Working Principle of Transistors

When no voltage is applied across the transistor, diffusion of free electrons across the junctions produces two depletion layers. For each depletion layer, the barrier potential is about 0.7 V at 25°C for a silicon transistor and 0.3 V for a germanium transistor.

Silicon transistors are more widely used than germanium transistors because of higher voltage rating, greater current ratings, and low temperature sensitivity.

Since the three regions in a transistor have different doping levels, the depletion layers have different widths. If a region is heavily doped, the concentration of ions near the junction will be more, resulting in thin depletion layer and vice versa.

Since the base is lightly doped as compared to emitter and collector, the depletion layers extend well into it, whereas penetration in emitter/collector regions is to a lesser extent. Moreover, the emitter depletion layer is narrower compared to collector depletion layer.

In order to make a transistor function properly, it is necessary to apply suitable voltages to its terminals. This is called biasing of the transistor.

A n-p-n Transistor

The emitter-base junction is forward biased while the collector-base junction is reverse biased. When forward bias is applied to the emitter, free electrons in the emitter have to overcome the barrier potential to enter the base region.

When VBE exceeds barrier potential (0.6 to 0.7 V for silicon transistor), these electrons enter the base region. Once inside the base, these electrons can flow either through the thin base into the external base lead or across the collector junction into the collector region.

The downward component of base current is called recombination current. It is small because the base is lightly doped and only a few holes are available. Since the base region is very thin and it receives a large number of electrons, for VBE > 0.7 V, most of these electrons diffuse into the collector depletion layer. The free electrons in this layer are pushed (by the depletion layer field) into the collector region and flow into the external collector lead.

So, a steady stream of electrons leaves the negative source terminal and enters the emitter region. The forward bias forces these electrons to enter the base region. Almost all these electrons diffuse into the collector depletion layer through the base. The depletion layer field then pushes a steady stream of electrons into the collector region.

In most transistors, more than 95 percent emitter-injected electrons flow to the collector; less than 5 percent flow to the external base lead.

The relation between collector current (IC) and emitter current (IE) is expressed in terms of signal current gain, α, of a transistor. It is defined as

α = IC/IE

The value of α is nearly equal to but always less than one.

Similarly, you can relate the collector current to the base current in a transistor.

β = IC/IB

Beta signifies the current gain of the transistor in common-emitter configuration. The value of β is significantly greater than one.

Since emitter current equals the sum of collector current and base current,

IE = IC + IB

On dividing throughout by IC,

1/α = 1 + 1/β

β = α/(1–α)

A p-n-p Transistor

In a p-n-p transistor, reverse the battery terminals when n-p-n transistor is substituted by p-n-p transistor. The emitter-base junction is forward biased by battery of voltage VEB and the collector-base junction is reverse biased by a battery of voltage VCB.

The resistance of the emitter-base junction is very small due to its forward bias as compared to the collector base junction (which is reverse biased). Therefore, you apply small forward bias voltage (0.6 V) to the emitter-base junction, whereas the reverse bias voltage applied to the collector-base junction is of much higher value (9 V).

The forward bias of emitter-base junction makes the majority carriers, that is the holes, in emitter (p-region), to diffuse to the base (n-region), on being repelled by the positive terminal of the battery. As width of the base is extremely thin and it is lightly doped, very few (two to five percent) of total holes that enter the base recombine with electrons and 95% to 98% reach the collector region.

Due to reverse bias of the collector-base region, the holes reaching this region are attracted by the negative potential applied to the collector, thereby increasing the collector current (IC). Therefore, increase in emitter current (IE) increases collector current.