Diodes1

In the following notes we will give an outline and salient features of various electronic devices. These notes are suitable for undergraduate courses taught in many engineering colleges and universities.
Following are some of the diodes used in practice:

• General-purpose and Rectifier Diodes

The general-purpose and rectifier diodes are the most common form. These diodes are essentially a PN junction, made from doped semiconductor. They are used to convert alternating current into direct current. The symbol of a diode is given below.

However there are general-purpose diodes used in limiting or clamping circuits. Limiting circuits are sometimes used to clip off parts of input voltages above or below certain levels. A clamper circuit is used to add a dc voltage level to an input signal voltage. Clamping action is used in  voltage multiplier circuits to increase peak voltages in rectifier circuits without increasing the input transformer's voltage rating. A multiplication of two, three or four may be used. These multipliers are found in TV receivers.

• Zener Diodes

Zener diodes are used for voltage regulation in dc power supplies. The zener diode is also a silicon PN junction device. It differs from rectifier diodes because it is designed to operate reverse biased in the reverse breakdown region. Its symbol is,

Zener diodes with breakdown voltages of less than 5 V operate in zener breakdown. Those with breakdown voltages greater than 5 V operate in avalanche breakdown. Zeners are available with breakdown voltages of 1.8 V to 200 V.

Zener diodes have the ability to keep the voltages across its terminals constant which is a key feature of the zener diode. A zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse-current values.

• Veractor Diodes

Veractor diodes also called variable capacitance diodes because the junction capacitance varies with the amount of reverse-bias voltage. These devices are commonly used in tuning circuits used in communication systems. Its symbol is

We see that the depletion region in a PN junction is widened by reverse bias and acts as a capacitor dielectric because of its nonconductive characteristic. The p and n region are conductive and can be thought as the capacitor plates.

Since depletion region widens with reverse-bias voltage, the plate separation increases, dielectric thickness increases thus decreasing the capacitance. A decrease in reverse-bias voltage narrows the depletion region and increases capacitance.

An important parameter of varactor diodes is tuning ratio which is the ratio of the diode capacitance at a minimum reverse voltage to the diode capacitance at a maximum reverse voltage. For example if the minimum and maximum reverse voltages are 4 V to 60 V and the corresponding diode capacitances are 6.8 pF and 2.3 pF, the tuning ratio is 6.8/2.3 = 2.9.

• Optical Diodes

The light-emitting diode (LED) and the photodiode are two types of optical diodes. The photodiode is also called a light detector as opposed to the LED. The symobl of LED is,

The LED operates in forward-biased operation. The wavelength of the emitted light is established by adding impurities during the doping process. The wavelength indicates the color of light and whether it is visible or invisible (infrared). LEDs are made of gallium arsenide (GaAs), gallium phosphide (GaP), or gallium arsenide phosphide (GaAsP). Silicon and germanium which are heat-producing materials are not used. GaAs LEDs produce invisible infrared radiation, while GaAsP produces red or yellow light, and GaP produces red or green light.

The photodiode or light-detecting diode is a PN junction device that operates in reverse-bias condition. Its symbol is

A photodiode has a small window that allows light to strike the PN junction. When the PN junction of a photodiode is exposed to light, the reverse current increases with the light intensity.