Characteristics, test methods and parameters of crystal diodes

1. Characteristics of diodes

The most important characteristic of diodes is unidirectional conductivity. Its volt-ampere characteristic curve is shown in Figure 1:

1. Forward characteristics

When the forward voltage (P is positive and N is negative) at both ends of the diode is very small (germanium tube is less than 0.1 volt, silicon tube is less than 0.5 volt), the tube is not conducting and is in a "dead zone" state. When the forward voltage exceeds a certain value, the tube will conduct. When the voltage increases slightly, the current will increase sharply (see curve section I). Diodes made of different materials have different starting voltages, about 0.5-.7 volts for silicon tubes and about 0.1-0.3 for germanium tubes.

2. Reverse characteristics

When a reverse voltage is applied to both ends of the diode, the reverse current is very small. When the reverse voltage gradually increases, the reverse current remains basically unchanged. The current at this time is called reverse saturation current (see curve section II). Diodes made of different materials have different reverse currents. The reverse current of silicon tubes is about 1 microampere to tens of microamperes, while that of germanium tubes can be as high as hundreds of microamperes. In addition, the reverse current is greatly affected by temperature changes, and the stability of germanium tubes is worse than that of silicon tubes.

3. Breakdown characteristics

When the reverse voltage increases to a certain value, the reverse current increases sharply. This phenomenon is called reverse breakdown (see curve III). The reverse voltage at this time is called reverse breakdown voltage. The reverse breakdown voltage values ​​of tubes made of different structures, processes and materials vary greatly, ranging from 1 volt to several hundred volts, or even up to several thousand volts.

Figure 1. The volt-ampere characteristic curve of a diode

4. Frequency characteristics

Due to the existence of junction capacitance, when the frequency is high to a certain extent, the capacitive reactance is small enough to short-circuit the PN junction. This causes the diode to lose its unidirectional conductivity and fail to work. The larger the PN junction area, the larger the junction capacitance, and the less able it is to work at high frequencies.

2. Simple test method for diodes

The polarity of the diode is usually marked on the tube shell. If there is no mark, you can use the multimeter resistance range to measure its forward and reverse resistance to determine (usually use R×100 or ×1K). The specific method is shown in the table.

Simple test method for diode

3. Main parameters of diodes

1. Forward current IF is

the current value allowed to pass through the diode at rated power.

2. Forward voltage drop VF is

the voltage drop between the two poles when the diode passes the rated forward current.

3. Maximum rectified current (average value) IOM is

the average value of the maximum half-wave current allowed under the condition of continuous operation of half-wave rectification.

4. Reverse breakdown voltage VB is

the reverse voltage value when the reverse current of the diode increases sharply to the point where breakdown occurs.

5. Forward reverse peak voltage VRM is

the reverse voltage peak allowed when the diode is working normally. Usually VRM is two-thirds of VP or slightly smaller.

6. Reverse current IR

is the reverse current value flowing through the diode under the specified reverse voltage conditions .

7. Junction capacitance C

Junction capacitance includes capacitance and diffusion capacitance. When used in high-frequency occasions, the junction capacitance is required to be less than a certain specified value.

8. Maximum operating frequency fm

is the frequency of the highest AC signal with unidirectional conductivity of the diode.