Semiconductors

Modern day electronics use semiconductors in their electrical circuits. Silicon (Group 4 in the periodic table) is a good element to use in semiconductors. It is atomic number 14, so it has 14 electrons; 2 in the first orbital, 8 in the second and 4 in the outer.

For the purposes of this discussion, let's concentrate only on the outer electrons. The inner electrons are still there, but let's talk about just the outer ones.

 

 

 

 

 

 

Notice that there are 4 electrons in the outer orbital. Remember, though, that atoms like to have their outer orbitals full of electrons. Because of this, atoms will "share" their outer electrons if it will allow them to fill the outer orbital. In the case of silicon, it has 4 electrons and needs 4 to finish filling it's orbital. Silicon atoms will share electrons with neighboring silicon atoms and thereby fill their outer shells. When they do this, they arrange themselves into a regular pattern called a "lattice" or "crystal" . It really happens in three dimensions, but for the sake of discussion (and an easier display) it's shown below in two dimensions. The atoms are said to "bond". When sharing electrons they form "covalent bonds".

LET'S REVIEW:
Atoms in a solid can form a lattice or crystal by sharing electrons. "Impurity" elements inserted in a crystal can leave it (the crystal) with a surplus of electrons or holes. A surplus of electrons on one side of the crystal and holes on the other side form a PN junction. The holes and electrons near the junction can diffuse across the junction, combine with atoms on the other side and form positive and negative ions. This region is called the depletion region and has an electric field associated with it due to the positive and negative ions. If a photon of light impacts the P or N type region, an electron and hole are formed. If this happens in the P-type region, the electric field accelerates the electron into the N-type region. If this happens in the N-type region, the electric field accelerates the hole into the P-type region.

Silicon atoms in a lattice

 

 

 

 

 

 

 

 

 

 

 

All of the outer shell electrons are held tightly in place because the shell is full. There aren't any free electrons for an electrical current.

Well, this is fine. But is it possible to have bonding between different elements? It sure is! Let's look at two different scenarios.

We can take an element from a different group, for example, Phosphorus (Group 5) and place it in the silicon lattice. There are 5 outer electrons in phosphorus. Four of them form covalent bonds with the surrounding Silicon atoms. However, one electron is left over. It's not involved in a bond and can roam freely in the lattice. When this is done, it's said an impurity has been "doped" into the lattice. When the doping impurity has excess electrons it is called an "N-type" semiconductor; "N" meaning "negative" because the electrons have a negative charge.

We can also take an element from Group 3, for example Gallium (with 3 outer electrons) and place it in a Silicon lattice. The Gallium will form 3 covalent bonds with the neighboring Silicon atoms. However, there will be one missing electron to form a bond with the fourth neighboring Silicon atom. Because of this there is a "hole" or vacancy where an electron should be. A neighboring electron may move into this hole to fill it, but the electron will leave a hole in the place it came from. Because of this, the doping impurity has an excess of holes (not electrons), so it's called a "P-type" semiconductor; "P" meaning "positive". These excess electrons and holes are considered to be charge carriers.

 

"N-type" material
Silicon doped with Phosphorus
(excess electrons)
   
"P-type" material
Silicon doped with Gallium
(excess holes)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In and of itself this is fine. But the interesting effects come about when a N-type material and a P-type material are brought together. This forms a "pn junction".

 

PN Junction
In the real world you can't just put two doped materials together and have a pn junction. But you can take a crystal and dope one side with an N-type material and the other side with a P-type material.

 

 

 

 

 

 

 

With a surplus of negative charges on one side of the crystal and positive charges on the other side, a charge gradient exists. This gradient will allow some of the electrons near the junction to diffuse (move) across the junction from the N-type side and fill in some of the holes on the P-type side. Also, some of the holes near the junction from the P-type side will diffuse across the junction to the N-type side. As an electron crosses the boundry and recombines with a hole, the atom takes on an overall negative charge (there are now more electrons in the atom than protons in the nucleus; it's no longer electrically neutral). It's now considered a "negative ion". Also, as holes cross the junction and recombine with electrons a "positive ion" is formed. When this happens the area around the junction becomes populated with positive and negative ions. This area is called the "depletion region".

 

 

 

 

 

 

 

Because there are ions of opposite charge in the depletion region, an electric field exists. As more and more holes and electrons diffuse into the junction area, the depletion region becomes larger and the electric field becomes stronger. The stronger the electric field becomes, the less likely a hole or electron can cross it. At some point, the electric field strength balances the charge gradient and diffusion stops.

 

 

 

 

 

 

 

Here's where things get interesting.

If you recall from the section where we talked about atoms, we said that it's possible for an electron to receive energy and leave it's orbital in the valence band and move into the conduction band. One of these ways is for an electron to absorb a photon of light (we'll talk about photons in a moment). When this happens, an electron and a hole pair are created. The balance between the charge gradient and electric field in the crystal is no longer even. Electrons that are created in the P-type region will diffuse back into the depletion region where the electric field moves them into the N-type region. Holes created in the N-type region diffuse back into the depletion region where the electric field moves them into the P-type region. Solar cells take advantage of this.

Photons of light that strike the material form a hole and electron pair.

 

 

 

 

 

 

 

 

 

 

 

 

Now let's look at the nature of light.