W. Shockley, J. Barden and W. Brattain invented the transistor in 1947. The term ‘transistor’ is derived from the words ‘transfer’ and ‘resistor.’ These words describe the operation of a BJT which is the transfer of an input signal from a low resistance circuit to a high resistance circuit. The abbreviation BJT, from bipolar junction transistor, is often applied to this three-terminal device. The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized mate

What is a Bipolar Junction Transistor (BJT)? 

A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector and the emitter. A signal of small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process. Bipolar transistors are manufactured in two types, PNP and NPN, and are available as separate components, usually in large quantities. The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers. They have a wide application in electronic devices like mobile phones, televisions, radio transmitters and industrial control. 

Bipolar Transistor Construction

 According to dopping: E>C>B 

 According to Width: C>E>B

ACTION OF BJT

Two batteries are used to simplify operation theory. Most applications require one voltage source. The negative terminal of the battery is connected to the N emitter.  The positive terminal of the same battery is connected to the P-type base. Therefore, the emitter-base circuit is forward-biased. 

 In the collector circuit, the N collector is connected to the positive battery terminal. The P base is connected to the negative terminal.  The collector-base circuit is reverse biased.  Electrons enter the emitter from the negative battery source and flow toward the junction. The forward bias has reduced the potential barrier of the first junction. 

 The electrons then combine with the hole carriers in the base to complete the emitter-base circuit. However, the base is a very thin section, about 0.001 inches

.  Most of the electrons flow on through to the collector as the collector terminal is connected to the positive terminal of the battery and this reverse-biased potential is very large so most of the majority of charge carriers are attracted and will cross the large base to the collector depletion region due to large reverse-biased potential.  

there will be an injection of minority carriers(holes) from the N-dopped collector into the P-type base region material as the collector terminal is connected to the positive terminal of the battery. Similarly, minority charge carriers(free electrons) in the base will move towards the collector.Due to this a current will flow from the collector terminal towards the base due to the presence of minority charge carriers.So the total collector current will be:

𝐼𝐶 =𝐼𝐶 majority + 𝐼𝐶𝑂 minority

The minority-current component is called leakage current and is given the symbol 𝐼𝐶𝑂 (𝐼𝐶 current with emitter terminal Open).  Approximately 95 to 98 per cent of the current through the transistor is from an emitter to a collector. About two to five per cent of the current moves between emitter and base.  A small change in the emitter to base bias voltage causes a somewhat larger change in emitter-collector current. This is what allows transistors to be used as amplifiers. The emitter-base current change, however, is quite small.

 𝐼𝐸 = 𝐼𝐶 +𝐼B

Advantages of BJT’S

  • BJTs have high switching frequencies since their turn-on and turn-off time are low.
  • The turn-on losses of a BJT are small.
  • BJT has controlled turn-on and turn-off characteristics since base drive control is possible.
  •  BJT does not require commutation circuits

Demerits of BJT

  • Drive circuit of BJT is complex.
  • It has the problem of charge storage which sets a limit on switching frequencies.
  •  It cannot be used in parallel operation due to problems of negative temperature coefficient.
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