Lecture - 6: Bipolar Transistor

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

Fig. 4

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

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

Thus, there is a steady stream of electrons leaving the negative source terminal and entering the emitter region. The V_EB 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 the 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 transistor, more than 95% of the emitter injected electrons flow to the collector, less than 5% fall into base holes and flow out the external base lead. But the collector current is less than emitter current.

Relation between different currents in a transistor:

The total current flowing into the transistor must be equal to the total current flowing out of it. Hence, the emitter current I_E is equal to the sum of the collector (I_C ) and base current (I_B). That is,

I_E = I_C + I_B

The currents directions are positive directions. The total collector current I_C is made up of two components.

1. The fraction of emitter (electron) current which reaches the collector ( a_dc I_E )

2. The normal reverse leakage current I_CO

a_dc is known as large signal current gain or dc alpha. It is always positive. Since collector current is almost equal to the I_E therefore αdc I_E varies from 0.9 to 0.98. Usually, the reverse leakage current is very small compared to the total collector current.

NOTE: The forward bias on the emitter diode controls the number of free electrons infected into the base. The larger (V_BE) forward voltage, the greater the number of injected electrons. The reverse bias on the collector diode has little influence on the number of electrons that enter the collector. Increasing V_CB does not change the number of free electrons arriving at the collector junction layer.

The symbol of npn and pnp transistors are shown in fig. 5.

Fig. 5

Breakdown Voltages:

Since the two halves of a transistor are diodes, two much reverse voltage on either diode can cause breakdown. The breakdown voltage depends on the width of the depletion layer and the doping levels. Because of the heavy doping level, the emitter diode has a low breakdown voltage approximately 5 to 30 V. The collector diode is less heavily doped so its breakdown voltage is higher around 20 to 300 V. 0http://ecmagic.blogspot.com