Semiconductor diode

Semiconductor diodes can be divided into two types according to their structures: point contact type and surface contact type.

Point contact diodes are made by welding a very thin metal contact wire (such as trivalent aluminum) and a semiconductor (such as germanium) to make corresponding electrode leads, and then sealing them with a tube shell. Its structure is shown in Figure (a). The inter-electrode capacitance of point contact diodes is very small and cannot withstand high reverse voltage and large current. They are often used for small current rectification, high-frequency detection and switching tubes.

The structure of the surface contact diode is shown in Figure (b). This type of diode has a large PN junction area and can withstand large currents, but the inter-electrode capacitance is also large. This type of device is suitable for rectification but not for high-frequency circuits.

Figure (c) is a structural diagram of a planar diode in an integrated circuit, and Figure (d) is the representative symbol of a diode.

2.3.2 V-I characteristics of diodes

The V–I characteristic of a semiconductor diode is shown in the figure. The V–I characteristic is explained in three parts below.

V–I characteristic curve of diode

1. Forward characteristic

The forward characteristic is shown as segment ① in the figure. When the forward voltage is small, the forward current is almost zero. This working area is called the dead zone. Vth is called the threshold voltage or dead zone voltage (this voltage is about 0.5V for silicon tubes and 0.1V for germanium tubes). When the forward voltage is greater than Vth, the internal electric field is weakened, and the current increases rapidly, showing a very small forward resistance.

2. Reverse characteristics

The reverse characteristic is shown as segment ② in the figure. Since it is a reverse saturation current formed by a minority current carrier, its value is very small. However, temperature has a great influence on it. When the temperature rises, the reverse current will increase accordingly.

3. Reverse breakdown characteristics

The reverse breakdown characteristic corresponds to segment ③ in the figure. When the reverse voltage increases to a certain value, the reverse current increases sharply and the diode is reversely broken down. The reason is the same as that of PN breakdown.

2.3.3 Main parameters of diode

The parameters of a device are a quantitative description of its characteristics and are also the basis for our correct use and reasonable selection of devices. The main parameters of semiconductor diodes are:

Maximum rectified current IF

Refers to the maximum average forward current allowed to pass through the diode during long-term operation. It is determined by the junction area of ​​the PN junction and the external heat dissipation conditions. In actual application, the average current of the diode cannot exceed this value and must meet the heat dissipation conditions, otherwise the diode will burn out.

Maximum reverse operating voltage VR

Refers to the maximum reverse voltage allowed when using a diode. If this value is exceeded, the diode is in danger of reverse breakdown. Usually half of the reverse breakdown voltage is taken as VR.

Reverse current IR

Refers to the reverse current value when the diode is not reversely broken down. The smaller this value is, the better the unidirectional conductivity of the diode is. This value is closely related to temperature, so special attention should be paid when running at high temperatures.

Maximum operating frequency fM

It is mainly determined by the junction capacitance of the PN junction. If this value is exceeded, the unidirectional conductivity of the diode will not be well reflected.

Due to the limitations of the manufacturing process, even for the same model of tubes, the dispersion of parameters is very large, and the manual often gives a range of parameters.

The types and parameters of diodes can be found in the product manual provided by the manufacturer .