How to conduct current in PN junction devices
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How to conduct current in PN junction devices

Posted Date: 2024-02-07

The PN junction is composed of an N-type doped region and a P-type doped region in close contact. Since the free electrons in the N-type region are majority carriers, the holes are almost zero, called minority carriers, while the holes in the P-type region As the majority carrier, the free electron is the minority carrier, and there is a concentration difference between electrons and holes at their junction.

Due to the difference in concentration of free electrons and holes, a space charge region is formed in the middle. After the space charge region is formed, due to the interaction between positive and negative charges, an internal electric field is formed in the space charge region, and its direction is from the positively charged N region to the negatively charged P region. Obviously, the direction of this electric field is opposite to the direction of carrier diffusion movement, preventing diffusion.

In order for the PN junction to conduct current and form a current, the resistance of the internal electric field in its space charge region must be eliminated.

At this time, the P region loses holes, leaving negatively charged impurity ions; the N region loses electrons, leaving positively charged impurity ions. How can they be eliminated?

Next we conduct experiments:

Experiment 1: What will happen if a negative voltage is applied to the p-type semiconductor?

At this time, electrons in the N-type region are attracted to the positive electrode, while holes in the P-type region are attracted to the negative electrode. Therefore, a thick wall-like area is formed in the middle, cutting off the supply of electrons. The thick wall in this middle part is called the depletion layer. In this state, because the applied voltage cannot flow current, it is called reverse voltage.

Experiment 2: Connect the P-type semiconductor to the positive electrode

When the P-type semiconductor is connected to the positive electrode, the width of the depletion layer becomes narrower. The holes in the P-type region move across the PN junction to the N-type region, and the electrons in the N-type region move to the P-type region.

Once this state is established, a very large current can flow by simply increasing the voltage slightly. This voltage loaded is called forward voltage.

It can be seen that the PN junction device has a rectifying effect. But when a reverse voltage is applied, a negligible amount of current still flows. This is because there are injected carriers that penetrate the depletion layer due to thermal energy. This is a current caused by thermal energy, independent of the applied voltage (under a reverse electric field, the dominant current is thermal energy).

On the other hand, when a forward voltage is applied, the number of holes and electrons flowing in increases exponentially with the voltage, and the current becomes very large. Therefore, it shows obvious rectification effect. The so-called rectification refers to the property of controlling the flow or non-flow of current by the polarity of the external applied voltage. A semiconductor element with two terminals that has a rectifying effect is called a diode.

The rectification characteristics shown in the figure can only roughly represent its tendency. If the reverse voltage is increased to a certain value, the current will suddenly increase. This phenomenon is called breakdown, and the voltage at this time is called Zener voltage.

Review Editor: Huang Fei


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