Through modern technology, we form a P-type semiconductor on one side of an intrinsic semiconductor and an N-type semiconductor on the other side, so that a P-N junction is formed at the junction of the two semiconductors. It is the basis for the formation of other semiconductors, and we must master its characteristics!
1. Phenomenon of special-shaped semiconductor contact
In the formed P-N junction, since the concentration of electrons and holes on both sides is very different, they will diffuse: electrons diffuse from the N region to the P region; holes diffuse from the P region to the N region. Because they are both charged particles, they diffuse to the other side while leaving positively charged holes in the N region and negatively charged impurity ions in the P region, thus forming a space charge region, that is, forming an electric field (self-built field). Their formation process is shown in Figures (1) and (2).
Under the action of the electric field, the carriers will drift, and their movement direction is opposite to the diffusion movement, which prevents the diffusion movement. The strength of the electric field is related to the degree of diffusion. The more diffusion, the stronger the electric field, and the greater the resistance to the diffusion movement. When the diffusion movement is equal to the drift movement, the number of carriers passing through the interface is 0. At this time, the junction area of the PN junction forms a high-resistance area lacking carriers, which we also call a barrier layer or depletion layer.
By applying voltages in different directions at both ends of a PN junction, we can destroy its original balance, causing it to exhibit unidirectional conductivity.
1. PN junction external forward voltage
The connection method of applying a forward voltage to the PN junction is to connect the P region to the positive pole of the power supply and the N region to the negative pole of the power supply. At this time, the direction of the electric field formed by the external voltage is opposite to the direction of the self-built field, thereby narrowing the barrier layer, and the diffusion effect is greater than the drift effect. Most carriers diffuse to the other area to form a forward current, and the direction is from the P region to the N region. As shown in Figure (1)
At this time, the PN junction is in the on state, and the resistance it presents is the forward resistance. The greater the forward voltage, the greater the current. Its relationship is an exponential relationship:
Where: ID is the current flowing through the PN junction, U is the voltage across the PN junction,
U T = kT/q is called the temperature voltage equivalent, where k is the Boltzmann constant, T is the absolute temperature, q is the electron charge, at room temperature (300K) U T = 26mv, and IS is the reverse saturation current. We must master this formula!
2. PN junction external reverse voltage
Its connection method is opposite to the forward direction, that is, the P region is connected to the negative pole of the power supply, and the N region is connected to the positive pole of the power supply. At this time, the direction of the electric field formed by the external voltage is the same as the direction of the self-built field, so that the barrier layer becomes wider, the drift effect is greater than the diffusion effect, and the minority carriers form a drift under the action of the electric field.
The reverse current is the reverse current, and its direction is opposite to the direction of the forward voltage, so it is also called the reverse current. Since the reverse current is formed by minority carriers, the reverse current is very small. Even if the reverse voltage increases, the minority carriers will not increase, and the reverse voltage will not increase, so it is also called the reverse saturation current. That is: I D = -I S
At this time, the PN junction is in the cut-off state, and the resistance presented is reverse resistance, and the resistance value is very high.
From the above we can see that: under the action of forward voltage, the PN junction is in the on state, and under the action of reverse voltage, it is in the off state, so the PN junction has unidirectional conductivity.
The general relationship between its current and voltage is:
It is called the volt-ampere characteristic equation, and Figure (3) shows the volt-ampere characteristic curve.
When the PN junction is in reverse bias, within a certain voltage range, the current flowing through the PN junction is very small, but when the voltage exceeds a certain value, the reverse current increases sharply. This phenomenon is called reverse breakdown. There are two types of breakdown: avalanche breakdown and Zener breakdown. For the PN junction of silicon material, avalanche breakdown occurs when the breakdown voltage is greater than 7v, and Zener breakdown occurs when the breakdown voltage is less than 4v. Between 4v and 7v, both types of breakdown occur. This phenomenon destroys the unidirectional conductivity of the PN junction, and we should avoid it when using it. Breakdown does not mean that the PN junction is burned out.
