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Carriers in Semiconductors Lesson

by Greg Lush

Lesson Topic: Carriers in Semiconductors

Reading Assignment: Section 2.2.3

Discussion Questions

1. What is a hole?
2. How are free electrons and broken valence bonds related?

Homework: None

What do you need to know for the exam?

1. Nothing much here. We are still building general understanding.



Summary
A carrier in a semiconductor is something that has positive or negative charge and is able to move around inside a semiconductor. There are electrons (negative) and holes (positive) in a semiconductor. A hole is the lack of an electron in the valence band. Electrons can move around when they are in the conduction band and holes move around in the valence band.

Carriers
The way the text begins talking about carriers is to imagine that the valence band is completely full of electrons—which means that there are no bonds broken (bonding model). This occurs when the semiconductor is at zero Kelvin. If the valence band is full of electrons, none of them can move around because there are no empty electronic states to move into. It is like being in a classroom where every seat is occupied by a student. You cannot move because there are no empty seats.

As we warm up a semiconductor, some of the electrons in the valence band can absorb enough thermal energy to move to the conduction band—the equivalent of a broken bond in the bonding model. Once the electron moves to the conduction band, it is surrounded by empty states so it can move around. We call an electron in the conduction band a free electron. The free electron leaves behind the lack of an electron, or hole, in the valence band. Any of the surrounding electrons can move into the hole, so electrons cannot move around the valence band as well. The creation of one free electron also creates one hole. Rather than thinking about electrons moving around in the semiconductor, however, it is easier to count the few missing electrons and worry only about holes in the valence band.

It is fairly simple to visualize an electron "jumping" to the conduction band and moving in any direction, since we know it is a physical object. Visualizing a hole, however, is something new. Since a hole is the absence of an electron, one way to look at it is by picturing a soft drink. When it is being poured, bubbles rise from the very bottom of the glass to the top and are released. As they rise to the top, the liquid is taking their place at the bottom. We can picture the glass to be the valence band and the atmosphere being the conduction band. The bubbles are the holes and the liquid is the electron. Since electrons like to be at the lowest energy state possible, they take the place of the hole in a lower state, making it rise to a higher state carrying the absence of an electron, a positive charge, with it.

Definitions:
Carrier: One that transports or conveys, a charge-carrying entity, especially an electron or a hole in a semiconductor.
Electron: A subatomic particle in the lepton family having a rest mass of 9.1066e-28 grams and a unit negative charge of approximately 1.602 x 10-19 coulomb.
Hole: A vacant electron energy state that is manifested as a charge defect in a crystalline solid, the defect behaving as a positive charge carrier with charge magnitude
equal to that of the electron.
Dopant: A small quantity of a substance, such as phosphorus, added to another substance, such as a semiconductor, to alter the latter's properties.

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