Semiconductor Technology

1
Chapter 8

• Semiconductor Technology

2
Basic Atomic Theory

• A basic understanding of atomic activity is
  necessary to understand the operation and
  application of semiconductor devices in
  electronic circuits.
• Semiconductor devices, such as transistors and
  diodes, form the basis of nearly all modern
  electronic systems.

3
Classifications of Material

• Materials can be classified in many ways.
• One way of classification is into solid, liquid,
  or gas states. The materials in this section are
  all classed as solid-state.
• Other methods of classification include
  electrical conductivity, color, density,
  hardness, resiliency, composition, and so on.
• Classes of material according to conductivity
  are insulators, conductors, semiconductors, and
  superconductors.

4
Review of Basic Atomic Model

• Atoms are comprised of electrons, neutrons, and
  protons.
• Electrons are found orbiting the nucleus of an at
  atom at specific intervals, based upon their
  energy levels.
• The outermost orbit is the valence orbit.

5
Energy Levels

• Valence band electrons are the furthest from the
  nucleus and have higher energy levels than
  electrons in lower orbits.
• The region beyond the valence band is called the
  conduction band.
• Electrons in the conduction band are easily made
  to be free electrons.

6
Intrinsic Semiconductors

• Silicon, germanium, and gallium arsenide are the
  primary materials used in semiconductor devices.
• Silicon and germanium are elements and are
  intrinsic semiconductors.
• In pure form, silicon and germanium do not
  exhibit the characteristics needed for practical
  solid-state devices.

7
Isolated Semiconductor Atoms

• Silicon and Germanium are electrically neutral
  that is, each has the same number of orbiting
  electrons as protons.
• Both silicon and germanium have four valence band
  electrons, and so they are referred to as
  tetravalent atoms. This is an important
  characteristic of semiconductor atoms.

8
Semiconductor Crystals

• Tetravalent atoms such as silicon, gallium
  arsenide, and germanium bond together to form a
  crystal or crystal lattice.
• Because of the crystalline structure of
  semiconductor materials, valence electrons are
  shared between atoms.
• This sharing of valence electrons is called
  covalent bonding. Covalent bonding makes it more
  difficult for materials to move their electrons
  into the conduction band.

9
Electron Distribution

• Considering the distribution of electrons at two
  temperatures
• Absolute zero - atoms at their lowest energy
  level.
• Room temperature - valence electrons have
  absorbed enough energy to move into the
  conduction band.
• Atoms with broken covalent bonds (missing an
  electron) have a hole present where the electron
  was. For every electron in the conduction band,
  there is a hole in the valence band. They are
  called electron-hole pairs (EPHs).

10
Electron Distribution

• As more energy is applied to a semiconductor,
  more electrons will move into the conduction band
  and current will flow more easily through the
  material.
• Therefore, the resistance of intrinsic
  semiconductor materials decreases with increasing
  temperature.
• This is a negative temperature coefficient.

11
Semiconductor Doping

• Impurities are added to intrinsic semiconductor
  materials to improve the electrical properties of
  the material.
• This process is referred to as doping and the
  resulting material is called extrinsic
  semiconductor.
• There are two major classifications of doping
  materials.
• Trivalent - aluminum, gallium, boron
• Pentavalent - antimony, arsenic, phosphorous

12
Trivalent Doping

• Silicon is the most widely used semiconductor
  material.
• By adding a trivalent material to the crystal
  structure, holes are introduced and provide a
  mechanism for conduction.
• Because trivalent materials can accept an
  additional electron, they are called acceptor
  atoms.
• A silicon crystal doped with trivalent material
  is called p-type material.

13
Trivalent Doping
14
Pentavalent Doping

• Doping silicon with pentavalent material results
  in extra electrons being available, improving the
  conduction characteristics.
• Pentavalent materials donate electrons, and
  therefore are called donor atoms.
• Once a silicon crystal has been doped with
  pentavalent materials, it is called n-type
  semiconductor material.

