1- Why is the photonic devices are considered direct semiconductors? Explain 2- Where does the potential barrier get constructed? 3- Can we consider an npn transistor as two diodes connected oppositely in series? Why?
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1- Because these materials have relatively small band gap compared to insulators. So, we know if the band gap decrease them, it's the amount of electrons in the conduction band will be increased because if the band of it is smaller, more electrons can be transmitted from the weight and spent two conduction bed, which will be to increase in conductivity.
2-
A diode is made by making a ohmic contact between a n-type material and a p-type material.
In intrinsic semiconductors, like pure germanium or pure silicon, each atom has 4 valence electrons which forms a bond with neighbours Si atoms satisfying the octal rule.
When pentavalent atoms like Phosphorus or arsenic is doped 4 valence electrons bond with Si and one extra electron is left. Hence excess of electrons leaves the semiconductor with a net negative charge and those atoms are called the donor atoms(they donate an electron).
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