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Forward bias and reverse bias diagram of diode

When there is a positive voltage bias outside, the mutual suppression and elimination of the external electric field and the self built electric field make the diffusion current of the carrier increase and cause a positive current. In the electronic circuit, if the positive terminal of the diode is connected to the high potential terminal and the negative terminal to the low potential terminal, the diode will turn on. This connection is called forward bias.

When the PN junction is positively biased, the direction of the external electric field is from the p region to the N region, which is obviously opposite to the direction of the internal electric field. At this time, the external electric field drives the holes in the p region to enter the space charge region to offset some negative space charges, while the free electrons in the N region enter the space charge region to offset some positive space charges. As a result, the space charge region is narrowed and the internal electric field is weakened.
The weakening of the internal electric field makes the diffusion motion of most carriers stronger and forms a larger diffusion current (the diffusion current is formed by the directional movement of many carriers, usually referred to as current). In a certain range, the stronger the external electric field is, the greater the forward current is, and the PN junction has a low resistance state to the forward current, which is called the forward conduction of PN junction in electronic technology.
In the case of no applied voltage, the diffusion motion and drift motion of semiconductor are in dynamic equilibrium, and the current passing through PN junction is zero in dynamic equilibrium. At this time, if voltage is applied at both ends of PN junction, the balance of diffusion and drift will be destroyed, and PN junction will show its unidirectional conductivity.
Compared with the forward bias, the positive and negative positions of the switching power supply, i.e. the p-area is connected with the negative pole of the power supply, and the n-area is connected with the positive pole of the power supply, constitute the reverse bias of the PN junction.

In some important applications of diodes, high-speed alternation between high resistance and low resistance is often required. In these applications, some voltage waveforms in the circuit take the form of pulses, that is, square waves varying between high level (usually 5V) and low level (usually 0V). The conversion frequency of these high and low voltage signals is very high, which makes the diode switch between "on" and "off" States at high speed.
When a resistance is connected to a silicon diode, the current at both ends of the resistance changes alternately when the supply voltage changes alternately from 0V to 5V. When e (z) = 5V, the diode is in the forward bias state, in the on state, the pin current flows through the resistance, and the voltage at both ends of the resistance is equal to 5-0.7 = 4.3v. When e (J) = 0V, the diode is in high resistance state, i.e. cut-off state; because there is no current flowing through the resistance, the voltage at both ends of the resistance is equal to zero.
This mode is very similar to the function of rectifier. This is the two extreme states in digital circuit - high level and low level. In other words, it is assumed that the voltage value is one of the two states. Because the role of diodes in these circuits is to turn on or off at different voltage levels, this application is also called switching circuit.
A typical diode switch circuit consists of two or more diodes, each of which is connected to an independent voltage source. To understand the operation process of switching circuit correctly, we must first determine which voltage source determines which diode is in the on state and which is in the off state.
The key to correctly distinguish which state is in is: if the anode of the diode is positive compared with the cathode potential, it is in a positive bias state, that is to say, when the anode potential of the diode (relative to the ground) is higher than the cathode (relative to the ground) potential, it is in a positive bias state.
Of course, it can also be said that the cathode potential (relative to the ground) of the diode is lower than that of the anode (relative to the ground). On the contrary, if we want the diode to be in the reverse bias state, let the anode of the diode be negative compared with the cathode potential, and the cathode of the diode be positive compared with the anode.
When the PN junction is biased in the reverse direction, the direction of the applied electric field is the same as that of the internal electric field in the space charge region, which also leads to the destruction of the equilibrium state of diffusion and drift motion. The external electric field drives the holes and free electrons on both sides of the space charge area to move away, making the space charge area wider and the internal electric field stronger, which makes the diffusion movement of most carriers difficult to carry out. At the same time, it strengthens the drift movement of a few carriers, forming the reverse current from the n area to the P area.
However, the reverse current is very small because a few carriers are constant and few at room temperature. The current fiction shows that the reverse resistance of PN junction is very high, and it can be generally considered that the reverse biased PN junction is not conductive and is basically in the cut-off state. This situation is called reverse blocking of PN junction in electronic technology. When the applied reverse voltage changes within a certain range, the reverse current hardly changes with the change of the applied voltage.
This is because the reverse current is formed by minority carrier drift. Under thermal excitation, the number of minority carriers increases, and the reverse current of PN junction increases. In other words, as long as the temperature does not change, the concentration of a few carriers will not change. Even if the reverse voltage increases within the allowable range, the number of minority carriers cannot be increased, and the reverse current tends to be constant. Therefore, the reverse current is also called reverse saturation current. It is worth noting that the reverse current is one of the main causes of circuit noise, so the temperature compensation must be considered in the design of the circuit.