Bipolar transistor:

A transistor is generally a Si on Ge crystal having three separate regions.  It  can either  be  NPN  or PNP type figure1. The middle region is called as base and the outer two regions are called as emitter and the collector. The outer layers although they are of the same type but their functions can't be changed. They have different physical and electrical properties both.

In    most   of the  transistors, emitter is very heavily doped. Its job is to emit or inject electrons into base. These bases are slightly doped and very thin, it passes most of the emitter injected electrons on to the collector. The doping level of collector is intermediate between the heavy doping of emitter and the light doping of base.

Fig. 1

The collector is named so because it collects electrons from the base. The collector is largest of the three regions; it must dissipate more heat than emitter or base. The transistor has 2 junctions. One between emitter and base and other between base and collector. Because of this the transistor is similar to two diodes, one emitter diode and the other collector base diode.

The depletion layers don't have the same width, because different regions have different doping levels.  The more heavily doped  a region  is,  greater the concentration of ions  near  the junction. This means that the depletion layer penetrates more deeply into the base and slightly into the emitter. Likewise, it penetration more into collector. The thickness of the collector depletion layer is large while the base depletion layer is small as shown in the figure 2. 

When a transistor  is  made,  diffusion  of  the free  electrons  across  junction  produces  two depletion layers. For each of the depletion layers, the barrier potential is 0.7 V for Si transistor and 0.3 V for the Ge transistor.

Figure 2

If both the junctions are forward biased using 2 DC sources, as shown in the figure 3a. free electrons enter emitter and collector of transistor, joins at base and come out of base. Because both the diodes are forward biased, emitter and collector currents are large.

                           Fig. 3a                                                                                                  Fig. 3b

If both the junction are reverse biased as shown in the figure 3b, then small currents flows through both junctions only  because of  thermally produced  minority carriers  and  surface  leakage.  Thermally produced  carriers  are  temperature dependent  it  nearly doubles  for  every  10  degree Celsius rise in the ambient temperature. The surface leakage current increases with voltage.

When emitter diode is forward biased and collector diode is reverse biased as shown in the figure 4 then one expect the large emitter current and small collector current but collector current is almost as large as the emitter current.

Figure 4

When  the emitter  diodes  are forward  biased  and  the  applied  voltage  is  over  0.7  V  (barrier potential)  then the larger number  of  majority  carriers  (electrons  in  N type)  diffuse  across  the junction.

Once the electrons are injected by emitter enter into base, they become minority carriers. These electrons don't have separate identities from those, which are generated thermally, in the base region itself. The base is made quite thin and is very lightly doped. Because of this only few electrons traveling from emitter to base region recombine with the holes. This gives rise to the recombination current. The rest of the electrons exist for more time. Since the collector diode is reverse biased, (n is connected to the positive supply) thus most of the electrons are pushed into collector layer. These collector elections can then flow into external collector lead.

Therefore, there is a steady stream of the electrons leaving negative source terminal and entering emitter region. The VEB forward bias forces these emitter electrons to enter the base region. The thin and lightly doped base gives almost all those electrons enough lifetime to diffuse into depletion layer. The depletion layer field pushes a steady stream of electron into the collector region. These electrons leave the collector and flow into the positive terminal of the voltage source. In most of the transistors, over 95% of the emitter injected electrons flow to collector, below 5% fall into base holes and flow out the external base lead. But the collector current is below emitter current.

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