Non-exam preparation materials
2021.1.15
Chapter 14 Semiconductor Devices
PN junction, positive direction: P is connected to positive and N is connected to negative. It has unidirectional conductivity and is the common foundation of various semiconductor devices.
Diode: PN junction + electrode lead + case, divided into point contact type, surface contact type, and plane type.
Point contact type: generally a germanium tube with small junction capacitance and good high-frequency performance. It can be used as a switching element in digital circuits.
Surface contact type: generally silicon tube, large junction capacitance, low operating frequency, generally used for rectification.
Plane type: It can be used as a switch tube in high-power rectifier tubes and digital circuits.
Volt-ampere characteristics:
Dead zone voltage (the size is related to the material and ambient temperature), the silicon tube is about 0.5V, and the germanium tube is about 0.1V.
Reverse current: It is related to temperature, if it exceeds a certain limit, it will break down the diode.
Main parameters:
Maximum rectified current: Maximum forward average current during long-term use.
Peak reverse working voltage: generally half or two-thirds of the reverse breakdown voltage.
Reverse peak current: the reverse current value when the reverse working peak voltage is added.
Diode applications: rectification (AC to DC), detection (detecting the low-frequency signal superimposed on the high-frequency carrier), amplitude limiting (limiting the amplitude of the signal within the required range, conducting or non-conducting) , Component protection (to form a loop, freewheeling) and as a switching component in digital circuits.
Expand knowledge:
1. Diodes mainly play the role of electronic switches in digital circuits. When the electromotive force of the power supply is greater than Vd(on), it will be turned on, otherwise it will be turned off, and it will not turn on when the power supply is reversed.
2. Junction capacitance: PN junction has a capacitance effect.
3. In digital circuits, triodes are used as switching elements and work in saturation and cut-off regions.
4. Distributed capacitance: exists between two insulated conductors with voltage difference. Therefore, in any circuit, distributed capacitance will be formed between any two insulated conductors with a voltage difference, which is just a matter of the size of the distributed capacitance. At high frequencies, its impact on the circuit cannot be ignored, so it is generally necessary to take measures in high frequency circuits and precision instruments to reduce the impact of distributed capacitance. For example, when wiring a digital signal, it is necessary to consider the influence of the distributed capacitance between the lines.
5. The better the high-frequency characteristics of the system, which means that when the input signal of the system is a high-frequency signal, its output signal is also ideal.
6. The function of the detection (also called demodulation) diode is to use its unidirectional conductivity to take out the low-frequency signal or audio signal in the high-frequency or intermediate-frequency radio signal, which is widely used in semiconductor radios, tape recorders, televisions and communication equipment In the small-signal circuit, the operating frequency is higher and the processing signal amplitude is weaker.