Transistor is a semiconductor device that can both conduct and insulate. A transistor can act as a switch and an amplifier. It converts audio waves into electronic waves and resistor, controlling electronic current.

Transistors have very long life, smaller in size, can operate on lower voltage supplies for greater safety and required no filament current. A transistor performs the same function as a vacuum tube triode, but using semiconductor junctions instead of heated electrodes in a vacuum chamber.

Most transistors are made from very pure silicon or germanium, but certain other semiconductor materials can also be used.


Transistor Basics:

A transistor is a three terminal device. Namely,

⇒  Base : This is responsible for activating the transistor.

⇒  Collector : This is the positive lead.

⇒  Emitter : This is the negative lead.

Types of Transistors

There are two types of transistors in present; they are bipolar junction transistor (BJT), field effect transistors (FET). A small current is flowing between the base and the emitter; base terminal can control a larger current flow between the collector and the emitter terminals. For a field-effect transistor, it also has the three terminals, they are gate, source, and drain, and a voltage at the gate can control a current between source and drain. The simple diagrams of BJT and FET are shown in figure below:

BJT Transistor

BJT Transistor

FET Transistor

FET transistor

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Bipolar Junction Transistor

A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. It comes with two types, P-N-P and N-P-N.

N-P-N Transistor

N-P-N transistor consisting a layer of P-doped semiconductor between two layers of N-doped material. By amplifying current the base we get the high collector and emitter current.

That is when NPN transistor is ON when its base is pulled low relative to the emitter. When the transistor is in ON state, current flow is in between the collector and emitter of the transistor. Based on minority carriers in P-type region the electrons moving from emitter to collector. It allows the greater current and faster operation; because of this reason most bipolar transistors used today are NPN.

N-P-N Transistor

P-N-P Transistor

P-N-P transistor consisting of a layer of N-doped semiconductor between two layers of P-doped material. The base current entering in the collector is amplified at its output.

That is when PNP transistor is ON when its base is pulled low relative to the emitter. The arrows of PNP transistor symbol the direction of current flow when the device is in forward active mode.




  • 1. Smaller mechanical sensitivity.
  • 2. Lower cost and smaller in size, especially in small-signal circuits.
  • 3. Low operating voltages for greater safety, lower costs and tighter clearances.
  • 4. Extremely long life.
  • 5. No power consumption by a cathode heater.
  • 6. Fast switching.

!!! Problem Faced !!!

Some time changes between base and collector legs of transistor because of multiple manufacturers...

I face that problem in transistor 2N3904