Light-emitting diodes (LEDs) contain \(\mathrm{P}-\mathrm{n}\) junctions. The
circuit in an LED is arranged so that electrons from the power source flow
into the conduction band of the n-type side. As electrons continue to flow,
they are pushed to the conduction band of the p-type side, which can hold more
electrons. These electrons enter the conduction band of the p-type side,
because they already occupy the higher-energy band in the n-type side.
However, once the electrons are in the higher-energy band of the p-type side,
they fall into the lower-energy band unless it is full. As these electrons
make transitions to the lower-energy band, energy is released in the form of
light. (a) Explain, in terms of the movement of electrons, why an LED cannot
be made from the junction of pure silicon and silicon doped with phosphorus.
(b) If the direction of the circuit in the LED is reversed, so that the
electrons flow from the power source into the \(\mathrm{p}\)-type side of the
\(\mathrm{p}-\mathrm{n}\) junction directly, where would the electrons go once
they enter the \(\mathrm{p}-\mathrm{n}\) junction? (That is, indicate which
bands would receive the electrons and whether the electrons would then migrate
to other bands.)
(c) Would you expect the LED to emit light when it is placed in the reverse
circuit described in part (b)? Explain your answer.