Semiconductors Flashcards
Why is Silicon (or an element in the same group) used as a semiconductor?
Because it has four valence electrons (needs four bonds to be satisfied).
It can thus bond with four other silicon atoms to create a grid like structure.
What happens when the Silicon is heated?
When heated, some of the silicon atoms may gain enough energy to break free, leaving behind positive holes.
These positive holes attract other free electrons, thus the current can be carried throughout the material.
Define ‘intrinsic’ conduction.
Conduction due to electrons and positive holes in a pure semiconductor material.
How can conductivity of a semiconductor be increased? List 3 ways.
- Increasing temperature (more holes, more free electrons).
- Increasing light exposure (for LDRs).
- Adding impurities (i.e. extrinsic conduction).
What is ‘doping’?
The addition of (controlled amounts of) an impurity to increase extrinsic conduction.
In an n-type diode, what is the impurity added, and why is that substance chosen?
Potassium (or any alkali metal), because it has 5 valence electrons.
This leaves an extra electron spare before any energy is needed. The electrons are the majority charge carriers.
In a p-type diode, what impurity is added and why is this substance used?
Boron (or any atom with 3 valence electrons) is used because there is an extra positive hole.
Again, the same result (more conductivity) before energy is added except now the majority charge carriers are positive holes.
What is a p-n diode?
A p-type and an n-type material joined to form a semiconductor (charge flows one way).
What is the ‘depletion layer’?
It is an area where there are very little charge carriers. It is formed when a p-type and n-type material combine.
What is the function/significance of this depletion layer?
It acts as an insulator (i.e. The circuit is broken, current will not flow).
What is a junction voltage, how is it formed and what is its significance?
It is a potential difference formed across the depletion layer because of a small accumulation of charge on the two sides of the depletion layer (electrons and positive holes).
This voltage (~0.7V for silicon) needs to be surpassed by the voltage applied to the semiconductor for a current to flow.
What happens when a p-n diode is connected in forward bias?
Positive terminal attached to p-type, negative to n-type. The like charges repel each other and shrink the depletion layer until (at junction voltage) a charge can flow.
What happens when a p-n diode is connected in reverse bias?
Postive terminal to n-type, negative to p-type. Opposite charges attract, depletion layer grows.
There is a leakage current (very small) but as voltage is increased a breakdown will occur and the diode will break.
What are semiconductors?
They are materials with a resistivity between that of a good insulator and a good conductor.
