Semiconductor devices Flashcards
Junction between 2 metals
When 2 dissimilar metals are placed in contact, electrons can flow from higher energy states in metal A to lower energy vacant states in metal B
This process continues until the highest occupied energy states on either side of the junction are the same - i.e. the metals have the same Fermi energies
Contact potential
Generated when electrons are transferred from A to B
Metal A is now positive and metal B is now negative
Magnitude of the contact potential
The difference between the work functions of the 2 metals
Contact potential definition
The difference in electrostatic potential between two metals that are in contact and are in thermodynamic equilibrium Specifically, it is the potential difference between a point close to the surface of the first metal, and a point close to the surface of the second metal
Describe p-n junction
When p- and n-type materials are initially placed in contact, almost all of the conduction electrons are on the n-side of the junction and most of the holes are on the p-side
Electrons from the n-region near the p–n interface diffuse into the p-region and recombine with holes in the p-region, forming negatively charged acceptor ions in the p-region
Likewise, holes from the p-type region near the p–n interface diffuse into the n-type region and recombine with electrons to form positive donor ions in the n-region
Therefore, there is a layer either side of the junction with a reduced carrier concentration compared to the rest of the crystal
This is called the depletion zone
An equilibrium is reached in which a potential difference (contact potential) is formed across the junction
Electric field
Goes from positive to negative
So goes from n to p in the depletion zone of the p-n junction
What happens when a p-n junction is connected to a battery source?
An additional energy source overcomes the potential barrier in the depletion zone, resulting in free electrons being able to cross the depletion region from one side to the other
Why are the conduction and valence bands higher energy on the p-side of the p-n junction?
Electron potential energy is higher on the p-side than the n-side
Bands bend up in the direction of the electric field
Electric field is positive to negative, opposite direction to potential
Define depletion zone
The insulating region in the middle of the p-n junction that contains no mobile charge carriers
Only contains ionised donors/acceptors
Why is the depletion zone named so?
Because it is formed from the removal of all free charge carriers, leaving none to carry a current
Charge of a p-n junction
The p- and n- regions are both neutral, but the regions near the p-n interface lose their neutrality due to the diffusion of the majority charge carriers
N-side is positively charged
P-side is negatively charged
What is the consequence of the p-n interface being charged?
It creates an electric field the opposes the charge diffusion
When this electric field is sufficiently strong, it prevents further diffusion of electrons and holes
i.e. the depletion region reaches equilibrium
Equilibrium state of a p-n junction
No matter how large the contact potential between the p- and n- sides of the junction, there is still a finite probability that some of the conduction electrons on the n-side have a greater energy than that of the conduction band edge on the p-side
These electrons can therefore diffuse across the junction into the p-side and give rise to a ‘diffusion current’
There is also a finite probability of finding some thermally-created electrons in the conduction band on the p-side
These electrons are attracted to the n-side due to the built-in electric field across the junction, giving rise to a ‘drift current’
Diffusion current
At equilibrium, some of the conduction electrons on the n-side of the junction have a greater energy than that of the conduction band edge on the p-side so can diffuse across the junction into the p-side
Drift current
At equilibrium, there is a finite probability of finding some thermally-created electrons in the conduction band on the p-side of the junction
These electrons are attracted to the n-side due to the built-in electric field across the junction, giving rise to a ‘drift current’
Magnitude of diffusion and drift currents at equilibrium
When the system is in equilibrium, there must be no net flow of charge
i.e. the flow of electrons from n to p must exactly equal the flow of electrons from p to n
i.e. the diffusion current must be equal in magnitude to the drift current
The p-n junction is in a state of dynamic equilibrium
Forward bias
Applying a positive voltage
Involves connecting the positive terminal of the battery to the p-end and the negative terminal of the battery to the n-end
This means the external voltage is in the opposite sense to the built-in voltage, thus reducing the voltage difference across the junction (the depletion zone is ‘pushed’ towards the centre of the junction so gets smaller)
This means more of the conduction electrons on the n-side can diffuse across the junction, i.e. the diffusion current is increased compared to when there is no applied voltage
The diffusion current becomes larger than the drift current so there is a net flow of electrons from n to p
Negative bias
Applying a negative voltage
Involves connecting the negative terminal of the battery to the p-end and the positive terminal of the battery to the n-end
This means the external voltage is in the same sense as the built-in voltage, thus the potential difference across the junction is increased
This means fewer conduction electrons on the n-side can diffuse across the junction, i.e. the diffusion current is decreased compared to when there is no applied voltage
