Electrons at Work Flashcards
What is a conductor?
A material that allows electrons to move relatively easily (has a low resistance)
What is an insulator?
A Material that does not allow electrons to move easily (has a high resistance)
What is a semiconductor?
A Material that can act as a conductor or an insulator under certain conditions
What are the energy levels like in a conductor atom?
The outer energy level contains electrons but not enough to fill it allowing a degree of movement for the electrons
What are the energy levels like in an insulator?
The outer energy level is full and the electrons are fixed or tied in the one level
What happens when atoms are brought together?
The outer energy levels form an energy band (the conduction band)
What happens in the outer band of a metal?
Electrons can move from atom to atom within the band
What is the band level below the conduction band?
The valence band
What happens at room temperature in the valence band in semi conductors?
The valence band slightly overlaps with the conduction band allowing electrons to move out to the conduction band and assist conduction
Why don’t insulators conduct?
Because the outer level is full so there is no transfer of movement of electrons and charge. This is the valence band meaning the conduction band us empty and without electrons
How can you force insulators to conduct?
By providing huge amounts of energy in the form of a large voltage
What is the breakdown voltage?
The voltage at which electrons in an unilateral leave the valence bad to jump to the conduction band
What happens if you increase the temperature of a semiconductor?
It can conduct more easily
What is the band gap like in insulators?
The gap between the valence and conduction band is large and electrons cannot move between them
How do thermistors work?
Raising the temperature of a semiconductor increases the number of active charge carriers promoting them to the conduction band so the material can conduct more current
What is an ntype semiconductor?
A semiconductor with the addition of an electron making the majority of charge carriers negative
How does ntype doping work?
The extra electron means there’s a surplus so can displace electrons in other atoms leading to a chain reaction of moving electrons allowing electricity to flow
What is a ptype semiconductor?
When there’s a missing electron in a material leading to the majority of charge carriers to be positive
How does ptype doping work?
Due to the missing electron surrounding electrons move in to try and stabilise the atom this leads to more electrons missing as they try to fill the surrounding ‘holes’ allowing electricity to flow (in the opposite direction)
What does doping do?
The addition of impurities to produce ntype or ptype semiconductors with increased conductivity
What is the depletion layer?
A layer formed by the electrostatic attraction of n-type and p-type semiconductors that have been brought together. Where conduction and valence bands ‘overlap’ in a p-n junction
What happens when light is incident on the depletion layer?
Photons can transfer energy to electrons allowing them to move to a higher energy band creating a potential difference and allowing currents to flow. (Photovoltaic effect)
How do LEDs work?
When current flows across the depletion layer electrons pass into holes in the layer and move to lower energy levels creating excess energy which is released as a photon
What determines the colour of an LED?
The amount of energy released by the electron moving to a lower level. Red light is the lowest frequency so required the least energy
