Review of the concepts of intrinsic semiconductors and PN junctions

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Review of the concepts of intrinsic semiconductors and PN junctions [Copy link]

semiconductor

The material is taken from the junction of metals and non-metals in the periodic table. At room temperature, the conductivity of semiconductors is between conductors and insulators.

Intrinsic semiconductors

A pure semiconductor with a crystal structure is called an intrinsic semiconductor. (Since it contains no impurities and has a crystal structure, its conductivity is worse than that of ordinary semiconductors.)

At room temperature, a few valence electrons gain enough energy due to thermal motion to break free from the constraints of covalent bonds and become free electrons. At this time, the covalent bond leaves an empty position, namely a hole. Atoms become positively charged due to the loss of electrons, or in other words, holes are positively charged. When an electric field is applied to an intrinsic semiconductor, free electrons will move in a directional manner to generate current; at the same time, valence electrons will fill holes in a certain direction in turn, which is equivalent to holes also moving in a directional manner, and in the opposite direction of electrons. The current of an intrinsic semiconductor is the sum of these two currents. Particles that carry charge are called carriers.

When a free electron is generated, a hole is bound to be generated, so the free electron and hole pair are born and die together. When a free electron fills a hole in motion, both disappear at the same time. This phenomenon is called recombination. At a certain temperature, the concentrations of the two carriers are the same, reaching a dynamic balance. When the temperature rises, the thermal motion will intensify, and more electrons will break free, causing the carrier concentration to rise, thus breaking this balance. When the temperature is constant, the balance will be established again.

Impurity semiconductor

Certain elements are doped into the intrinsic semiconductor through the diffusion process.

1. N-type semiconductor

When the +5-valent element phosphorus is added to the intrinsic semiconductor, an extra electron will be generated after the covalent bond is formed due to the addition of an element with 5 electrons in the outermost layer. This electron becomes a free electron. Because the number of free electrons in this semiconductor is greater than the number of holes, and the electrons are negatively charged, it is called an N (negative) type semiconductor.

2. P-type semiconductor

When the +3 valence element boron is added to the intrinsic semiconductor, an extra vacancy will be created after the covalent bond is formed due to the addition of an element with 3 electrons in the outermost layer. The outermost electrons of the silicon atom will fill this vacancy, thus creating an extra hole. The hole is positively charged, so it is called a P (positive) type semiconductor.

In N-type semiconductors, free electrons are the majority carriers and holes are the minority carriers; in P-type semiconductors, holes are the majority carriers and free electrons are the minority carriers.

Formation of PN Junction

Using a certain process, P-type semiconductors and N-type semiconductors can be made on the same silicon wafer.

Due to the concentration difference, diffusion movement will occur. At the same time, at the junction of the P region and the N region, the majority carrier concentration decreases, a positive ion region appears in the P region, and a negative ion region appears in the N region, and an internal electric field is generated inside. This electric field will produce a movement to prevent the diffusion movement, which is called drift movement. The number of carriers participating in the diffusion movement and the drift movement is the same, and a dynamic balance is reached to form a PN junction.

Unidirectional conductivity of PN junction

Capacitance Effect of PN Junction

There is an equivalent capacitance at the PN junction (barrier capacitance and diffusion capacitance, the sum of the two is called junction capacitance, the details are omitted). Since the capacitive reactance is inversely proportional to the frequency, when the frequency of the alternating current applied to the PN junction is high, the alternating current can form a path through the capacitance of the PN junction, and the PN junction will lose its unidirectional conductive property.