Diode Tutorial: Types, Characteristics, and Applications

This diode tutorial covers different types of diodes, including junction diodes, point contact diodes, Zener diodes, varicap (varactor) diodes, Gunn diodes, etc. A diode allows current to flow in one direction while blocking it in the opposite direction.

• It has very low resistance in the forward direction.
• It has very high resistance in the reverse or opposite direction.

Most semiconductor diodes are made of silicon or germanium.

diode tutorial

A diode has two leads: an anode and a cathode. The figure above depicts the diode symbol. The cathode is marked by a band at one end. The band indicates the lead from which current will leave the diode when it is forward biased (i.e., anode is positive and cathode is negative).

There are several types of diodes constructed based on their applications, such as junction diodes, point contact diodes, Zener diodes, varactor diodes, Gunn diodes, etc.

Diode Identification Codes

There are two main identification codes for diodes: the American code and the conventional system code.

• American System: Starts with “1N” followed by the diode’s serial number. For example, 1N4001.
• Conventional System: The first letter indicates the material (e.g., “A” for germanium, “B” for silicon), and the second letter indicates the diode type (e.g., “A” for signal diode, “Y” for rectifier diode, “Z” for Zener diode). For example, AA119 is a germanium signal diode.

Diode Parameters

The following two electrical parameters are considered when selecting a diode for a design:

• $I_F$ (Average Forward Current):
• $V_{RRM}$ (Maximum Reverse Voltage):

These values should not be exceeded under normal conditions. For example, the 1N4001 diode has an $I_F$ of 1A and a $V_{RRM}$ of about 50V.

Junction Diode

junction diode characteristics

A junction diode consists of a p-n junction with one lead connected to the p-side (referred to as the anode, A) and the other lead connected to the n-side (referred to as the cathode, K). The figure above depicts a typical characteristic curve for silicon and germanium diodes at a temperature of 25°C.

The junction diode is used as a rectifier in power supplies for AC to DC conversion. Larger junction areas can handle larger forward currents.

Silicon is preferable over germanium in diode construction due to the following:

• Lower reverse current (making it a more efficient rectifier)
• Higher breakdown voltage
• Ability to operate at higher temperatures

Point Contact Diode

point contact diode

The figure above depicts a germanium point contact diode. As shown, the tip of a gold or tungsten wire is connected to a pellet of n-type germanium. A p-n junction is formed when a brief current is passed through it, resulting in a tiny p-region around the tip.

Point contact diodes are used to detect radio frequency signals due to their very low capacitance or tiny junction area. Germanium is used as a signal diode because of its lower turn-on voltage compared to silicon, allowing it to start conducting in the forward direction at lower signal voltages.

Zener Diode

In a normal diode, if the reverse bias is increased until the depletion layer breaks down, the diode suffers permanent damage. A Zener diode is designed to work in the breakdown region as long as a resistor limits the current. It looks like a rectifier diode. Zener diodes are used in reverse bias at their breakdown voltage.

Varicap (Varactor) Diode

A varactor diode is used to tune TV and VHF radio stations, which helps lock the frequency to desired stations. This is known as “automatic frequency control” or AFC.

When reverse biased, a normal junction diode behaves as a capacitor because its depletion layer acts as an insulator between conductors. The conductors are the p-type and n-type regions here. The greater the area of the junction and the thinner the depletion layer, the greater the capacitance.

Most diodes are designed to have minimum capacitance, but varactor diodes are designed to have capacitance in the range of 2 to 10 pF. The value of the capacitance can be changed by varying the reverse voltage, which in turn changes the width of the depletion layer.

Gunn Diode

A Gunn diode is made using n-type gallium arsenide sandwiched between metal electrodes. It is used in microwave oscillators.