Transistor bipolar junction transistor There's Big [desired result]

Bipolar Junction Transistor (BJT) 

      

Bipolar junction transistor construction

• BJT is a three-layer semiconductor device consisting of two PN junctions. 
•  N-type material is sandwiched between two layers of P-type, the transistor is known as the PNP transistor.
• On the other hand, a layer of P-type material is sandwiched between two layers of N-Type, the transistor is known as NPN transistor. 
• The center layer is the base, the left layer is the emitter and the right layer is  a collector.
•  EMITTER = E,
• BASE=B,
• COLLECTOR=C,
•  The emitter layer is heavily doped it emits electrons if the transistor is of NPN type (emits holes if the transistor is of PNP type) to the base.
•  The base is lightly doped and the doping level of the collector is intermediate. 
• The collector is so named because it collects electrons if the transistor is of NPN type (collects holes if the transistor is of PNP type) from the base. 
• The collector is the largest of the three regions. 
• To have a better understanding of transistors we will concentrate on NPN transistors in the next sections.

        

Bipolar junction transistor construction

• Let us consider the NPN transistor shown in Fig. 
• There are two junctions in it.
•  One is between emitter and base and the other is between collector and base. 
• Because of their repulsion, the free electrons on then sides diffuse across the emitter-base junction and recombine with the holes in the base.
•  Similarly, the electrons in the collector diffuse across the collector-base junction. 
• When a free electron in the n-layer diffuses across the junction, it leaves the parent pentavalent atom in the n-layer and makes it a positive ion. 
• After diffusion, it combines with the parent trivalent atom in the p-region making it negative.
•  As a result, a layer of depleted carriers is formed at the junction.
• This layer of depletion without free charge carriers is known as the depletion layer. 
• Beyond a certain point, the depletion layer acts as a barrier to further diffusion of free electrons across the Junction.
• The difference of potential across the diffusion layer is called barrier potential. 
• Since the doping levels of the three layers are different, the thickness of depletion layers does not have the same width. 
• As the emitter is heavily doped, the concentration of ions near the junction is greater; hence the depletion layer penetrates slightly into the emitter region.
•  But the depletion region penetrates deeply into the base, as it is lightly doped. 
• As the collector layer is moderately doped, the depletion layer width in the collector is more than the depletion layer width in the emitter. 
• Hence the depletion layer width in the emitter-base junction is less than the depletion layer width in the collector-base junction.

         

Bipolar junction transistor block diagram

 Transistor Biasing

         

Transistor with EB, CB junctions reverse biased

Transistor with EB, CB junctions reverse biased

Transistor with EB and CB junctions forward biased

• For the proper operation of a transistor, the two PN junctions must be correctly biased with external de voltages. Fig shows an NPN transistor in which the emitter-base and collector-base junction are forward biased. 
• Since both junctions are forward biased the emitter and collector currents are large. Fig shows the transistor in which both emitter-base and collector-base junctions are reverse biased. 
• In this case, the currents are due to thermally generated minority carriers.
•  The third type of bias is most commonly used to operate the transistor as an amplifier. In this bias, the emitter-base junction is forward biased and the collector-base junction is reverse biased.

Operation of NPN Transistor

           

Transistor with forward-biased EB junction and reverse biased CB junction

Transistor with reverse biased EB junction and reverse biased CB junction

Transistor with forward-biased EB junction and reverse biased CB junction 

• Now we discuss the basics of the operation of an NPN transistor. Fig Shows an NPN with a forward-biased emitter-base (EB) junction and a forward-biased collector-base(CB) junction. 
• Due to forward bias, the barrier potential at the EB junction and at CB Junction reduces and electrons flow from n-type to p-type.
•  Both the emitter and collector currents are large.
• When both junctions are reverse biased the flow of current is due to thermally generated minority carriers. 
• This thermally produced current is temperature-dependent.
•  It approximately doubles for every 10°C rises in ambient temperature.
• For a normal transistor operation, the EB junction is forward biased and the CB junction is reverse biased. 
• When EB junction is forward biased the barrier potential reduces and causes a large number of electrons to flow from the n-type emitter to the p-type base. 
• Since the base is very thin and lightly doped a small portion (1% to 4%)of base electrons will recombine with holes and constitute a base current. 
• The magnitude of base current is typically in the order of microamperes. 
• The electrons that are not recombined in the base region are collected by the collector and constitute a collector current.

Operation of a PNP Transistor

     

Biasing of pnp transistor

• The operation of a PNP transistor can be explained using Fig. In Fig the emitter-base junction and collector-base junctions are forward biased. 
• Hence both the emitter and collector currents are large.
•  Fig shows the PNP transistor when both the junctions are reverse biased.
•  In this case, the emitter and collector currents are small and are due to thermally generated minority carriers.
•  In Fig the emitter-base junction is forward biased and collector-base junction is reverse biased.
•  The forward biased emitter-base junction causes a lot of holes to cross from the emitter region to the collector region. 
• Since the base is lightly doped the number of electrons in this region is small. 
• Hence the number of holes combining with electrons in the base is also small which results in a small base current. 
• The remaining holes that are not combined (more than 95%) cross collector-base junction to constitute a collector current.