Transient voltage suppressor semiconductor basic terms

Impurities in semiconductors have a very large effect on the resistivity. When a trace impurity is added to a semiconductor, the periodic potential field near the impurity atom is disturbed and an additional bound state is formed, and the impurity level is added in the forbidden band. For example, when impurity atoms such as phosphorus, arsenic, and antimony are added to the quaternary element germanium or silicon crystal, the impurity atom is a molecule of the lattice, and four of its five valence electrons form a covalent bond with the surrounding germanium (or silicon) atom, and the excess electron is bound to the impurity atom near the hydrogen-like energy level.

The impurity level is located above the forbidden band and near the conduction band bottom. Electrons at the impurity level are easily excited to the conduction band to become electron carriers. This impurity that can provide electron carriers is called the donor, and the corresponding energy level is called the donor level. The energy required for an electron at the donor level to transition to the conduction band is much less than that required for excitation from the valence band to the conduction band.

When a trace trivalent element boron, aluminum, gallium and other impurity atoms are added to a germanium or silicon crystal, the impurity atom forms a covalent bond with the surrounding four germanium (or silicon) atoms, and there is a vacancy, and the corresponding energy state of the vacancy is the impurity level, usually located below the band gap near the valence band. Electrons in the valence band are easily excited to the impurity level to fill this gap, causing the impurity atom to become a negative ion. A hole carrier is formed in the valence band due to the absence of an electron. Such impurities that can provide holes are called acceptor impurities. In the presence of acceptor impurities, the energy required to form a hole carrier in the valence band is much less than in the case of intrinsic semiconductors.

The resistivity of the doped semiconductor decreases greatly. Thermal excitation or photoexcitation generated by heating or illumination will increase the number of free carriers and lead to a decrease in resistivity, and semiconductor thermistors and photoresistors are made according to this principle. For semiconductors doped with donor impurities, the conducting carriers are mainly electrons in the conduction band, which are electron-type conductors, called N-type semiconductors (Figure 3). The semiconductor doped with the acceptor impurity is a cavity type conduction, called P-type semiconductor.

Semiconductors can produce electron-hole pairs at any temperature, so a small number of conducting holes can exist in N-type semiconductors, and a small number of conducting electrons can exist in P-type semiconductors, which are called minority carriers. In the various effects of semiconductor devices, a small number of carriers often play an important role.

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