Semiconductors and Diodes

1
Semiconductors and Diodes

• Bands, gaps, etc.

2
Semiconductor

• Semiconductors are materials, which are neither
  good conductors (like copper, gold, silver) nor
  good insulators (like rubber and glass).
• The most common semiconductors are silicon and
  germanium.
• Silicon is directly under Carbon on the periodic
  table, Germanium is under Silicon.

3
The Importance of Being Semiconducting

• Much of a computers circuitry is made of
  semiconductors.
• Before semiconductor (solid-state) transistors,
  there were vacuum tubes which were huge by
  comparison.
• The ENIAC was made from tubes.
• In addition to the obvious benefit of taking up
  less space, smaller devices tend to be faster and
  require less energy.

4
ENIAC
By today's standards for electronic computers the
ENIAC was a grotesque monster. Its thirty
separate units, plus power supply and forced-air
cooling, weighed over thirty tons. Its 19,000
vacuum tubes, 1,500 relays, and hundreds of
thousands of resistors, capacitors, and inductors
consumed almost 200 kilowatts of electrical
power.
5
Why are they useful?

• Semiconductors can be made to conduct better
  under the right circumstances it is this control
  over the conductivity that makes them useful.
• A semiconductor devices ability to carry current
  can depend on the current or voltage applied.
• They are NOT Ohmic (i.e they do not obey Ohms
  law VIR).
• Their properties may also depend on the whether
  or not light is shining on them, how much light,
  what color light, etc.

6
Doping

• One way to change a semiconductors properties is
  to dope it.
• Doping is adding another substance to a pure
  semiconductor.
• Typically one dopes with an element that lies
  either to the right or left of the pure element
  on the Periodic Table.

7
Periodic Table
B Boron C Carbon N Nitrogen O Oxygen
Al Aluminum Si Silicon P Phosphorus S Sulfur
Ga Gallium Ge Germanium As Arsenic Se Selenium
In Indium Sn Tin Sb Antimony Te Tellurium
8
n and p doping

• Doping with elements on the right increases the
  number of free (valence) electrons and is called
  n doping.
• Doping with elements on the left decreases the
  number of free (valence) electrons and is called
  p doping.
• The material added is called a dopant.
• Both kinds of doping tend to increase the
  conductivity.

9
Energy levels

• The electrons in atoms do not have arbitrary
  amounts of energy.
• There are very precise energy levels that the
  electrons are allowed to have.
• (You might recall 1s, 2s, 2p, 3s, 3p, 3d, etc.
  from chemistry.)
• The energy is said to be quantized.

10
Bands

• When many atoms come together to form a solid,
  there are still quantized energy levels, just
  many, many more of them.
• For some energy levels, the next highest or
  lowest energy level is so close, its almost
  continuous (sometimes called quasi-continuous).
• The closely packed energy levels are said to form
  a band.

11
Gap

• There are still some energies that the electrons
  are not allowed to have.
• Thus there are jumps in energy between the
  highest energy in one band and the lowest energy
  in the next band.
• This jump in energy is called the band gap or
  just the gap.

12
Pauli Exclusion Principle

• Electrons have a property called spin which can
  only have two values up and down.
• The Pauli Exclusion Principle says that two
  electrons with the same spin cannot occupy the
  same level.
• Thus there can only be two electrons per level
  one with spin up, one with spin down.

13
Filling in the bands (not the gaps)

• Imagine the energy levels are all empty, and we
  begin putting in the electrons.
• The first two electrons go into the lowest level.
• The next two go into the next lowest level, etc.

14
Where does it all end?

• A materials conductivity depends on whether the
  last filled energy level is in the middle of a
  band (a metal) or at the top of a band just
  before a gap (semiconductors and insulators).
• The size of the gap determines whether the
  material is a semiconductor (small gap) or
  insulator (large gap).
• (This is over-simplified.)

15
Introducing a current

• To have current, one must move electrons around.
• To move electrons around, one must give them more
  energy.
• With more energy they go to higher energy levels.
• The energy level an electron goes into cannot
  already be occupied.

16
Band versus Gap

• If the highest energy level (the Fermi level)
  falls within a band, then one only needs to add a
  little energy to cause a current (a conductor).
• If the Fermi level falls in a gap, then one has
  to add a lot of energy to cause a current (a
  semiconductor or insulator).

17
Valence and Conduction Bands

• If the Fermi level falls in the gap, the highest
  filled band is called the valence band and the
  lowest unoccupied band is called the conduction
  band.
• In such a case, electrons must be promoted to the
  conduction band in order to move around.

