Several common structures of semiconductor diodes
Several common structures of diodes are shown in Figure 1.2.2(a)(c), and the symbols are shown in Figure (d). The point contact diode shown in Figure (a) consists of a metal wire connected to the semiconductor surface through a special process to form a PN junction. Therefore, the junction area is small and large current cannot pass. However, its junction capacitance is small, generally below 1 pF, and its operating frequency can reach more than 100 MHz. Therefore it is suitable for high frequency circuits and low power rectification. The surface contact diode shown in Figure (b) is made by alloying process. The junction area is large and a large current can flow through it, but its junction capacitance is large, so it can only work at a lower frequency and is generally only used as a rectifier. The planar diode shown in Figure (c) is made using the diffusion method. Those with larger junction areas can be used for high-power rectification, while those with smaller junction areas can be used as switching tubes in pulse digital circuits.
1.2.2 Voltage-ampere characteristics of diode 1. The difference in volt-ampere characteristics between diode and PN junction is the same as that of PN junction. Diode has unidirectional conductivity. However, due to the semiconductor body resistance and lead resistance of the diode, when a forward voltage is applied, the terminal voltage of the diode is greater than the voltage drop on the PN junction when the current is the same; or in other words, when the forward voltage is the same Under high current conditions, the forward current of the diode is smaller than the current of the PN junction; under high current conditions, this effect is more obvious. In addition, due to the leakage current on the diode surface, the reverse current increases when a reverse voltage is applied. In the approximate analysis, the PN junction current equations (1.1.2) and (1.1.3) are still used to describe the volt-ampere characteristics of the diode. When measuring the volt-ampere characteristics of the diode, it was found that only when the forward voltage is large enough, the forward current increases exponentially from zero with the terminal voltage. The critical voltage that causes the diode to start conducting is called the turn-on voltage i80C20°CU, as shown in Figure 1.2.3. When the value of the reverse voltage applied to the diode is large enough, the reverse current is, I. If the reverse voltage is too large, the diode will break down. The breakdown voltage of different types of diodes varies greatly, from tens of volts to several thousand volts. 7.U(8N) Table 1.2.1 lists the order of magnitude of the turn-on voltage, forward conduction voltage range, and reverse saturation current of low-power diodes of two materials. Since the depletion layer potential U of the silicon material is larger than that of the germanium material when the PN junction of the silicon material is balanced, U of the silicon material is larger than that of the germanium material. OUM Figure 1.2.3 Voltage-ampere characteristics of the diode.
2. The influence of temperature on the volt-ampere characteristics of the diode. When the ambient temperature increases, the forward characteristic curve of the diode will shift to the left and the reverse characteristic curve will shift downward (as shown by the dotted line in Figure 1.2.3). Near room temperature, if the forward current remains unchanged, the forward voltage drop decreases by 2~2.5 mV for every 1C temperature increase; the reverse current I approximately doubles for every 10C temperature increase. It can be seen that the characteristics of the diode are very sensitive to temperature. 1.2.3 The main parameters of the diode To describe the performance of the diode, the following main parameters are often quoted: 1. The rectified current l, l. is the forward average current allowed to pass through the diode during long-term operation. Its value is related to the PN junction area and external Cooling conditions, etc. Under specified heat dissipation conditions, if the average forward current of the diode exceeds this value, it will burn out due to excessive junction temperature rise.
2. Reverse working voltage U
U is the reverse voltage allowed to be applied when the diode is working. When this value is exceeded, the diode may be damaged due to reverse breakdown. Usually (U is half of the breakdown voltage U(BB). 3. The reverse current Il is the reverse current when the diode does not break down. The smaller I is, the better the unidirectional conductivity of the diode is, and l is very sensitive to temperature.
4. The operating frequency f/s is the upper limit cutoff frequency of the diode operation. When this value is exceeded, the diode will not be able to perform unidirectional conductivity well due to the effect of junction capacitance.
It should be pointed out that due to the limitations of the manufacturing process, semiconductor device parameters are dispersed, and the parameter values of the same type of tubes will also have considerable differences. Therefore, the upper limit, lower limit or range of the parameters are often given in the manual. In addition, special attention should be paid to the test conditions of each parameter in the manual when using it. When the use conditions are different from the test conditions, the parameters will also change.
In practical applications, diodes that meet the requirements should be selected according to the conditions in which the tube is used, the reverse voltage, forward average current, operating frequency, ambient temperature and other conditions it withstands.
#common #structures #semiconductor #diodes
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