Video Transcript
Which of the following most
correctly describes the net direction of motion of free electrons within a silicon
lattice? (A) The net motion of free
electrons is across the longest side of the lattice. (B) The net motion of free
electrons is across the shortest side of the lattice. (C) There is no net motion of free
electrons. Or (D) the net motion of free
electrons is periodic.
To begin, we should recall that
free electrons are higher-energy electrons that aren’t strictly bound to any one
particular atom. Rather, they′re known as free
charge carriers, since they’re able to move relatively freely around the silicon
lattice, helping to make a substance more conductive. This is why free electrons are
useful to a semiconductor. They’re charged particles that are
available to easily respond to an applied potential difference. Thus, the semiconductor is able to
conduct electricity or facilitate the flow of electric charge.
But note that in this question,
there’s no mention of a potential difference being applied to the lattice, which
would move the electrons in one direction, and also no mention of a difference in
density of electrons in one area of the lattice compared to another. While either of these would
influence the overall direction of motion of free electrons in the lattice, that’s
not what this question is asking about.
But also, on their own, free
electrons don’t just stay still. Rather, they’re constantly moving
around due to their thermal energy. And they repeatedly collide with
other free electrons and atoms in the lattice. This type of motion, in which a
particle is rapidly moving around and experiencing collisions, is characterized by
its apparent randomness. A free electron is equally likely
to be redirected in any direction at any time, meaning that any given free electron
tends to have net zero displacement.
Knowing this, answer option (C)
seems like the best choice. It is true that, generally, free
electrons experience random motion due to repeated collisions. And such random movement does not
constitute the net movement of electrons in any one direction. Thus, we know that answer choice
(C) is correct. There is no net motion of free
electrons in a silicon lattice.