18
An Analogy

• Assume a university offers 100-, 200-, 300- and
  400- level courses.
• That all 100-level courses are of a similar
  difficulty, but that 200-level courses are
  significantly harder.
• The university offers 10 classes at each level
  and each class has a strict cap of 20 students.

19
Analogy continued

• Each level has an occupancy of 200 (10 classes
  ? 20 students).
• The course levels (100, 200, etc.) correspond to
  the bands.
• The change in difficulty between 100 and 200
  corresponds to a gap.

20
The Semiconductor University

• The students at this university must all take
  four courses.
• This is analogous to Silicons having four
  valence electrons.
• Say there are 100 students at the university,
  they must take 400 courses all together (100
  students ? 4 courses each).

21
The lazy students

• The students all register for the easiest courses
  first, but they fill up.
• The students end up only taking 100- and 200-
  level courses.

22
An afternoon job

• A student wants to take an afternoon job, but has
  a schedule conflict.
• In order to have some flexibility in his/her
  schedule, such a student would have to take a
  substantially harder course (the only open
  courses are in the 300 and 400 level).
• The schedule flexibility is like conductivity
  (the ease of motion or lack thereof).

23
The exchange program

• Now say a percentage of the 100 students belong
  to an exchange program and that they only take
  three courses.
• In such a case, not all of the 200-level courses
  would be filled.
• And the student wanting the afternoon job would
  not have to take a significantly harder course
  (assuming he/she was already taking one of the
  harder 200-level courses).
• So there is more flexibility (higher
  conductivity).
• This is analogous to p-doping.

24
The three-year program

• A different possibility is that a percentage of
  the 100 students belong to an 3-year program and
  take five courses.
• In such a case, some of the 300-level courses
  would be filled.
• If the student wanting the afternoon job already
  had a 300-level course, he/she would not have to
  take a significantly harder course.
• So there is more flexibility (higher
  conductivity).
• This is analogous to n-doping.

25
At this juncture (or is that junction?)

• We can change the conductivity, so what?
• The interesting phenomena occurs when a p-doped
  semiconductor meets an n-doped semiconductor.
• This is called a p-n junction.

26
Two nearby universities

• To pursue the analogy, imagine two universities
  in the same city. The p-doped University has an
  exchange program, n-doped University has a 3-year
  program.
• Some students taking 300-level courses at N.U.
  notice there are open slots in the 200-level
  courses at P.U.
• There are unoccupied low-energy levels in the
  p-doped material.

27
The penalty

• For 300-level-taking N.U. students who live in
  between the universities, there is a benefit to
  taking 200-level classes at P.U., but for others
  there is a penalty (a longer commute).
• In the materials the penalty is charge
  separation.
• Recall with batteries and capacitors it required
  energy to separate charges.

28
Violating Neutrality

• Although one might think of n-doped materials as
  having excess electrons, they are neutral.
• Similarly p-doped materials may be thought of as
  deficient in electrons, but they too are
  neutral.
• But when electrons from n-doped go over to the
  p-doped side, the n-doped side is left with a
  positive charge and the p-doped side gets a
  negative charge.

29
Like a little capacitor

• There is an energy cost involved in separating
  charges.
• The p-n junction is like a little capacitor

n-doped
n-doped
30
A loss in flexibility

• With some N.U. students now taking P.U. classes,
  there is a loss in schedule flexibility
  (conductivity) especially among students who live
  between the universities (at the p-n junction).
• There is a loss in conductivity at the p-n
  junction.

31
Applying a voltage

• Connecting a p-n junction to a battery as shown
  below adds positive charges on the n-doped side
  making the region of poor conductivity larger.
• This is called reverse bias.

n-doped
?
32
Applying a voltage II

• Connecting a p-n junction to a battery as shown
  below adds positive charges on the p-doped side
  making the region of poor conductivity smaller
  (oversimplified).
• This is called forward bias.

n-doped
?
33
Diode

• Thus current at p-n junction flows readily in one
  direction and poorly if at all in the other
  direction.
• Devices with this property are called diodes.
• Its like a valve in the heart that only allows
  blood to flow in one direction.

34
LED

• Some diodes are configured such that when
  electrons cross the junction they must give off a
  photon (light).
• Thus they glow when they are conducting.
• They are known as light emitting diodes or LEDs.

35
Photoconductors

• An electron below the gap can be given the boost
  it needs to jump the gap by absorbing the energy
  from a photon (light).
• Once in the conduction band, the excited electron
  is free to move around.
• If the boost is provided by visible light, the
  material is called a photoconductor.

36
Night and day

• A photoconductor is a poor conductor in the dark
  but a reasonable conductor in the light.
• Photoconductors are the key technology in
  photocopiers, laser printers, scanners and
  digital cameras.