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A semiconductor is a material with electrical conductivity between that of conductors and insulators. It contains particles that carry electric charge, known as electron carriers (negatively charged free electrons) and hole carriers (positively charged holes). Silicon, germanium, selenium, and most metal oxides and sulfides are semiconductors.
A crystal is a substance in which atoms are arranged in a regular, orderly pattern. Semiconductors typically have this kind of structure, which is why they are often referred to as crystals.
An intrinsic semiconductor is a pure substance (containing no other elements) with a crystalline structure. In intrinsic semiconductors, the number of electrons and holes is always equal under any circumstances. Examples include single-crystal germanium and silicon.
Doping refers to introducing specific types and quantities of other elements into intrinsic semiconductors. The introduced elements are called impurities. Doping is done to enhance the semiconductor’s conductivity, either by increasing the number of electrons or holes on top of the existing “electron-hole pairs.”
When a small amount of impurity is introduced into an intrinsic semiconductor to introduce a large number of electrons, the resulting semiconductor is called an “electron-type semiconductor” or “N-type semiconductor.” In an N-type semiconductor, the majority carrier is electrons, and the minority carrier is holes.
When a significant number of holes are introduced into an intrinsic semiconductor through doping, the resulting semiconductor is called a “hole-type semiconductor” or “P-type semiconductor.” In a P-type semiconductor, the majority carrier is holes, and the minority carrier is electrons.
When a P-type semiconductor and an N-type semiconductor are joined together using a specific process, the abundance of holes in the P-type region and electrons in the N-type region causes majority carriers to diffuse across the junction. This forms a charged region called a “space charge region” or PN junction.
II. Unidirectional Conductivity of PN Junction
One side of the PN junction’s space charge region is positively charged, while the other side is negatively charged. This creates an internal electric field in the PN junction, with the positive charge of the N-region facing the negative charge of the P-region. This field inhibits the further diffusion of holes from the P-region into the N-region and electrons from the N-region into the P-region.
If the P-region of the PN junction is connected to the positive terminal of a power supply and the N-region to the negative terminal, as shown in figure (a), the external electric field opposes the internal electric field. Under the influence of the external electric field, majority carriers cross the PN junction, resulting in forward current. This connection is known as the forward bias of the PN junction. The PN junction offers minimal resistance to forward current, allowing it to flow easily.
Conversely, if the external voltage is reversed, as in figure (b), the external electric field aligns with the internal electric field of the PN junction. This strengthens the blocking effect on majority carriers, resulting in a minimal current flow known as reverse leakage current. The resistance to current when the PN junction is under reverse voltage is large, indicating the basic principle of unidirectional conductivity of semiconductor PN junction.
III. Conductive Properties of Semiconductors