N-type crystalline silicon and P-type crystalline silicon

N-type crystalline silicon and P-type crystalline silicon

(1) N-type crystalline silicon When crystalline silicon is doped with a small amount of impurity group V elements (such as P), its 5 valence electrons form 4 covalent bonds with silicon atoms, and the V group ion nucleus has one more positive charge, forming a positive center, and one more valence electron. This electron is bound by the positive center, forming a bound electron with an energy level below the bottom of the conduction band.

When the electron gains energy from the bondage of the positive center (group V impurity atom), it becomes an electron that can conduct electricity. Such positively charged centers (impurities) are called donors. The semiconductor silicon dominated by donor impurities is called N-type semiconductor silicon. The electron energy level (ED) bound to the donor is called the donor level. The donor level is located in the forbidden band. Donor impurities near the bottom of the conduction band are called shallow donor impurities. Since ΔED<<Eg, electrons bound to the donor are easily excited from the donor level to the conduction band at room temperature (kT=0.026eV). The number of carriers in N-type crystalline silicon depends on the number of such donor impurity atoms.

(2) P-type crystalline silicon When crystalline silicon is doped with group III impurity atoms (such as B), there are only 3 valence electrons in the impurity atom, and only 3 covalent bonds can be formed with the silicon atom, and a vacancy appears in the valence bond, which is called “hole”. Holes are equivalent to positive charges. The ion nucleus of group III atoms has only 3 positive charges, forming a negative center in the lattice, which binds holes.

The hole level ground state is located at EA above the top of the valence band at the bottom of the forbidden band. For group III impurities that form negatively charged centers, they can accept electrons in the valence band, which are called acceptor impurities, and their energy levels are called acceptor levels. The acceptor-based semiconductor silicon is called P-type semiconductor silicon. Acceptor impurities near the top of the valence band are called shallow acceptor impurities. Shallow acceptor impurities can significantly change the conductivity of silicon due to ΔEA<<Eg, as shown in Figure 1.

N-type crystalline silicon and P-type crystalline silicon
Figure 1 – Donor and acceptor energy levels

Typically, there are both donor and acceptor impurities in silicon. Since the donor energy level is higher than the acceptor energy level, electrons on the donor energy level will fill the acceptor energy level first, resulting in a compensation effect. When the donor concentration nD is much greater than the acceptor concentration nA (ie nD>>nA), in addition to filling the acceptor energy level, there are still a lot of electrons on the donor energy level, which can transition to the conduction band to become carriers. At this time, the crystalline silicon is still an N-type semiconductor. Conversely, when nA>>nD, crystalline silicon is still a P-type semiconductor. After compensation, the net impurity concentration in the semiconductor is the effective impurity concentration. When nD>nA, then nD-nA is the effective donor concentration; when nA>nD, then nA-nD is the effective acceptor concentration.

For more information on the structure of crystalline silicon, you can click here.

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