Section 11 - Conduction in solids
11.1 What is the longest wavelength that a photon can have to promote an electron from the top of the valence band to the bottom
of the conduction band in silicon? Assume a band gap of 1.12eV for silicon.
m
11.2 What is the longest wavelength that a photon can have to promote an electron from the top of the valence band to the bottom
of the conduction band in silicon dioxide? Assume an energy gap of 8.0 eV between the valence and conduction bands.
m
Section 14 - p-n junction
14.1 Diodes in forward-biased circuits emit
electromagnetic radiation. Specifically, when
an electron from the conduction band in the n-
type semiconductor of the diode fills a hole in
the valence band of the p-type semiconductor
of the diode, it emits a photon whose energy is
equal to the difference in energy between the
two bands. The energy required for this
emission is provided by whatever is driving the
electric current in the circuit.
The accompanying photograph shows a circuit
containing green light emitting diodes (LEDs). LEDs actually look a lot like computer chips, but they are enclosed in protective
plastic lenses like the ones you see here so that they will project a bright light. Light emitting diodes are made from non-silicon
semiconductors such as gallium, and the energy difference between their conduction and valence bands is such that the
photons they emit have wavelengths in the visible light spectrum. LEDs are used for applications ranging from jumbo TV
screens in sports stadiums to the digits of glow-in-the-dark clocks.
The LEDs in the photograph emit green light with a wavelength of 520 nm. What is the energy difference between their
conduction and valence bands?
eV