Introductory Electrical Engineering With Math Explained in Accessible Language

14Diodes and Transistors: Active Components

14.1 Introduction

In this chapter, we will examine diodes and transistors, the first two practical applications coming from the invention of N‐type and P‐type materials.

14.2 Diodes

We have explained before that PN junctions allow current to flow in one direction but not on the other. This is the exact definition of a diode.

When a diode is forward biased, current flows and behaves, roughly, like a closed switch or like a resistor of small value.

However, if reverse biased, a diode will block current flow and behave like an open switch or a resistor of large value.

Figure 14.1 shows the symbol used for diodes. The triangle is the anode and the line represents the cathode, equivalent to materials P and N, respectively. The anode is the positive side and the cathode is the negative side.

Figure 14.1 Diode symbol.

14.3 NPN Junction

Suppose we add one more N‐type block to the PN structure and create an NPN block like shown in Figure 14.2. What happens now?

Figure 14.2 NPN structure.

Now, we have two electrical barriers between the two N‐type and the P‐type materials.

If the junction PN is equal to a diode, this NPN block is equal to two connected diodes like shown in Figure 14.3.

Figure 14.3 NPN junction “diodes.”

Instead ...