Semiconductor Basics

5The pn‐Junction

OBJECTIVES OF THIS CHAPTER

After the digression in Chapter 4, talking about one of the applications that can be understood with only the knowledge of semiconductor materials, its electrical properties and the concept of energy bands and energy gaps, we are now ready to see how by combining two different semiconductor types, one p and the other n, one with extra free holes and the other with extra free electrons at room temperature, we can create very useful devices.

In this chapter I explain how a juxtaposition of one p‐ and one n‐type semiconductor creates a pn‐junction or a semiconductor diode, a device that lets the current go in just one direction. The pn‐junction is the fundamental concept that explains all the semiconductor devices we use today.

5.1 The pn‐Junction

Consider two boxes, one full of sand and the other empty, the situation I show at the top of Figure 5.1. What happens when I bring the two boxes together and remove any barrier between them? The sand from the left‐hand box spills over to the empty box, as I show in the lower part of the figure. The sand moves to the right because of a density gradient.

Three points are very important:

• First, the left‐hand box has lost sand and the right‐hand box has gained some.
• Second, there is a force that limits and prevents more sand from sliding toward the empty box. If instead of sand I had used water, both boxes would reach the same level. This does not happen with sand because of friction, ...