Video Transcript
Which of the following correctly describes an n-type semiconductor? A) An n-type semiconductor is a semiconductor material that carries a net negative charge. B) An n-type semiconductor is a semiconductor material that contains an impurity such that there are more free electrons in the material then there would be in the pure semiconductor. C) An n-type semiconductor is a semiconductor material that contains an impurity such that there are fewer free electrons in the material than there would be in the pure semiconductor. D) An n-type semiconductor is a semiconductor made from an element in period four of the periodic table. E) An n-type semiconductor is a semiconductor made from an element in period five of the periodic table.
Okay, so in this question, we’re trying to find out which of the options A to E most correctly describes an n-type semiconductor. So, let’s start by recalling what an n-type semiconductor actually is. Now to do this, we first need to recall what a pure semiconductor is.
So, a semiconductor is a material that has an electrical conductivity that’s better than the electrical conductivity of an insulator but not quite as good as the conductivity of a conductor. Most commonly, a pure semiconductor material is made from silicone. And this is done because each silicon atom has four electrons in its outermost shell. This way, each silicon atom can bond with four other silicon atoms, resulting in, now, eight electrons in the outer shell.
So, if we think about the silicon atom at the centre of this diagram that we’ve drawn, four of those electrons are originally from the silicon atom itself and the other four electrons are taken from the four bonded silicon atoms. Each silicon atom provides one electron to each bond.
Now this bonding of silicon results in a bulk structure, such that each silicon atom that is surrounded by four other silicon atoms has a full outer electron shell. It’s only the ones on the edges of the bulk that don’t quite have full electron shells. However, this material extends for millions of silicon atoms in all directions. And so, the majority of the silicon atoms have full outer electron shells.
So, this bulk material is known as a pure semiconductor. However, if we were to take one of these silicon atoms and replace it with an atom from another element, then this is where things get interesting.
So, let’s say we take one of these silicon atoms in the bulk and replace it with an atom of phosphorus. Phosphorus doesn’t have four electrons in its outer shell, but rather has five. So, if we now replace the silicon atom with a phosphorus atom, then the phosphorus atom does bond with four silicon atoms around it. But then it’s also got the extra electron in its outer shell, because it had originally five in its outer shell.
And it turns out that this extra electron must go to a higher energy level because the outer shell cannot have more than eight electrons. And so, this extra electron becomes free to move around inside the bulk material. And since electrons are charged particles, this electron can then be used to marshal a current through this bulk material.
So, every time we replace one of the silicon atoms with a phosphorus atom, for example, or for that matter any atom with five electrons in its outer shell, then that atom will donate an electron to the bulk material. So, these extra electrons actually increase the conductivity of our material. And because electrons are negatively charged and are provided by this replacement atom, the semiconductor as a whole is known as an n-type semiconductor, or negative type.
And it’s also worth noting that whenever we replace a silicon atom in the bulk with another atom, this process is known as doping. And the atom being used to replace the silicon atom is known as the dopant. But anyway, so the point is, that an n-type semiconductor is one where it’s not pure silicone. But instead, some of the silicon atoms in the bulk have been replaced with another atom that has five electrons in its outer shell.
Now it’s important to know that although the phosphorus atom in this case is donating an electron to our bulk, this doesn’t make our bulk negatively charged. Because remember, although phosphorus has five electrons in its outer shell, all of these electronic charges are balanced by the corresponding charges in the nucleus of the phosphorus atom. And so, although there is an excess of electrons flying around in this bulk, compared to what there would be if it was just pure silicone, that does not make this bulk negatively charged. Because the extra charge is balanced by the protons in the nucleus of the phosphorus atom.
So, based on this information, let’s now look at options A to E and see which one answer that question the best. Option A says that an n-type semiconductor is a semiconductor material that carries a net negative charge. Well, we’ve just said that this is not the case. The semiconductor is neutral. And hence, option A is not the answer to our question.
Option B says that an n-type semiconductor is a semiconductor material that contains an impurity, such that there are more free electrons in the material than there would be in the pure semiconductor. Well, this is a very good description. Because the dopant, in fact, is the impurity that we’re talking about. And the dopant for an n-type semiconductor does indeed provide more free electrons than there would be in a pure semiconductor. So, it looks like option B is the answer to our question. Let’s quickly look at options C, D, and E to make sure that they’re wrong as well.
Option C says that an n-type semiconductor is a semiconductor material that contains an impurity, such that there are fewer free electrons in the material than there would be in the pure semiconductor. Well, that’s definitely not the case because, in this situation, we’ve got more free electrons. And a semiconductor with fewer free electrons is actually known as a p-type, or positive type, semiconductor. So, option C is not what we’re looking for either.
Option D says that an n-type semiconductor is a semiconductor made from an element in period four of the periodic table. Well, silicon itself is not in period four of the periodic table. And sometimes the dopant can be from period four, but this is not relevant. The only relevant thing is that the original semiconductor material, in this case silicone, contains four electrons in its outer shell. And for an n-type semiconductor, the dopant material contains five electrons in its outer shell.
So, both options D and E, which talk about elements from period four or period five of the periodic table, are not relevant answers when it comes to talking about n-type semiconductors. Hence, our final answer is there an n-type semiconductor is a semiconductor material that contains an impurity, such that there are more free electrons in the material than there would be in the pure semiconductor.
