What is a transistor and types of transistors?

Transistor is a semiconductor device capable of transferring the signal from high resistance to low resistance or vice versa. Transistor has three terminals and is used in electronic circuits as amplifiers and switches. There are mainly three types of transistors used in electronic circuits they are a) bipolar junction transistor b) Junction field effect transistor c) Metal oxide semiconductor field effect transistor.

What is bipolar junction transistor?

The name Bipolar came from the fact that the device operation depends on movement of charge carriers with both polarities (holes and electrons). BJT has three terminals a) Emitter b) base c) collector analogous to cathode gate and anode in vacuum tube. Emitter is heavily doped collector is moderately doped and base is lightly doped as we need most of the current from emitter should reach collector; base current is needed only as a pilot signal for collector current variation. Transistor is active device which has the ability to control the electron flow through it. BJT is of two types 1) PNP transistor 2) NPN transistor .PNP transistor is formed by sandwiching n-type semiconductor between two P-type semiconductors. Similarly PNP transistor is formed by sandwiching n-type semiconductor between two P type semiconductors.

What is power dissipated by transistor in active region?

The power dissipation across the transistor mainly occurs at the collector junction. This is due to the fact that the collector junction is reverse biased, so offers more resistance to current flow. Hence more voltage drop occurs across the collector junction. Power dissipation across the transistor is given by

              { VBE is neglected as VBE << VCE }

 Why inverse active mode of transistor is not useful?

Bipolar junction transistor internal design is in such a way that it will have high gain in normal active mode. Also when you interchange the roles of emitter and collector with emitter base junction reverse biased then break down voltage decreases as break down voltage is inversely proportional to the amount of doping. As emitter is highly doped compared to collector it is advantageous to reverse bias collector base junction in order to have the advantage of high breakdown voltages.

What are the types of transistor configurations?

BJT can be operated in three configurations with one terminal common to both input and output. They are a) Common Base b) Common emitter c) Common collector configurations.

What are α,β and γ in a transistor ?

β is current gain of transistor in common emitter configuration. Common emitter current gain beta (β) = Ic/Ib as in common emitter configuration input current is Ib and output current is Ic. α is large signal current gain of transistor in common base configuration α = (IcIco) / (IE-0). Alpha typically varies from 0.9 to 0.995.If we neglect reverse saturation current Ico then beta can be represented in terms of alpha, β= α / (1- α). In common collector configuration output is taken at emitter terminal, hence common collector current gain is given γ = IE/IB (input current is IB and output current is IE). γ is also called emitter injection ratio.

What is the relation between α,β and γ in a transistor?


What are the values of collector to emitter, Base to emitter saturation, active, cut in, cut off voltages?

Typical n-p-n, p-n-p transistor junction voltages at 25 Deg C

VCE,sat VBE,sat VCE,active VCE,cutin VCE,cutoff
Silicon 0.2 V, -0.2 V 0.8 V, -0.8 V 0.7 V, -0.7 V 0.5 V, -0.5 V 0,0
Germanium 0.1 V, -0.1 V 0.3 V, -0.3 V 0.2 V, -0.2 V 0.1 V, -0.1 V 0.1 V, -0.1 V

What is ICBO and ICEO in a transistor what is relation between ICEO, ICBO and ICO?

ICBO is the collector current with collector junction reverse biased and base open-circuited. ICEO is the collector current with collector junction reverse biased and emitter open-circuited.

Why ICBO is greater than ICO?

ICBO is greater than ICO for two reasons

  1. In addition to the reverse current through the junction, there exists a leakage current which flows around the junction and across the surfaces. Ileak α  Vjunction
  2. New carriers may be generated by collision in the collector junction transition region.
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