Figure 2-6.The reverse-biased junction in an NPN transistor.
At this point you may wonder why the second PN junction (base-to-collector) is not forward biased like
the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a
tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section
(base). In essence, we would have two junction diodes possessing a common base, thus eliminating any
amplification and defeating the purpose of the transistor. A word of caution is in order at this time. If you
should mistakenly bias the second PN junction in the forward direction, the excessive current could develop
enough heat to destroy the junctions, making the transistor useless. Therefore, be sure your bias voltage
polarities are correct before making any electrical connections.
NPN JUNCTION INTERACTION.We are now ready to see what happens when we place the two
junctions of the NPN transistor in operation at the same time. For a better understanding of just how the two
junctions work together, refer to figure 2-7 during the discussion.