Electronics 1a - Semiconductor theory Flashcards
Describe the electron bands for a conductor, insulator and semiconductor and any key information.
The dotted line represents the conductive band (range where electrons are have enough energy to move provided an electric field is present).
The normal line represents the valence band, where there is a full valence shell.
Conductor:
* Electrons are less than the valence band, as the valence shell isn’t full, and in the range of the conductive band.
Insulator:
* Electrons are in the valence band, and there is a wide gap between the valance and the conductive band, so no charge flows across it.
* This of course can be jumped provided very high energy levels are given, this is how air can ionise (lightining) even though we think of it as insulative.
Semiconductors:
* Electrons are in the valence band, however the gap between the conductive and valence band is narrow and can be jumped provided sufficient energy is given.
* This energy can come from temperature or even occur at room temperature
How does current flow compared to holes and electrons in semiconductors?
When an electric field is provided across a semiconductor, and some electrons may be excited at room temperature, those electrons flow from the negative to the postive potential, leaving holes. Holes then get filled by more electrons, so seemingly the holes flow to the negative while electrons to the positive. Current is always from positive to negative, so positive current is the direction of the holes.
What is the difference between intrinsic vs extrinsic semiconductors?
- Pure (intrinsic) semiconductors: overall current is produced by thermal excitement of electrons, usually very small current flow.
- ‘Doped’ semiconductors (extrinsic): have increased levels of charged carriers by doping, so we change how it works slightly.
What is an n-type doping of a semiconductor?
Remember: doping increases the level of charge carriers (either electrons or holes) so that thermal excitation or applying an electronic field has more of an effect then just an intrinsic semiconductor
This is when the type-4 silicon (has 4 valence electrons in the valence shells) is bonded with a type-5 elements (like phosphorus) in a crystal lattice structure.
This leaves some extra electrons, while this is the case the overall charge for silicon and phosophorus are neutral as the electrons are balanced by the nucleaus.
The ‘donor’ impurities (extra electrons) are at a higher energy level then the rest, closer to the conductive band (called the donor level) than the valance band.
This means those ‘donor’ impurities (extra electrons) are easiliy excited to the conduction band (up the energy levels). Can be excited by thermal excitation or electronic fields.
What is an p-type doping of a semiconductor?
Remember: doping increases the level of charge carriers (either electrons or holes) so that thermal excitation or applying an electronic field has more of an effect then just an intrinsic semiconductor
This is when the type-4 silicon (has 4 valence electrons in the valence shells) is bonded with a type-3 elements (like boron) in a crystal lattice structure.
This leaves some holes (absences of electrons), while this is the case the overall charge for silicon and boron are neutral as the electrons are balanced by the nucleaus.
The ‘acceptor’ impurities (holes) are at a higher energy level then the rest, but still closer to the valance band then the conductive band (this energy level is called the acceptor level.
This means those ‘acceptor’ impurities (holes) travel down the energy levels to the valance band, where they conduct. The extra states within the band gap increase the carrier population at thermal equilibrium.
What are the minority and the majority charge carriers for both n-type and p-type semiconductors?
n-type semiconductors:
* minority : the few holes left from electrons excited from thermal excitation
* majority: the large number of free electrons as a result of having extra electrons from the phosophorus
p-type semiconductors:
* minority: the few free electrons that are present from thermal excitation
* majority: the large number of holes in the valence band from having holes from the boron present.
How does a p-n junction (diode) work?
at thermal equilibrium and not connected to a power supply
An n-type and p-type semiconductor is connected next to each other.
The electrons want will diffuse to the right filling up the holes in the p-type, while leaving a hole in the n-type.
This creates an electric field (called the depletion region) where it’s no longer charge neutral. There is now positive charge near the boundary on the n-type side, and a negative charge near the boundary on the p-type side.
depletion region is important as it’s has a ‘built-in’ potential difference.
There is also some drift caused by the recombination effect, resulting in the minority charge carriers respectively in each semiconductor being pushed into the other side so the two semiconductors balance.
How does a diode (p-n junction) have a forward and a reversed bias?
Reversed bias: When the n-type is connected to the positive terminal of the power supply and the p-type is connected to the negative side, results in:
* Electrons in the n-type are attracted towards the power supply
* Holes in the p-type are attracted towards the other side of the power supply
* Increases the depletion region making it harder for current to flow through.
Forward bias: When the n-type is connected to the negative terminal of the power supply and the p-type is connected to the positive terminal, results in:
* Electrons are repelled by the negative terminal pushing electrons towards the centre
* Holes are repelled by the positive terminal pushing holes towards the centre
* Decreases the depletion region, making it easier for current to flow when a potential difference is provided greater then the electronic field p.d. generated (usually 0.7v).
State the defining characteristic of a diode.
The diode has a small resistance to current flow in one direction (forward bias) and a very high resistance to current flow in the other direction (reversed bias).
What does a diodes V-I graph look like, mention key points?
