Bipolar Junction Transistors (BJTs)

The term transistor implies a silicon bipolar junction transistor (BJT) unless modified by an adjective such as JFET or MOSFET. BJTs can be can be made in two different configurations: NPN and PNP. Figure 1 shows a section view of an NPN BJT transistor. Here the letter N indicates silicon doped with an N-type material, which, by convention, means that it contains an excess of negatively charged electrons. The letter P indicates silicon doped with a P-type material, which means it has an excess of positively charged holes.


Figure 1 NPN bipolar junction transistor (BJT) structure.

A voltage applied to the P-type base in the NPN transistor causes electrons to flow from the N-type emitter through the base to the N-type collector. (Conventional current is considered to flow in the opposite direction). This BJT has vertical topology, so its metal base contact is deposited on the P-type base next to the metal emitter contact on the N-type emitter, while the collector contact is a metal layer on the bottom of the N-type collector.

Electrons in an NPN transistor cannot flow from the emitter to the collector through the P-type base unless a positive bias is placed on the base contact and a positive voltage is applied to the collector contact. Then holes, repelled by the positive bias, enter the emitter region while electrons flow from the emitter region to the base region. Most of the injected electrons complete the transit through the base region into the N-type collector region and are collected at its contact.



Figure 2 NPN bipolar junction transistor: (a) section view, and (b) symbol.

Figure 2a shows a simplified section view of the NPN BJT, and Fig. 2b shows its schematic symbol. The direction of the arrow represents conventional current flow directed from its P-type base to its N-type emitter.



Figure 3 PNP bipolar junction transistor: (a) section view, and (b) symbol.

Figure 3a shows a simplified section view of a PNP BJT, and Fig. 3b shows its schematic symbol. It can be seen that the polarities and doping of NPN and PNP transistors are reversed. The PNP BJT schematic symbol has its arrow directed from its P-type emitter to its N-type base.