Electronic Devices Question 225
Question: The typical ionisation energy of a donor in silicon is
[IIT-JEE 1992]
Options:
A) $ 10.0eV $
B) $ 1.0eV $
C) $ 0.1eV $
D) $ 0.001eV $
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Answer:
Correct Answer: C
Solution:
When donor impurity (+5 valence) added to a pure silicon (+4 valence), the +5 valence donor atom sits in the place of + 4 valence silicon atom.
So it has a net additional + 1 electronic charge.
The four valence electron form covalent bond and get fixed in the lattice.
The fifth electron (with net - 1 electronic charge) can be approximated to revolve around + 1 additional charge.
The situation is like the hydrogen atom for which energy is given by $ E=-\frac{13.6}{n^{2}}eV $ .
For the case of hydrogen, the permittivity was taken as e0.
However, if the medium has a permittivity er, relative to e0, then $ E=-\frac{13.6}{\varepsilon _{r}^{2}n^{2}}eV $ For Si, er = 12 and for n = 1, $ E\tilde{}0.1eV $
