Selection Criteria for Diodes in Circuit Design

    Diodes, also known as crystal diodes, are abbreviated as "diodes." Additionally, there are early vacuum electronic diodes. A diode is an electronic device with unidirectional conductivity. Inside a semiconductor diode, there is a PN junction with two lead terminals, and this electronic device exhibits unidirectional conductivity based on the direction of the applied voltage. Generally, a crystal diode is a PN junction interface formed by sintering a p-type semiconductor and an n-type semiconductor. On both sides of this interface, a space charge region is formed, creating a built-in electric field. When the applied voltage is zero, the diffusion current caused by the carrier concentration difference on both sides of the PN junction equals the drift current caused by the built-in electric field, resulting in electrical equilibrium. This is the characteristic of a diode under normal conditions.

1. Selection Based on Circuit Function

For high-frequency detection circuits, germanium detection diodes should be selected. Their characteristics include high operating frequency, low forward voltage drop, and low junction capacitance. 2AP1117 are used for frequencies below 40M, 2AP910 for frequencies below 100M, 2AP1~8 for frequencies below 150M, and 2AP30 for frequencies below 400M.

2. Selection Based on the Size of the Complete Unit

As the trend for complete units moves towards miniaturization, thinning, and lightening, the supporting diodes also need to be miniaturized and chip-sized. DO-35 type switching diodes and frequency band switching diodes have glass lengths of 3.8mm, DO-34 frequency band switching diodes have a glass length of 2.2mm, and SOD-23 type plastic encapsulated varactor diodes have a length of 4mm.

3. Selection Based on Cost-Performance Ratio

Select diodes rationally based on the cost-performance ratio of the complete unit and the role of supporting diodes in the complete unit. The following are guidelines for using diodes in circuit installations:

1. Use diodes below the maximum rated value.
2. Use diodes with a de-rating.
3. Use diodes well below the maximum junction temperature.
4. Properly cut, form, and install diodes.
5. Usage considerations.

Due to the trend towards miniaturization, ultra-miniaturization, and chip-size, special attention should be paid to the following:

1. For point contact type and glass diodes, prevent them from falling onto hard surfaces.
2. For glass diodes, prevent direct contact with the glass shell during soldering.
3. Do not apply forward voltage to Zener diodes.
4. Schottky diodes are susceptible to static damage; both people and equipment should be grounded.
5. For chip diodes, pay attention to the expansion coefficient of the diode itself and the printed circuit board.
6. For diodes requiring matching, prevent mixing during use to avoid affecting debugging.