Basic knowledge and detection methods of crystal diodes 4

3. Common knowledge of diode selection

Several aspects to consider when selecting a transistor:

(1) Positive 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 volts, silicon tube is less than 0.5 volts), the tube does not conduct and is in a "dead zone" state. When the forward voltage exceeds a certain value, the tube conducts, and the voltage slightly increases, causing a sharp increase in current (see curve segment I). The starting voltage of diodes made of different materials varies, with silicon tubes ranging from 0.5-7 volts and germanium tubes ranging from 0.1-0.3 volts.

(2) Reverse characteristic

When a reverse voltage is applied across the diode, the reverse current is very small. As the reverse voltage gradually increases, the reverse current remains basically unchanged, and this current is called the reverse saturation current (see curve segment II). The reverse current of diodes made of different materials varies, with silicon tubes ranging from 1 microampere to several tens of microamperes and germanium tubes reaching up to several hundred 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, and this phenomenon is called reverse breakdown (see curve III). The reverse voltage at this point is called the reverse breakdown voltage. The reverse breakdown voltage values of pipes made of different structures, processes, and materials vary greatly, ranging from 1 volt to several hundred volts, and even up to several thousand volts.diode

(4) Frequency characteristics

Due to the presence of junction capacitance, when the frequency reaches a certain level, the capacitance impedance is small enough to short-circuit the PN junction. Causing the diode to lose unidirectional conductivity and fail to operate, the larger the PN junction area, the larger the junction capacitance, and the less it can operate at high frequencies.diode