4: Capacitance effect of PN junction
Since the change of voltage will cause the change of charge, thus the capacitance effect appears. There is a change of charge inside the PN junction, so it has a capacitance effect. There are two types of capacitance effects: barrier capacitance and diffusion capacitance. The barrier capacitance is caused by the space charge in the barrier layer. The diffusion capacitance is caused by the accumulation of charge caused by the majority carriers in the diffusion process under the action of the forward voltage of the PN junction. When the PN junction is forward biased, the diffusion capacitance plays a major role, and when the PN junction is reverse biased, the barrier capacitance plays a major role.
5. Semiconductor diode
Semiconductor diodes are composed of PN junctions, leads and tube shells. There are many types of them.
According to manufacturing materials: silicon diodes and germanium diodes.
According to the structure of the tube, there are point contact diodes and surface contact diodes.
The logical symbol for a diode is:
1. Characteristics of diodes
Forward characteristics: When the forward voltage is lower than a certain value, the forward current is very small. Only when the forward voltage is higher than a certain value, the diode has a significant forward current. This voltage is called the conduction voltage, which we also call the threshold voltage or dead zone voltage. It is generally represented by U ON . At room temperature, the U ON of silicon tube is about 0.6-0.8 V, and the U ON of germanium tube is about 0.1-0.3v. We generally believe that when the forward voltage is greater than U ON , the diode is turned on. Otherwise, it is cut off. Reverse characteristics: When the reverse voltage of the diode is constant, the reverse current is very small and does not change much (reverse saturation current), but when the reverse voltage is greater than a certain value, the reverse current increases sharply and breakdown occurs. Temperature characteristics: The diode is very sensitive to temperature. Near room temperature, the forward voltage will decrease by 2-2.5mV for every 1 degree increase in temperature, and the reverse current will approximately double for every 10 degree increase in temperature.
2. Main parameters of diode
The physical quantity that we use to describe the characteristics of a device is called the characteristics of the device. The characteristics of a diode are:
The maximum rectified current I F is the maximum average forward current allowed to pass through the diode.
The maximum reverse operating voltage UR is the maximum operating voltage allowed by the diode. We generally take the breakdown voltage as UR .
The reverse current IR is the current when the diode is not broken down. The smaller it is, the better the unidirectional conductivity of the diode.
The maximum operating frequency f M depends on the size of the PN junction capacitance. The larger the capacitance, the higher the frequency.
The DC resistance RD of a diode is the ratio of the DC voltage applied to the two ends of the tube to the DC current, which is called DC resistance. It can be expressed as: RD = UF / IF . It is nonlinear. The greater the difference between the forward and reverse resistance values, the better the performance of the diode.
The AC resistance r of a diode is the ratio of a slight change in voltage near the diode's operating point to the corresponding slight change in current value, which is called the AC resistance at that point.
6. Zener diode
The voltage stabilizing diode utilizes the breakdown characteristics of the diode. This is because the diode works in the reverse breakdown region. When the reverse current changes greatly, the reverse voltage changes very little, thus showing a good voltage stabilizing characteristic.
7. Application of diode
We use diodes mainly for their unidirectional conductivity. When it is conducting, we can replace it with a short line, and when it is off, we can consider it as an open circuit .
When the input signal voltage changes within a certain range, the output voltage also changes accordingly with the input voltage; when the input voltage is higher than a certain value, the output voltage remains unchanged. This is the limiting circuit. We call the voltage that does not change at the beginning the limiting level. It is divided into upper limit and lower limit.
Example 1. Analyze the amplitude limiting circuit shown in Figure (1). The waveform of the input voltage is shown in Figure (2). Draw the waveform of its amplitude limiting circuit.
(1) When E=0, the limit level is 0v. When u i >0, the diode is turned on, u o =0, and when u i <0, the diode is cut off, u o =u i . Its waveform is as shown in Figure (3).
(2) When 0<E<U M , the limit level is +E. When u i <+E, the diode is cut off, u o =u i ; when u i >+E, the diode is turned on, u o =E, and its wave
The shape diagram is as shown in Figure (4)
(3) When -U M <E <0, the limit level is negative, and its waveform is as shown in Figure (5)
2. Diode gate circuit
The gate circuit composed of diodes can realize logical operations. For the circuit shown in Figure (6), as long as one circuit input is low level, the output is low level, and only when all inputs are high level, the output is high level. It realizes the logical "AND" operation.
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The barrier capacitance is caused by the space charge in the barrier layer. 
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