15
Pentavalent Doping
16
Energy Levels
17
Current Flow in a Semiconductor

• When a doped semiconductor has a voltage applied
  to it, current will flow from negative to
  positive, regardless of whether it is p- or
  n-type material.
• The current flow is radically different for the
  two types of material.

18
Current Flow Through N-Type Material

• N-type material has many conduction band
  electrons.
• If a voltage is connected across n-type crystal,
  free electrons will move toward the positive
  terminal.
• As electrons are moved from one atom towards the
  positive terminal, a hole is left behind,
  allowing more electrons to shift towards the
  source voltage.

19
Current Flow Through P-Type Material

• Current flow in p-type material causes the shift
  of holes towards the negative terminal because
  of the shifting of the covalent electrons.
• Hole flow moves from positive to negative in a
  p-type semiconductor material.
• Actual current flow is still electron current
  flow from negative to positive.

20
Electron versus Hole Flow

• Electron flow in p-type material occurs in the
  valence band electron movement in n-type
  material occurs in the conduction band
• Electrons are the majority carriers in n-type
  material they are holes in p-type material.

21
Semiconductor Junctions

• When p-type material meets n-type material within
  a single silicon crystal, a pn junction is
  formed.

Fig 8-16
22
Unbiased Junction

• The pn junction is formed in the process of
  creating the semiconductor device.
• Before carrier migration, there are equal numbers
  of holes and electrons on either side of the
  junction.
• Because of random thermal energy, some electrons
  will pass across the pn junction mating with
  holes on the other side. This is recombination.

23
Unbiased Junction

• After a time, the region will be depleted of
  charge carriers because of the migration of
  electrons and holes.
• This leaves an area known as the depletion region
  in the pn junction.
• Further electron migration will not take place
  until the barrier potential is overcome.
• In silicon, the potential is 0.60.7 V in
  germanium, it is 0.20.3 V.

24
Forward Biased Junction

• An external source can either oppose or aid the
  barrier potential.
• If the positive side of the voltage is connected
  to the p-type material, and the negative side to
  the n-type material, then the junction is said to
  be forward biased.

25
Forward Biased Junction

• In a forward biased junction, the following
  conditions exist
• Forward bias overcomes barrier potential.
• Forward bias narrows the depletion region.
• There is maximum current flow with forward bias.

26
Reverse Biased Junction

• Reverse bias occurs when the negative source is
  connected to the p-type material and the positive
  source is connected to the n-type material.
• Reverse bias strengthens the barrier potential.
• Reverse bias widens the depletion region.
• Current flow is minimum.

27
Reverse Biased Junction

• A reversed biased junction has zero current flow
  (ideally).
• Reverse current is temperature dependent.
• If reverse biased is increased enough, the
  reverse current increases dramatically.
• This breakdown is called junction breakdown. The
  voltage required to reach this point is the
  reverse breakdown voltage.
• As the breakdown occurs, avalanche may occur and
  destroy the device if uncontrolled.

28
Troubleshooting Semiconductors

• Mechanical considerations
• Leads should be bent with needle-nose pliers.
• Some semiconductors have glass packages and need
  to be handled with care.
• Repetitious bending of the leads can cause them
  to break.

29
Soldering/Desoldering

• Troubleshooting often requires the component to
  be soldered or desoldered into or from the
  circuit board.
• Semiconductor devices are temperature sensitive
  and caution should be used in soldering/desolderin
  g.
• Heat sinking solid-state devices is essential to
  protect them from thermal damage.

30
Electrical Considerations

• Junction voltage measurements are useful in
  troubleshooting semiconductor devices.
• A forward-biased silicon junction will have
  approximately 0.7 V across it if germanium, it
  will have approximately 0.3 V.
• An ohmmeter can be used to test a pn junction.
• A front-to-back ration of at least 110 should be
  the result of an ohmmeter test.