Characteristics and Applications of Diodes

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Characteristics and Applications of Diodes

Semiconductor diodes are used in almost all electronic circuits. They play an important role in many circuits. They are one of the earliest semiconductor devices and are widely used.

Working Principle of Diode

A crystal diode is a pn junction formed by a p-type semiconductor and an n-type semiconductor. Space charge layers are formed on both sides of the interface, and a self-built electric field is built. When there is no external voltage, the diffusion current caused by the difference in carrier concentration on both sides of the pn junction and the drift current caused by the self-built electric field are equal, and the diode is in an electrical equilibrium state.

When there is a forward voltage bias in the outside world, the mutual cancellation of the external electric field and the self-built electric field increases the diffusion current of the carriers and causes a forward current.

When there is a reverse voltage bias in the outside world, the external electric field and the self-built electric field are further strengthened, forming a reverse saturation current I0 that is independent of the reverse bias voltage value within a certain reverse voltage range.

When the external reverse voltage is high enough, the electric field strength in the space charge layer of the pn junction reaches a critical value, resulting in a carrier multiplication process, generating a large number of electron-hole pairs, and generating a large reverse breakdown current, which is called the breakdown phenomenon of the diode.

Types of diodes

There are many types of diodes. According to the semiconductor materials used, they can be divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes). According to their different uses, they can be divided into detector diodes, rectifier diodes, voltage regulator diodes, switching diodes, etc. According to the core structure, they can be divided into point contact diodes, surface contact diodes and planar diodes. Point contact diodes use a very thin metal wire to press on the surface of a smooth semiconductor wafer, pass a pulse current, so that one end of the wire is firmly sintered with the wafer to form a "PN junction". Because it is a point contact, only a small current (no more than tens of milliamperes) is allowed to pass through, which is suitable for high-frequency and small current circuits, such as radio detection.

The "PN junction" area of the surface contact diode is larger, allowing a larger current (several amperes to tens of amperes) to pass through. It is mainly used in the "rectifier" circuit that converts alternating current into direct current.

A planar diode is a special silicon diode that can not only pass a large current, but also has stable and reliable performance. It is mostly used in switching, pulse and high-frequency circuits.

Conductive properties of a diode

The most important characteristic of a diode is its unidirectional conductivity. In a circuit, current can only flow into the diode from the positive electrode and out of the negative electrode. The following simple experiment will illustrate the forward and reverse characteristics of a diode.

1. Forward characteristics　
　　　　

In electronic circuits, if the positive pole of a diode is connected to the high potential end and the negative pole is connected to the low potential end, the diode will conduct. This connection method is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode still cannot conduct, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called the "threshold voltage", which is about 0.2V for germanium tubes and about 0.6V for silicon tubes) can the diode conduct directly. After conduction, the voltage across the diode remains basically unchanged (about 0.3V for germanium tubes and about 0.7V for silicon tubes), which is called the "forward voltage drop" of the diode.

2. Reverse characteristics

In electronic circuits, the positive pole of a diode is connected to the low potential end, and the negative pole is connected to the high potential end. At this time, almost no current flows through the diode, and the diode is in a cut-off state. This connection method is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, which is called leakage current. When the reverse voltage across the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unidirectional conductive properties. This state is called diode breakdown.

The main parameters of the diode

The technical indicators used to indicate the performance and application range of diodes are called diode parameters. Different types of diodes have different characteristic parameters. For beginners, it is necessary to understand the following main parameters:

1. Rated forward operating current

Refers to the maximum forward current value allowed to pass through the diode when it is working continuously for a long time. Because when the current passes through the tube, the tube core will heat up and the temperature will rise. When the temperature exceeds the allowable limit (about 140 for silicon tubes and about 90 for germanium tubes), the tube core will overheat and be damaged. Therefore, do not exceed the rated forward working current value of the diode when using it. For example, the rated forward working current of the commonly used IN4001-4007 germanium diode is 1A.

2. Maximum reverse operating voltage

When the reverse voltage applied to both ends of the diode reaches a certain value, the diode will be broken down and lose its unidirectional conductivity. In order to ensure safe use, the maximum reverse working voltage value is specified. For example, the reverse withstand voltage of IN4001 diode is 50V, and the reverse withstand voltage of IN4007 is 1000V.

3. Reverse current

Reverse current refers to the reverse current flowing through the diode under the specified temperature and maximum reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current is closely related to temperature. The reverse current doubles for every 10% increase in temperature. For example, for a 2AP1 germanium diode, if the reverse current is 250uA at 25°C, the reverse current will rise to 500uA when the temperature rises to 35°C. Similarly, at 75°C, its reverse current has reached 8mA, which not only loses its unidirectional conductivity, but also causes the tube to overheat and be damaged. For another example, a 2CP10 silicon diode has a reverse current of only 5uA at 25°C, and when the temperature rises to 75°C, the reverse current is no more than 160uA. Therefore, silicon diodes have better stability at high temperatures than germanium diodes.

Testing the diode

Beginners can use a multimeter to test the performance of diodes in amateur conditions. Before testing, first turn the multimeter's switch to the RX1K position of the ohm range (be careful not to use the RX1 position to avoid excessive current burning the diode), then short-circuit the red and black test leads and adjust the ohm zero.

1. Forward characteristics test

Touch the black probe (positive pole inside the meter) of the multimeter to the positive pole of the diode, and the red probe (negative pole inside the meter) to the negative pole of the diode. If the needle does not swing to 0 but stops in the middle of the dial, the resistance value at this time is the forward resistance of the diode. Generally, the smaller the forward resistance, the better. If the forward resistance is 0, it means that the tube core is short-circuited and damaged. If the forward resistance is close to infinity, it means that the tube core is open-circuited. Short-circuited and open-circuited tubes cannot be used.

2. Reverse characteristic test

Touch the red probe of the multimeter to the positive pole of the diode and the black probe to the negative pole of the diode. If the needle points to infinity or close to infinity, the tube is qualified.

Application of diode

1. Rectifier diode

By utilizing the unidirectional conductivity of the diode, alternating current with alternating direction can be converted into pulsating direct current with a single direction.

2. Switching elements

Under the action of forward voltage, the resistance of the diode is very small, and it is in the on state, which is equivalent to a switched on switch; under the action of reverse voltage, the resistance is very large, and it is in the off state, like a switched off switch. Various logic circuits can be formed by using the switching characteristics of the diode.

3. Limiting element

When the diode is forward-conducting, its forward voltage drop remains basically unchanged (0.7V for silicon tubes and 0.3V for germanium tubes). Using this characteristic, it can be used as a limiting element in the circuit to limit the signal amplitude within a certain range.

4. Relay diode

It plays a relaying role in the inductance of switching power supplies and inductive loads such as relays.

5. Detection diode

It plays the role of detection in the radio.

6. Varactor diode

Used in the high-frequency head of the television.

scsh

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icy

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