What is Bipolar Junction Transistor(BJT)?

The Bipolar
Junction Transistor:
The bipolar junction transistor consists of three regions of semiconductor material. One type is called a p-n-p transistor, in which two regions of p-type material sandwich a very thin layer of n-type material. A second type is called an n-p-n transistor, in which two regions of n-type material sandwich a very thin layer of p-type material. Both of these types of transistors consist of two
p-n junctions placed very close to one another in a back-to-back arrangement on a single piece of semiconductor material. Diagrams depicting these two types of transistors are shown in Figure 1.
The two p-type material regions of the p-n-p transistor are called the emitter and collector and the n-type material is called the base. Similarly, the two n-type material regions of the n-p-n transistor are called the emitter and collector and the p-type material region is called the base, as shown in Figure 1.
Transistors have three connecting leads and in operation an electrical input to one pair of connections, say the emitter and base connections can control the output from another pair, say the collector and emitter connections. This type of operation is achieved by appropriately biasing the two internal p-n junctions. When batteries and resistors are connected to a p-n-p transistor, as shown in Figure 2(a), the base-emitter junction is forward biased and the base-collector junction is reverse biased. Similarly, an n-p-n transistor has its base-emitter junction forward biased and its base-collector junction reverse biased when the batteries are connected as shown in Figure 2(b).

Figure 1
For a silicon p-n-p transistor, biased as shown in Figure 2(a), if the
base-emitter junction is considered on its own, it is forward biased and a current flows. This is depicted in Figure 3(a). For example, if RE is 1000 Ω  , the battery is 4.5 V and the voltage drop across the junction is taken as 0.7 V, the current flowing is given by (4.5-0.7) /1000 = 3.8 mA.

Figure 2
When the base-collector junction is considered on its own, as shown in Figure 3(b), it is reverse biased and the collector current is something less than 1 µA. However, when both external circuits are connected to the transistor, most of the 3.8 mA of current flowing in the emitter, which previously flowed from the base connection, now flows out through the collector connection due to transistor action.

Figure 3