Almost in all electronic circuits, semiconductor diodes are used. They play an important role in many circuits and are one of the earliest born semiconductor devices with a very wide range of applications.
Working Principle of Diode: A crystal diode is a p-n junction formed by p-type and n-type semiconductors. At the interface, a space charge layer forms on both sides and creates a self-built electric field. In the absence of external voltage, due to the carrier concentration difference on both sides of the p-n junction, the diffusion current equals the drift current caused by the self-built electric field, maintaining an electrical balance. When there is a forward bias voltage from the outside, the mutual cancellation effect of the external and self-built electric fields increases the diffusion current of carriers, causing a forward current. When there is a reverse bias voltage from the outside, the external and self-built electric fields further strengthen, forming a reverse saturation current I0 within a certain reverse voltage range that is independent of the value of the reverse bias voltage. When the applied reverse voltage reaches a certain level, the electric field intensity in the p-n junction space charge layer reaches a critical value, leading to carrier multiplication, producing a large number of electron-hole pairs, resulting in 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 material used, they can be divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes). Depending on their different uses, they can be classified as detector diodes, rectifier diodes, regulator diodes, switch diodes, etc. According to the core structure, they can also be divided into point-contact diodes, face-contact diodes, and planar diodes. The point-contact diode is formed by pressing a very thin metal wire onto the surface of a clean semiconductor wafer and passing a pulse current, firmly bonding one end of the contact wire to the wafer, forming a "PN junction". Since it's a point contact, only a small current (not exceeding tens of milliamperes) is allowed, suitable for high-frequency low-current circuits such as radio detectors.
The "PN junction" of the face-contact diode has a larger area, allowing for higher currents (from several amperes to dozens of amperes), mainly used in "rectification" circuits that convert alternating current into direct current.
Planar diodes are specially made silicon diodes that not only can handle larger currents but also have stable and reliable performance, often used in switching, pulse, and high-frequency circuits.
Conduction Characteristics of Diodes: The most important characteristic of a diode is its unidirectional conduction. In a circuit, current can only flow into the positive terminal and out of the negative terminal of the diode. Below, we will explain the forward and reverse characteristics of diodes through simple experiments.
Forward Characteristics: In electronic circuits, connecting the positive terminal of the diode to the high potential end and the negative terminal to the low potential end will cause the diode to conduct. This connection method is called forward bias. It must be noted that when the forward voltage across the diode is very small, the diode still cannot conduct, and the forward current flowing through the diode is extremely weak. Only when the forward voltage reaches a certain value (this value is called the "threshold voltage", approximately 0.2V for germanium tubes and 0.6V for silicon tubes) does the diode truly conduct. After conducting, the voltage across the diode remains basically unchanged (approximately 0.3V for germanium tubes and 0.7V for silicon tubes), known as the "forward voltage drop" of the diode.
Reverse Characteristics: In electronic circuits, connecting the positive terminal of the diode to the low potential end and the negative terminal to the high potential end results in almost no current flowing through the diode. In this state, the diode is in a cut-off state, and this connection method is called reverse bias. When the diode is under reverse bias, there is still a weak reverse current flowing through the diode, known as the leakage current. When the reverse voltage across the diode reaches a certain value, the reverse current will increase sharply, and the diode will lose its unidirectional conduction characteristic, entering a breakdown state.
Main Parameters of Diodes: Technical indicators used to indicate the quality and applicable 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 Working Current: This is the maximum forward current that a diode can continuously pass through during long-term operation. Because current passing through the tube causes the core to heat up, and if the temperature exceeds the allowable limit (approximately 140 degrees Celsius for silicon tubes and 90 degrees Celsius for germanium tubes), the core will overheat and damage. Therefore, during use, the rated forward working current value of the diode should not be exceeded. For example, the commonly used IN4001-4007 type germanium diode has a rated forward working current of 1A.
2. Maximum Reverse Working Voltage: If the reverse voltage applied across the terminals of the diode reaches a certain value, the tube will break down and lose its unidirectional conduction ability. To ensure safe use, a maximum reverse working voltage value is specified. For example, the IN4001 diode has a reverse withstand voltage of 50V, while the IN4007 has a reverse withstand voltage of 1000V.
3. Reverse Current: The reverse current is the current flowing through the diode under specified temperature and maximum reverse voltage conditions. The smaller the reverse current, the better the unidirectional conduction performance of the tube. It is worth noting that reverse current has a close relationship with temperature; approximately every 10-degree Celsius increase doubles the reverse current. For example, for the 2AP1 type germanium diode, at 25 degrees Celsius, the reverse current is 250uA, and at 35 degrees Celsius, the reverse current rises to 500uA, and so on. At 75 degrees Celsius, its reverse current has already reached 8mA, not only losing its unidirectional conduction characteristic but also potentially overheating and damaging the tube. Another example is the 2CP10 type silicon diode, which has a reverse current of only 5uA at 25 degrees Celsius and 160uA at 75 degrees Celsius. Therefore, silicon diodes have better stability at high temperatures compared to germanium diodes.
Testing the Quality of Diodes: Beginners can use a multimeter to test the performance of diodes under amateur conditions. Before testing, set the multimeter's conversion switch to the RX1K ohm range (note not to use the RX1 range to avoid excessive current burning out the diode), then short-circuit the red and black probes and adjust the ohms to zero.
1. Forward Characteristic Test: Touch the black probe (positive inside the meter) to the positive terminal of the diode and the red probe (negative inside the meter) to the negative terminal of the diode. If the needle does not swing to 0 but stops in the middle of the scale, the resistance 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 indicates a short circuit in the core, and if the forward resistance approaches infinity, it indicates an open circuit in the core. Both short-circuited and open-circuited diodes cannot be used.
2. Reverse Characteristic Test: Touch the red probe to the positive terminal of the diode and the black probe to the negative terminal of the diode. If the needle points to infinity or near infinity, the diode is qualified.
Applications of Diodes:
1. Rectifier Diodes: Using the unidirectional conductivity of diodes, alternating current that changes direction alternately can be converted into pulsating direct current in a single direction.
2. Switching Elements: Under forward voltage, the resistance of the diode is very small, in a conducting state, equivalent to a connected switch; under reverse voltage, the resistance is very large, in a cutoff state, like a disconnected switch. Using the switching characteristics of diodes, various logic circuits can be composed.
3. Limiting Elements: After the diode conducts forward, its forward voltage drop basically remains unchanged (0.7V for silicon tubes, 0.3V for germanium tubes). Using this characteristic, it can be used as a limiting element in the circuit, restricting the signal amplitude within a certain range.
4. Continuation Diodes: In the inductor of a switch power supply and other inductive loads like relays, it plays a continuation role.
5. Detector Diodes: Plays a detection role in radios.
6. Varactor Diodes: Used in the high-frequency head of TVs.
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