Bipolar Junction Transistors (BJT)

By placing two PN junctions together we can create a bipolar junction transistor. In a PNP transistor the majority charger carriers are holes and typically germanium is favored for these devices. In NPN transistors the majority charger carriers are electrons; for this case, silicon is typically used.

The thin and lightly doped central region is known as the base (B) and has the majority charge carriers of opposite polarity to those in the surrounding material. The two outer regions are known as the emitter an (E) and the collector (C). Under the proper operating conditions the emitter will emit or inject majority charge carriers into the base region, and because the base is very thin, most will ultimately reach the collector. The emitter is highly doped to reduce resistance. The collector is lightly doped to reduce the junction capacitance of the collector-base junction. The emitter-base junction is forward biased while the collector-base junction is reversed biased. The resistance emitter-base junction is very small as compared to collector-base junction. Therefore, the forward bias applied to the emitter-base junction is generally small whereas reverse bias on the collector-base junction is much higher.

Related: The Basics of Semiconductor Physics as the Foundation of Electronics

The Transistor Operation (NPN)

The figure below shows the NPN transistor with forward bias to emitter-base junction and reverse bias to collector-base junction.

The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current I_E. As these electrons flow through the p-type base, they tend to combine with the holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with the holes to constitute base current I_B. In other words, the electrons which combine with holes become valence electrons. Then as valence electrons, they flow down through holes and into the external base lead. This constitutes base current I_B. The remainder (i.e. more than 95%) cross over into the collector region to constitute collector current I_C. In this way, almost the entire emitter current flows in the collector circuit. Hence the emitter current is the sum of collector and base currents:

I_E = I_B + I_C

Most of the electrons from emitter continue their journey through the base to collector to form collector current because:

• The base is lightly doped and very thin; therefore, there are few holes which find enough time combine with electrons.
• The reverse bias on collector is quite high and exerts attractive forces on these electrons.

Transistor Operation (PNP)

The figure below shows the basic connection of a PNP transistor.

The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current I_E.

As these holes cross into the n-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, hence, only a few holes (less than 5%) combine with the electrons. The remainder (more than 95%) cross into the collector region to constitute the collector current I_C. As a result, almost the entire emitter current flows in the collector circuit. You may note that, current conduction within PNP transistor is by holes. However, in the external connecting wires the current is still by electrons.

Related: Semiconductor Diodes

Bottom Line

The input circuit (emitter-base junction) has low resistance because of forward bias whereas the output circuit (collector-base junction) has high resistance due to reverse bias.

From our discussion above, the input emitter current almost entirely flows in the collector circuit. Therefore, a transistor transfers the input signal current from a low resistance circuit to a high resistance circuit. This is an important factor responsible for the amplifying capability of the transistor