9/22/2004 EE 42 fall 2004 lecture 10 1

Lecture #10 Electrons, Atoms, and Materials

Reading: Malvino chapter 2 (semiconductors)

9/22/2004 EE 42 fall 2004 lecture 10 2

Electrons

• Electrons are not point particles• Can think of them like a vibrating jelly• Electrons take spatial shapes sometime

known as orbitals—would have been better to call them states or modes

• Only one electron of each of two types can be in the same mode (the types are called spin up and spin down)

• This is called Fermi exclusion

9/22/2004 EE 42 fall 2004 lecture 10 3

Here is what some of the orbitals look like:

9/22/2004 EE 42 fall 2004 lecture 10 4

Occupation

• Atomic nuclei have varying numbers of protons in them, which have a positive charge which has the same magnitude as that on the electron.

• A nuclei will attract electrons to it until it becomes neutral, filling the electronic states around it, making an atom.

9/22/2004 EE 42 fall 2004 lecture 10 5

• Since only two electrons can be in each spatial state, they fill up the orbitals in order.

• Lowest energy state first: 1S (spherical)

• One proton→one electron Hydrogen

• Two protons→two electrons Helium

9/22/2004 EE 42 fall 2004 lecture 10 6

Chemistry

• Interactions of electrons in orbitals is what we call chemistry

• The chemical reactions which a atom takes part in are determined by the outer orbitals.

• Because the inner orbitals are all full, and that keeps out other electrons like a shield.

9/22/2004 EE 42 fall 2004 lecture 10 7

Filling of orbitals

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d

The orbitals fill up in the order shown (simplified a bit)

Since the p, d, f orbitals have similar shapes and they get filled up in sequence, we get a periodicity in the chemical properties, giving us the periodic table

9/22/2004 EE 42 fall 2004 lecture 10 8

Periodic table

The number in each element’s box is the atomic number, which is the number of protons, and consequently the number of electrons needed for neutrality

9/22/2004 EE 42 fall 2004 lecture 10 9

Bonding

• “Bonding” is due to the fact that when two atoms are close together, the outer orbitals are wrapped around both of the nuclei, and the electons are in these “shared” orbitals.

• These orbitals can have lower energy than those of the two atoms would have if they were farther away from each other.

• Since the energy decreases as the atoms get close together, this provides a bonding force

• The shared electrons act like glue!

9/22/2004 EE 42 fall 2004 lecture 10 10

Column 4 of the periodic table

IV

9/22/2004 EE 42 fall 2004 lecture 10 11

Orbitals

• The periodic table can be understood by the following

• The single S orbital can hold 2 electrons

• The three P orbitals can hold 6 electrons

• If the orbitals are all full, the atom does not share and play well with friends. (Neon)

• While the other orbitals are filling, we get a whole bunch of different metals.

9/22/2004 EE 42 fall 2004 lecture 10 12

Silicon

• The elements of column four of the periodic table can bond with each other in a regular structure (crystallize)

• each atom bonded to the four nearest neighbors

9/22/2004 EE 42 fall 2004 lecture 10 13

Silicon structure and bonding

• There are four nearest neighbors, four orbitals to share, and 4 electrons to contribute

• Since each neighbor contributes one electron for each orbital, the orbitals are all full and the lattice is strong and stable

• Carbon in this form is called Diamond

9/22/2004 EE 42 fall 2004 lecture 10 14

TEM picture of silicon

9/22/2004 EE 42 fall 2004 lecture 10 15

Semiconductor

• Interestingly enough, even though this electronic glue is everywhere in the crystal, they can not contribute to current because they are locked into this pattern.

• These electrons are said to be in the “valence band”

• If there were a few extra electrons, they could wander about.

• How do we get extra electrons into our crystal?

9/22/2004 EE 42 fall 2004 lecture 10 16

Column 5 of the periodic tableV

If we look at column 5 of the periodic table, we can see that these elements are very similar to silicon, except that they have one extra proton—an extra quantum of positive charge.

9/22/2004 EE 42 fall 2004 lecture 10 17

• So a column 5 element substituted for a silicon atom results in what looks like a silicon crystal but which has a fixed postive charge.

• This is called “doping” the silicon with Arsenic or phosphorus.

• If the crystal is to be electrically neutral, then there will be a mobile electron, called a conduction band electron, hanging around.

9/22/2004 EE 42 fall 2004 lecture 10 18

N type silicon

• So if we take a silicon crystal, and add a very small amount of atoms of arsenic or phosphorous to it, then it will have extra electrons which can wander around.

• Since these extra electrons can move, they can conduct a current

• This is called N type because the carriers that can move are negative.

• They are at a higher energy than they could have if they could fall down into a bond

• Electrons which can wander around above a full set of orbitals are said to be in the “conduction band”

9/22/2004 EE 42 fall 2004 lecture 10 19

P type

But wait a minute: the electrons are always what moves through a crystal, and they are always negative!

• Lets look at what happens if we put a few atoms from column 3 of the periodic table into the crystal

9/22/2004 EE 42 fall 2004 lecture 10 20

Column 3 of the periodic tableIII

If we look at column 3 of the periodic table, we can see that these elements are very similar to silicon, except that they have one fewer protons—one less quantum of positive charge.

9/22/2004 EE 42 fall 2004 lecture 10 21

P type semiconductors

• If we have a silicon crystal with a few Boron, Aluminum, or Gallium atoms in it, then a few orbitals will be missing an electron.

• Other electrons can hop into that orbital• Since the electrons can now move, the crystal

can conduct an electrical current• Since it is an unoccupied orbital that is moving, it

is called a “Hole”• It moves like a positively charged particle would.

9/22/2004 EE 42 fall 2004 lecture 10 22

Electrons and Holes

• If you were to have both electrons and holes in the crystal, the electrons could fill up the holes until one or the other was depleted.

• Silicon crystals with quite a few extra electrons running around are called N-type, and they only have a few holes in the valence band.

• Silicon crystals with many holes running around are called P-type, and they only have a few electrons in the conduction band.

9/22/2004 EE 42 fall 2004 lecture 10 23

Applications

• We can take silicon wafers and put patterns of doping on their surface, and control its conductivity.

• We can further control the conductivity by applying electric fields

• We can make use of the difference between P and N type carriers moving through the crystal.

• This ability to control the conduction of silicon is the basis for the function of transistors, the foundation of the entire electronics industry