When a diode is placed in a circuit, the N-material is connected to the negative side of the circuit and the positive side of the circuit is connected to the P-material. In this configuration, which is known as forward bias, the diode is a good conductor. This is because the positively charged holes in the P-type material move toward the junction and fill these holes using them to move across the P-material. If the connections to the diodes are reversed, current flow will be blocked. This design is known as reverse bias. When the diode is connected backwards, the positively charged holes are attracted away from the junction to the negative terminal and the free electrons in the N-material are attracted away from the junction to the positive terminal. Without the presence of holes at the junction, the electrons are not able to cross it.

Zener Diodes
A zener diode (fig. 1-9) is a special type of diode that conducts current in the reverse direction as long as the voltage is above a predetermined value that is built into the device during manufacturing. For instance, a certain zener diode may not conduct current if the reverse bias voltage is below 6 volts. As the voltage

Figure 1-8.- Diode operation.

increases to 6 volts or more, the diode suddenly will begin to conduct reverse bias current. This device is used in control circuits, such as voltage regulators.