| what are most semi conductors made out of | elements with four outer electrons
eg. silicon |
| what is the structure of bonded atoms with 4 outer electrons | they form a crystal lattice structure where each electron is bonded to another (no free electrons) |
| what happens if you introduce an atom with 5 outer electrons into a semi conductor | there will still be a lattice structure but one atom will have an additional electron |
| what is a semi conductor called when it has one extra electron | n-type |
| what can the extra electron in n type semi conductors do | displace electrons from silicon atoms
these electrons can in turn displace other electrons |
| what happens when an atom of valency three is added to a semi conductor | there is a missing electron in the lattice so this leaves a hole |
| what is the name of semi conductors where a valence 3 atom has been introduced | p-type semi conductor |
| why do p-type semi conductors conduct | the hole appears to move due to electrons moving in the opposite direction, causing the material to conduct |
| what is doping | the adding of impurities to pure silicon to produce n-type or p-type silicon |
| what effect does doping have on the elctrical properties of semi conductors | it increases their conductivity, hence reducing their resistance |
| what is the charge of n type semiconductors | neutral |
| what is the overall charge of p type semi conductors | neutral |
| what is a p-n junction | when n type and p type semi conductors are combined
one half of the semi conductor is made of p type material, and the other half n-type material |
| what is a depletion layer | a region where excess holes from the p type material combine with excess electrons from the n type material (the charges combine) |
| are there any free charge carriers in the depletion layer | no |
| at a p-n junction what is the charge of the n-type side | there is a region of positive charge on the n type side |
| at a p-n junction what is the charge of the p type side | there is a region of negative charge on the p type side |
| what is reverse bias | when a positive voltage is applied to the n type side and a negative voltage is applied to the p type side of the material |
| what is the effect of reverse bias on the size of the depletion layer | size of depletion layer increases |
| when a p-n junction is reverse biased can any current flow across the junction? | no |
| what is forward bias | when a negative voltage is applied to the n type side of the material and a positive voltage is applied to the p type side of the material |
| what is the effect of forward bias on size of the depletion layer | depletion layer shrinks |
| when a p-n junction is forward bias, can current flow across junction? | yes
current flows across and junction can conduct |
| how do leds work? | when a p-n junction is forward biased, holes combine with electrons at the junction and release energy.
in leds this energy is released as light. |
| what does the colour of led light depend on | the amount of energy released when electrons and holes combine at a forward biased p-n junction |
| what is a diode | a component which only allows current to flow when connected the right way to a power supply |
| what is a solar cell | a semi conductor device that produces a voltage when photons enter the semi conductor junction |
| what happens when a photon of light enters a solar cell? | an electron in the valence band gains enough energy to jump across the energy gap to the conduction band, leaving a hole and creating an e.m.f across the p-n junction |
| how is an e.m.f created across a pn junction | when a photon of light is incident on a photodiode, an electron gains enough energy to move from the valence band to the conduction band
electrons then move towards the n type semi conductor, this creates a potential difference
this is called the photovoltaic effect |
| explain in terms of band theory how an led emits light | when a voltage is applied to the diode, the electrons move from the conduction band of the n-type semi conductor towards the conduction band of the p-type semi conductor
electrons fall from the conduction band to the valence band on either side of the junction
photons are emitted |
