3.5 Flashcards
What is a conductor?
a material with very low resistance to electricity
What is a semiconductor?
a material with a conductivity for electricity between that of a resistor and conductor
What is an insulator?
a material with a very high resistance to electricity
Describe band theory of conduction
When large collections of atoms come together, the electrons rearrange from orbiting individual nuclei to creating two bands, the conduction band and the valance band.
The valance band represents the lower energy level for electrons and the conduction band represents the higher energy level for electrons.
Electrons cannot exist in the gap between these energy levels, and a small or non-existent gap beteen them allows the flow of electrons from the valance band into the conduction band.
What two things are required for conduction?
Free electrons and accessible empty states
In terms of band theory, explain why conductors have low resistance
In conductors, the valance and conduction bands partially overlap and there is no band gap. This allows electron to flow easly without resistance. The conduction band is only partially filled, meaning that the electrons are free to move when they come from the valance band.
In terms of band theory, explain why insulators have a high resistance
There is a large band gap between the valance band and conduction band. This means that the valance band is full as electrons cannot travel to the conduction band, and the conduction band is empty.
Describe the band gap of semiconductors
The band gap in semiconductors is small, which allows for some electrons to travel from the valance band to the conduction band.
As temperature increases, the conductivity of semiconductors increases as electrons with increased energy can pass the band gap with more ease.
What is doping?
when impurities are added to semiconductors to affect the energy bands and increase conductivity.
What is a p-type semiconductor?
impurities with 3 outer shell electrons are added to the material, and three impurities will fit the cyrstalline structure, but with one electron missing. This creates holes for electrons to move to in the valance band, and therefore conductivity is increased.
What is a n-type semiconductor?
impurities with 5 outer shell electrons can be added to the material, causing there to be extra, non-bonded electrons. This increases the elctrons’ ability to move between bands, and so more electrons are available in the conduction band and conductivity increases.
What is a p-n junction diode?
a material where one half of the semiconductor is doped with a p-type impurity and the other half is doped with an n-type impurity. A depletion layer is formed between the p and n-type materials due to the imbalance in free electrons due to doping.
What is an unbiased p-n juction?
when no voltage is applied to the material
What is a reversed biased diode?
a p-n junction is reverse biased when the n-type side is connected to the positive supply terminal and the p-type is connected to the negative supply terminal. This causes the electric field across the depletion layer to increase, forming a greater barrier to electron flow and increasing its resistance.
What is a forward biased diode?
A p-n junction is forward bias when the n-type side is connected to the negative supply terminal and the p-type side is connected to the positive supply terminal. Electrons from the conduction band of the n-type travel to the conduction band of the p-type. Electrons drop from the conduction band of the p-type material to the valance band, releasing energy.
The electric field is therefore reduced across the depletion layer in a forward bias p-n junction, increasing conductivity.
