The simplest type of solid-state device is the diode, which is represented by the symbol shown in Fig. 9.

 Structurally, the diode is basically a p-n junction similar to those shown in Fig. 7. The n-type material which serves as the negative electrode is referred to as the cathode, and the p-type material which serves as the positive electrode is referred to as the anode. The arrow symbol used for the anode represents the direction of "conventional  current flow"; electron current flows in a direction opposite to the arrow.

Because the junction diode conducts current more easily in one direction than in the other, it is an effective rectifying  device. If an ac signal is applied, as shown in Fig. 10, electron current flows freely during the positive half cycle, but little or no current flows during the negative half cycle.

One of the most widely used types of solid-state diode is the silicon rectifier. These devices are available in a wide range of current capabilities, ranging from tenths of an ampere to several hundred amperes or more, and are  capable of operation at voltages as high as 1000 volts or more. Parallel and series arrangements of silicon rectifiers permit even further extension of current and voltage limits.

On to Zener Diodes