Characteristics and Applications of Diodes

Semiconductor diodes are used in virtually all electronic circuits, playing a crucial role in many applications. As one of the earliest semiconductor devices, they enjoy an exceptionally broad range of uses.

The operating principle of a diode

A crystal diode consists of a p–n junction formed by p-type and n-type semiconductors. At the interface, a space‑charge region develops on both sides, giving rise to an internal electric field. In the absence of an external voltage, the diffusion current driven by the carrier concentration gradient across the p–n junction is balanced by the drift current induced by the internal electric field, resulting in electrical equilibrium. When a forward bias is applied, the external electric field opposes the internal field, reducing the barrier to carrier diffusion and thereby increasing the forward current. Conversely, under reverse bias, the external field reinforces the internal field, leading to a reverse saturation current that remains essentially constant over a certain range of reverse voltages. When the reverse voltage exceeds a critical threshold, the electric field within the space‑charge region reaches a critical value, triggering an avalanche multiplication process that generates a large number of electron–hole pairs and produces a substantial reverse breakdown current—this phenomenon is known as diode breakdown.

Classification of Diodes

There are many types of diodes. Based on the semiconductor material used, they can be classified into germanium diodes (Ge diodes) and silicon diodes (Si diodes).

Depending on their specific applications, diodes can be classified into detection diodes, rectifier diodes, Zener diodes, switching diodes, and others.

According to their chip structure, diodes can be further classified into point-contact diodes, surface-contact diodes, and planar diodes. In a point-contact diode, a very fine metal wire is pressed against the polished surface of a semiconductor wafer, and a pulsed current is passed through it, causing one end of the wire to firmly weld to the wafer and form a “PN junction.” Because this is a point contact, such diodes can only handle relatively small currents (no more than several tens of milliamperes) and are suitable for high-frequency, low-current circuits, such as demodulation in radios. Surface-contact diodes have a larger “PN junction” area, allowing them to carry higher currents (from several amperes to tens of amperes), and are primarily used in rectifier circuits to convert alternating current into direct current. Planar diodes are specially manufactured silicon diodes that not only can handle substantial currents but also exhibit stable and reliable performance, making them widely employed in switching, pulse, and high-frequency circuits.

The conductive properties of a diode

The most important characteristic of a diode is its unidirectional conductivity. In a circuit, current can flow only from the diode’s anode to its cathode. Below, we will demonstrate the diode’s forward and reverse characteristics through a simple experiment.

I. Positive Characteristics:

In an electronic circuit, when the diode’s anode is connected to the higher potential and its cathode to the lower potential, the diode conducts; this connection configuration is called forward bias. It should be noted that when the forward voltage applied across the diode is very small, the diode still does not conduct, and the forward current through the diode is extremely weak. Only when the forward voltage reaches a certain value—known as the “threshold voltage” (approximately 0.2 V for germanium diodes and about 0.6 V for silicon diodes)—does the diode begin to conduct significantly. Once conducting, the voltage across the diode remains essentially constant—around 0.3 V for germanium diodes and approximately 0.7 V for silicon diodes—and this is referred to as the diode’s “forward voltage drop.”

II. Reverse Characteristics:

In an electronic circuit, when the diode’s anode is connected to the low-potential terminal and its cathode to the high-potential terminal, virtually no current flows through the diode, placing it in the cutoff state. This configuration is referred to as reverse bias. Even under reverse bias, a small reverse current—known as leakage current—still flows. When the reverse voltage across the diode increases to a certain threshold, the reverse current rises sharply, and the diode loses its unidirectional conduction characteristic; this condition is called diode breakdown.

Main parameters of a diode

The technical specifications used to characterize a diode’s performance and its applicable operating range are referred to as the diode’s parameters. Different types of diodes have distinct characteristic parameters. For beginners, it is essential to understand the following key parameters:

1. Rated forward operating current

It refers to the maximum forward current that a diode is permitted to carry during continuous, long-term operation. When current flows through the device, the junction heats up, causing its temperature to rise. If the temperature exceeds the allowable limit—approximately 140°C for silicon diodes and about 90°C for germanium diodes—the junction can overheat and be damaged. Therefore, during operation, the diode’s forward current should never exceed its rated value. For example, the commonly used IN4001–IN4007 series of germanium diodes have a rated forward current of 1 A.

2. Maximum Reverse Operating Voltage

When the reverse voltage applied across a diode exceeds a certain threshold, the device will break down, losing its unidirectional conduction capability. To ensure safe operation, a maximum reverse operating voltage is specified; for example, the IN4001 diode has a reverse breakdown voltage of 50 V, while the IN4007 has a reverse breakdown voltage of 1000 V.

3. Reverse Current

Reverse current refers to the reverse current that flows through a diode under specified temperature and maximum reverse voltage conditions. The smaller the reverse current, the better the diode’s unidirectional conduction characteristics. It is worth noting that reverse current is closely related to temperature: for every 10°C increase in temperature, the reverse current approximately doubles. For example, a 2AP1 germanium diode has a reverse current of 250 µA at 25°C; when the temperature rises to 35°C, the reverse current increases to 500 µA, and so on. At 75°C, its reverse current reaches 8 mA, at which point the diode not only loses its unidirectional conduction property but may also overheat and be damaged. In contrast, a 2CP10 silicon diode exhibits a reverse current of only 5 µA at 25°C, and even at 75°C, the reverse current remains no more than 160 µA. Consequently, silicon diodes demonstrate superior thermal stability compared to germanium diodes under high‑temperature conditions.