to another part of the circuit. Voltage level varies from 0 to 5 volts.


  1. BJT
  • NPN
  • PNP

2. FET

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BJT or Bipolar Junction Transistor: A BJT is a current controlled device which consists of a layer of n-type semiconductor material sandwiched between two layers of p-type semiconductor material. It consists of three terminals – The emitter, base, and collector. Among three terminals collector is less doped.


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Types of configuration

CB configuration: called grounded base configuration. The base is common terminal, emitter is input and collector are output.

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CE configuration: called grounded emitter configuration. The base is input and collector are output.

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CC configuration: Collector is common terminal. The base is input and emitter is output.

Comparison between configurations:

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FET or Field Effect Transistor: A FET is a voltage controlled device. The ohmic contacts are taken from the two sides of the n-type bar. It consists of three terminals – Gate, Drain, and Source.  The voltage applied across the Gate-Source and the Drain-Source terminal controls the flow of current through the device. It is generally a high resistance device.


JFET (junction Field effect transistor) which consists of an n-type substrate, on the side of which a bar of the opposite type is deposited

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Symbolic representation:


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Features of JFET:

  • Operated only in depletion mode
  • Conductivity is controlled by the reverse biasing of the gate
  • It has High Gate current
  • High Input Impedance
  • High Drain resistance
  • Simple manufacturing process
  • Three terminal

MOSFET (Metal Oxide Semiconductor FET) which consists of an insulated layer of silicon oxide between the metallic Gate contact and the substrate

G- gate




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  • Operated in both Enhancement and depletion mode
  • Conductivity is controlled by the carriers induced in the channel
  • It has low Gate current
  • Very High Input Impedance
  • Low Drain resistance
  • Difficult manufacturing process
  • Four Terminals


  • In a depletion-mode MOSFET, the device is normally ON at zero gate–source voltage. Such devices are used as load “resistors” in logic circuits. For N-type depletion-load devices, the threshold voltage might be about –3 V, so it could be turned off by pulling the gate 3 V negative. In PMOS, the polarities are reversed.
  • Enhancement-mode MOSFETs are the common switching elements in most MOS. These devices are off at zero gate–source voltage, and can be turned on by pulling the gate voltage either higher than the source voltage, for NMOS, or lower than the source voltage, for PMOS. In most circuits, this means pulling an enhancement-mode MOSFET’s gate voltage towards its drain voltage turns it ON.

TRIAC’S or SCR: An SCR or Silicon Controlled Rectifier is a three terminal device which is generally used as a switch in power electronics. It is a combination of two back to back diodes having 3 junctions. The current through the SCR flows because of the voltage applied across anode and cathode and is controlled by the voltage applied to the Gate terminal. It is also used as a rectifier in AC circuits.

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