1
\$\begingroup\$

Suppose I have an extrinsic semiconductor made of silicon and doped with phosphorous. Now, phosphorous has one more electron than silicon. After replacing one silicon atom, and taking its place, the phosphorous safely form covalent bonds with four surrounding silicon atoms, with one extra electron. At a certain temperature, this extra electron gains enough energy to jump into the conduction band and move around the semiconductor. However, the moment this electron leaves the current position, the phosphorous atom becomes positively charged. We call this an n-type semiconductor and electrons are the majority charge carriers.

However, when the electron moves away from the starting position, why don't we say that a hole has been created, instead of saying that we have a cation now?

Similarly, if the silicon is doped with boron, the boron forms bonds with three silicon atoms, and one 'incomplete' bond due to a lack of a single electron. Some nearby silicon jumps in to fill this spot, and that position becomes empty. The boron becomes negatively charged. However, the surrounding silicon has just lost an electron. In the previous case, when the phosphorous lost an electron, we said that it has become a cation. In this case, however, we say that an electron-hole has been created, instead of saying that the silicon has become a cation.

So, when do we say a hole has been created vs an atom has become positively charged. My intuition says, that when an electron is removed from a covalent bond, we say hole has been created. However, when an electron is removed, that wasn't part of a bond, we just say that the atom has become positively charged. Can someone verify this for me ?

In this p-type semiconductor, why do we call the holes to be the majority carriers ? Since the electrons jump from one position to another, thus making it look like the holes are jumping, we could have easily said that electrons are the majority charge carriers.

In the n-type, this is somewhat obvious as the electrons are mobile, while the hole(of the cation) stays in place, as the surrounding electrons do not rush in to occupy the position. But in case of the p-type, both the electrons and the holes move. SO, why do we call holes the majority charge carriers, even though it is actually the electrons that are moving ?

\$\endgroup\$

1 Answer 1

2
\$\begingroup\$

why don't we say that a hole has been created,

Although the Phosphorus atom is positively charged, it cannot move from its location in the crystal lattice. Nor does a Silicon atom some short distance from the Phosphorus want to donate one of its electrons to the Phosphorus atom to make the Phosphorus atom neutral. Thus the positively charged Phosphorus atom is not a carrier. Because it is not a carrier we don't consider it a hole. Holes are mobile.

Whenever a hole jumps from one place to another, so does an electron in the opposite direction. So why do we call holes majority charge carriers, even though electrons are moving too ?

When a hole moves from atom A to atom B, an electron moves from atom B to atom A. When the hole moves again from atom B to atom C, a different electron moves from C to B. (That is a different electron than the one that moved from B to A.) That is what distinguishes hole current from electron current, even though in both cases it is electrons that are moving.

In case of Boron, the neighboring atoms do donate electrons.

When a neighboring Silicon atom donates electrons to a Boron atom, that Silicon atom becomes a hole. Other Silicon atoms happily allow their electrons to jump and replace the missing electron, resulting in hole movement. But the Boron atom remain where it is, and the extra electron attached to the Boron is used to make a covalent bond with a neighboring Silicon atom, so it also remains where it is. So holes move in Boron doped Silicon, but electrons do not move from one to the next, to the next, to the next, so electrons are not called carriers in this case.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you so much for this, however can you answer the second part of my question. In case of Boron, the neighboring atoms do donate electrons. Thus in this case, both the electrons and the hole is moving. Why do we call hole as majority carrier ? For every moving hole, there is an electron moving too. Whenever a hole jumps from one place to another, so does an electron in the opposite direction. So why do we call holes majority charge carriers, even though electrons are moving too ? \$\endgroup\$ Sep 30, 2021 at 20:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.