Jun 18, 2016

Principle Operation of Transistor



Principle Operation of Transistor (P-N-P)

The basic connection of a p-n-p transistor is shown in the figure below.
principle operation of transistor p-n-p

Let us consider the principle operation of transistor action at emitter-base junction. It is forward biased and when the collector is left floating then majority carriers (holes) from the emitter diffuse into the base and majority carriers (electrons) from the base diffuse into the emitter.
Now if the base-collector is reverse biased and the emitter is left floating, then the width of the depletion layer at the base collector junction increases hence there is no flow of majority carriers. But there is a flow of current due to thermally generate minority carriers.

Now consider the action of a p-n-p transistor whose emitter-base junction is forward biased and collector –base junction is reverse biased. A large number if majority carriers (holes, say X in the emitter and the electrons, Y in the base will diffuse across the forward biased p-n junction.
Hence the total current flowing across this junction is the sum of hole diffusion current and electron diffusion current. This total current is called the emitter current, denoted by IE,
Emitter Current IE= IP(x) + In(Y)
Since the central region, i.e. base is very thin and lightly doped, the majority carriers that starts from the emitter get very little chance to recombine in the base and across the base region to reach the collector. Only a few majority carriers (x) are lost due to recombination. In practice about 2 percent of ht emajority carres which diffuse into the base, recombine with electrons.
The total loss of the number of electrons is (Y+y) is made up by the flow of an equal number of electrons from the negative terminal of a dc battery VEE in to the base. The base by such electrons constitutes the base current IB. Hence the magnitude of the base current, IB is very small and is the order of micro-amperes.
Base current IB = In(Y+y)

If X(=y) is the number of holes that combines with the electrons then the loss of the total number of holes in the emitter is (X+x). This loss made up of flow of an equal number of electrons from the emitter to the positive terminal of the dc battery VEE. These electrons from the emitter to the positive terminal of the dc battery VEE. These electrons are released form covalent bonds of the crystal atoms in the emitter and an equal number of holes is created.
The potential VCC in the right hand section os such that the carriers reaching that the section get easily attracted and as current starts flowing. This current is called the collector current Ic which is almost equal to emitter current.
The collector current has also a second component of current. This current is due to minority carriers called reverse saturation current, ICBO flows through a reverse biased collector diode. Hence the collector current IC therefore, is sum of the leakage current, ICBO and remaining emitter current [=IE(X-y)]
Collector current, I= ICBO(minority) + IC(majority)
                                    = IC(X-y) + ICBO

The larger the number of carriers reaching the collector from the emitter, the larger power gain if the device. For this, the base should be very thin and the area of collector–base junction should be large. So the collector current is only slightly than the emitter current.
Applying Kirchoff’s current law  to the transistor, we get,
IE = IC + IB.  

Principle Operation of N-P-N Transistor

principle operation of n-p-n transistor

The figure above shows the n-p-n trnasistor with forward bias to emitter-base junction and reverse bias to collector base junction. The forward bias VEE causes the majority carriers (electrons) in the n-type emitter to diffuse into the base and holes from the base diffuse into the emitter.
The flow of majority carries constitutes the emitter current. As these electrons flows through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, only a few combine with holes. As the base is slightly doped and very thin, only a few electrons combine with holes to form base current IB.

The remaining electrons cross over into the collector region to constitute collector current IC. This collector current is also called injected current because this current is produced due to electrons injected from the emitter region.
It is clear that emitter current is the sum of collector current and base current.
i.e. IE=IC + IB

Points to remember:

  • The emitter current of a transistor consist of two components namely base current and collector current.
  • The base current is a very small fraction of emitter current i.e. about 2% of the emitter current and collector current current is about 98% of the emitter current.
  • The collector current is mainly due to injected electrons (majority carriers) from the emitter. The collector current has also a second component due to thermally generated minority carriers, called reverse saturation current.
  • Emitter-base junction is always forward-biased and collector-base junction is always reverse biased.


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