Introduction:
A bipolar junction transistor (BJT) is a three-layer semiconductor device that consists of two PN junctions. It is widely used in electronic circuits for amplification, switching, and signal processing. Understanding the number of PN junctions in a BJT is crucial for analyzing its behavior and characteristics.

Explanation:
A BJT is composed of three semiconductor regions, namely the emitter, base, and collector. Each region is doped differently to create the necessary PN junctions. Let's examine the PN junctions in a BJT in detail:

1. Emitter-Base Junction:
The emitter-base junction is formed between the heavily doped emitter region and the lightly doped base region. This junction is a PN junction, where the P-region is the base and the N-region is the emitter. The emitter-base junction is forward-biased to allow the flow of majority carriers (electrons in an NPN transistor) from the emitter to the base.

2. Base-Collector Junction:
The base-collector junction is formed between the lightly doped base region and the moderately doped collector region. This junction is also a PN junction, where the P-region is the base and the N-region is the collector. The base-collector junction is reverse-biased to prevent the majority carriers (electrons) injected from the emitter from flowing into the collector.

Summary:
In a bipolar junction transistor, there are two PN junctions:
1. Emitter-Base Junction
2. Base-Collector Junction