Operation of BJT in active, saturation and cuttoff region

Operation of BJT

Bipolar junction transistors has two junctions base emitter junction, base collector junction. Accordingly there are four different regions of operation in which either of the two junctions are forward biased reverse biased or both. But the BJT can be effectively operated in there different modes according to the external bias voltage applied at each junction. i.e. Transistor in active region, saturation and cutoff. The other region of operation of BJT is called as inverse active region.The operation of transistor in these modes is explained below.

Transistor in active region

Active region is one in which Base emitter junction is forward biased and Base Collector junction will be reverse biased in a transistor. In NPN transistor when you bias it in active region the currents flowing through it will be as follows

 

The currents flowing through the three terminals of BJT are

Emitter current : A forward current flows from emitter into base consisting of electrons and hole current flowing from base to emitter.

Base current : A recombination current flows from the base which in the external circuit appears as base current supplied by power supply which is exactly equal to the rate at which charge carriers (holes) are lost in base due to recombination. This current will be small as base is lightly doped and numbers of charge carriers are less. Also a reverse saturation electron current flows base to collector as base collector junction is reverse biased.

Collector current : The collector current consists of two components

a) Reverse saturation current through reverse biased base collector junction. The base collector junction can be thought of as reverse biased diode. Then the current through the base collector junction from the diode current equation is given as

Irev,c = Ico (1-exp (Vbc/Vt)) in case of PNP transistor as reverse current flows from the base to collector and Vbc is negative for a reverse biased PN junction.

For a NPN transistor Irev,c = Ico*(1-exp(Vcb/Vt)) as reverse current flows from the collector to base and Vcb is negative for a reverse biased PN junction.

where Ico is reverse saturation current,Vt is voltage equivalent of temperature = k*T/e = 26 mV at 300 Deg C, k is Boltzmann’s constant =1.38*10-23 Joule/Kelvin, T is absolute temperature in kelvin,e is electronic charge = 1.6*10-19 C .

b) The emitter current left after recombination base current flows into collector. The fraction of emitter current is quantified in terms of a parameter termed as alpha.Alpha(α) is the large signal current gain which is defined as ratio of collector current increment from cut off to emitter current increment from cut off. In cut off IE = 0 amps and Ic = Ico.

α = (Ic -Ico) / (IE-0)

Large signal current gain of common base transistor α = (Ic -Ico) / (IE-0). Alpha typically varies from 0.9 to 0.995.

Summing up, the collector current is given as 

Ic= -α*IE + Ico (1-eVc/Vt)

If we neglect reverse saturation current Ico then beta can be represented in terms of alpha, β= α / (1- α) and  α = β/(1+β).

substituting the value of  α in terms of β in the equation for collector current and assuming the reverse current is ~Ico

Ic= -β*IE/(1+β) – Ico 

since IC+IB+IE =0 we will get Ic= β*(IC+IB)/(1+β) + Ico

rearranging the terms Ic= (β*IB) + Ico*(1+β)

By neglecting the reverse saturation current term there exists a linear relationship between collector current and base current in a common emitter transistor described by a parameter β also called as large signal current gain in common emitter configuration as in common emitter configuration input current is Ib and output current is Ic..

large signal current gain in common emitter configuration (β) = Ic/Ib

Transistor in Saturation region

Saturation region is one in which both Emitter Base and Base Collector junctions of the transistor are forward biased. In this region high currents flows through the transistor, as both junctions of the transistor are forward biased and bulk resistance offered is very much less.Transistor in saturation region is considered as on state in digital logic.

A transistor is said to be in saturation if and only if

 β > Ic/Ib

This is due to the fact that as both junctions of transistor are forward biased along with electron current flowing from emitter to base in active region there will be additional component of electron current flowing from collector to base.Small changes in Collector to base forward voltage leads to large variations in collector currents(variations in currents will be exponential as mentioned before through diode current equation).

Transistor in Cutoff region

In this region both junctions of the transistor are reverse biased. Hence transistor in cut off does not conduct any currents expect for small reverse saturation currents that flow across junctions. In cutoff condition emitter current is zero and the collector current consists of small reverse saturation currents. The transistor when used as switch is operated in cutoff on condition and saturation regions which corresponds to switch off an on condition respectively.

Inverse active region of transistor 

In inverse active region is just inverse or complementary to active region. In inverse active region the Base emitter junction is forward biased and Base Collector junction will be reverse biased. 

Disadvantages of operating BJT in inverse active region

Bipolar Junction Transistor is not operated in inverse active region due to following reasons

  • Bipolar junction transistor internal design is in such a way that it will have high gain in normal active mode.
  • 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 (Vbreak down α 1/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.

The different regions of operation of transistor in common emitter configuration is shown in the figure below

Output Characteristics of a Common emitter transistor

 

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