Why do point contact diode junctions have small capacitance and high operating frequency? Please tel

Point contact diode is to place a metal wire (generally with gold as material) as the electrode (positive electrode) on the germanium (silicon) plane, because the contact surface of this diode is very small, so the formation of junction capacitance is also very small (the capacity of the capacitor depends on the relative area and relative distance of the capacitor plate, the larger the relative area, the larger the capacitance, the smaller the relative distance, the larger the capacitance). The junction capacitance of the diode has little effect when the diode operates at a low frequency, but at a high operating frequency, the effect is very prominent (the higher the frequency of the capacitance, the smaller the capacitive reactance), and the rectification effect will be lost to the high-frequency signal. Capacitive reactance is the resistance of a capacitor to alternating current.

Point contact diodes are generally used as frequency modulation detection circuits, such as the detection level of radios, the monitoring frequency of TV signals from intermediate frequency signals to audio signals, etc.

Since the contact surface of the point contact diode is very small, the point contact diode cannot pass large currents, so it cannot be used in rectifier circuits with large currents.

Point-contact diodes are formed by pressing a metal pin on a single wafer made of germanium or silicon and then passing the galvanic method. Therefore, the capacitance of the p-n junction is small, making it suitable for high-frequency circuits. However, compared to the surface junction type, the forward and reverse characteristics of the point-contact diode are poor, so they cannot be used for high currents and rectification.

Because of the simple construction, it is cheap. It is a type with a wide range of applications for general purposes such as detection, rectification, modulation, mixing, and limiting of small signals.

1: Point contact diode: due to the small junction area, the junction capacitance is small, in addition to being used as an ordinary diode, it can also be used for detection

At this time, the diode is equivalent to a capacitor) [can filter out low-frequency signals and detect high-frequency signals, why? The capacitance of the capacitance for the signal is 1 jwc (w is the frequency, c is the capacitance), for the low frequency, w is small, c is also very small, the capacitive reactance will be very large, then the low frequency will naturally not pass, and it will be directly filtered out, and for the high-frequency signal, because w is very large, c is small, the capacitive reactance can be very small, then the high frequency can be passed through, and it is detected].

Rectification: (at this time, it is equivalent to a capacitor and an ideal diode in parallel) [because of the unidirectional conductivity of the diode, the AC signal is integrated into a DC signal, but it can only be used for low-frequency rectification, and the high-frequency signal can pass through (from the capacitor in parallel), and the purpose of rectification cannot be achieved], so in a nutshell, it can be used for high-frequency detection and low-frequency low-power rectification in addition to ordinary diodes.

2: surface contact diode (equivalent to a capacitor and an ideal diode in parallel): due to the large junction area, the junction capacitance is also large, if used for high-frequency rectification, because the capacitive reactance of the junction capacitor for the high-frequency signal is 1 jwc is very small (w is larger, c is larger), the high-frequency signal passes directly (from the parallel capacitor), can not achieve the purpose of rectification, and for the low-frequency signal, the capacitive reactance of 1 jwc is larger (w is smaller, c is larger), can not pass from the capacitor in parallel, then pass through the ideal diode below, Rounding AC to DC (diode unidirectional conductivity).

Answered

One of the functions of the capacitor is to pass on the high frequency and resist the low frequency, the higher the frequency, the smaller the capacitive reactance, so the larger the capacitance of the diode junction, the stronger the conduction ability, which destroys the characteristics of the diode itself.

On the other hand, the diode junction capacitance is very large, and when it is passed into high frequencies, it will accumulate a large amount of charge, and its release takes time, which is equivalent to adding a capacitor to an oscillation circuit, changing its oscillation frequency and destroying the wave pattern.

The effect of junction capacitance on diodes is mainly the second type!