Characteristics and Applications of Diodes

Operating Principle of Diode

Crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. Space charge layer is formed on both sides of its interface and self-built electric field is built. When there is no applied voltage, the diffusion current caused by carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the built-in electric field, so it is in the state of electric equilibrium.

When there is a positive voltage bias, the interaction between the external electric field and the self-built electric field causes the increase of carrier diffusion current and the positive current.

When there is reverse voltage bias, the external electric field and the self-built electric field are further strengthened, and the reverse saturated current I0 independent of the reverse bias voltage is formed in a certain range of reverse voltage.

When the applied reverse voltage is high to a certain extent, the electric field intensity in the space charge layer of p-n junction reaches the critical value, resulting in the multiplying process of carriers, a large number of electron hole pairs and a large number of reverse breakdown currents, which are called diode breakdown phenomena.

Types of Diodes

There are many kinds of diodes. According to the semiconductor materials used, they can be divided into germanium diode (Ge tube) and silicon diode (Si tube). According to its different uses, it can be divided into detector diode, rectifier diode, regulator diode, switch diode and so on. According to the core structure, it can be divided into point contact diode, surface contact diode and planar diode. The point contact diode is a kind of semiconductor wafer with a very thin wire pressed on the surface of the wafer, and connected with a pulse current, so that one end of the contact wire and the wafer are firmly sintered together, forming a "PN junction". Because of point contact, only small currents (no more than tens of milliamperes) are allowed to pass through, which is suitable for high frequency low current circuits, such as radio detection.

The "PN junction" area of the surface contact diode is large, which permits the passage of large currents (several to dozens of amperes). It is mainly used in the "rectifier" circuit for converting alternating current into direct current.

Planar diode is a special silicon diode. It can not only pass through large current, but also has stable and reliable performance. It is mostly used in switching, pulse and high frequency circuit.

Conductivity Characteristics of Diodes

The most important characteristic of a diode is its unilateral conductivity. In the circuit, the current can only flow from the positive pole to the negative pole of the diode. Following is a simple experiment to illustrate the forward and reverse characteristics of the diode.

Forward characteristic

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

2. Reverse characteristics

In the electronic circuit, the positive pole of the diode is connected to the low potential end, and the negative pole is connected to the high potential end. At this time, there is almost no current flowing through the diode. At this time, the diode is in the cut-off state, which is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage at both ends of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unilateral conductivity. This state is called the breakdown of the diode.

Main parameters of diodes

The technical specifications used to indicate the performance and scope of application of the diode are called the parameters of the diode. Different types of diodes have different characteristic parameters. For beginners, the following main parameters must be understood:

1. rated forward working current

The maximum forward current allowed for a diode to pass in long-term continuous operation. Because when the current passes through the tube, the core will be heated and the temperature will rise. When the temperature exceeds the allowable limit (about 140 silicon tube and 90 germanium tube), the core will be overheated and damaged. Therefore, the rated forward working current of the diode should not be exceeded in the use of the diode. For example, the rated forward operating current of the commonly used IN4001-4007 Germanium Diode is 1A.

2. Maximum reverse operating voltage

When the reverse voltage at both ends of the diode reaches a certain value, it will break down the tube and lose the single-guide electric power. In order to ensure safe operation, the maximum reverse operating voltage is specified. For example, the reverse voltage withstand of IN4001 diode is 50V, and that of IN4007 diode 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 is, the better the unidirectional conductivity of the tube is. It is noteworthy that the reverse current has a close relationship with temperature, and the reverse current doubles every 10% rise in temperature. For example, if the reverse current of 2AP1 Germanium Diode is 250uA at 25 o'clock and the temperature rises to 35, the reverse current will rise to 500uA, and so on. At 75 o'clock, the reverse current of 2AP1 Germanium Diode reaches 8mA, which not only loses the unilateral conductivity, but also causes the tube to be overheated and damaged. For example, the reverse current of 2CP10 silicon diode is only 5uA at 25 and 160uA at 75. Therefore, silicon diode has better stability than germanium diode at high temperature.

Testing diodes

Beginners can use multimeter to test the performance of diodes in amateur conditions. Before testing, switch the multimeter to RX1K gear of Ohm gear (pay attention not to use RX1 gear, in order to avoid excessive current burning diodes), and then short circuit the red and black pens to zero ohm.

1. Forward Characteristic Testing

The black pen of the multimeter (positive in the meter) touches the positive of the diode, and the red pen (negative in the meter) touches the negative of the diode. If the needle is stopped in the middle of the dial instead of setting to zero, then the resistance is the forward resistance of the diode. Generally, the smaller the forward resistance, the better. If the forward resistance is zero, it means that the core is damaged by short circuit. If the forward resistance is close to infinite value, it means that the core is broken. Short-circuit and open-circuit pipes can not be used.

2. Reverse Characteristic Testing

If the red pen of a multimeter touches the positive pole of the diode and the black pen touches the negative pole of the diode, the tube will be qualified if the needle points at or near the infinite value.

Application of Diodes

1. Rectifier Diode
By using the single conductivity of the diode, alternating alternating direction AC can be converted into pulsating DC in a single direction.

2. Switching Elements
The resistance of the diode under forward voltage is very small, and it is in the on state, which is equivalent to a switch on. Under reverse voltage, the resistance is very large, and it is in the cut-off state, just like a disconnected switch. Using the switching characteristics of the diode, various logic circuits can be formed.

3. Limiting element
The forward voltage drop of the diode remains unchanged (0.7V for silicon tube and 0.3V for germanium tube). Using this characteristic, as a limiting element in the circuit, the signal amplitude can be limited to a certain range.

4. Relay Diode
It plays a relay current role in inductance neutralization of switching power supply and inductive loads such as relays.

5. Detector Diode
It acts as a demodulator in the radio.

6. Varistor Diode
High Frequency Head for TV Set