Week 8 - Semiconductor Flashcards
Pure Si is also an intrinsic semiconductor, but with a band gap of 1.11eV. How do you expect the electrical resistivity of Intrinsic Si to compare with that of intrinsic Ge (0.67eV)?
The resistivity of intrinsic Si will always be greater than that of intrinsic Ge. The larger band gap for Si means more thermal energy is required to activate the same number of charge carrying electrons into the conduction band. At any temperature, Si will have fewer charge carriers than intrinsic Ge and this leads to a lower conductivity and hence higher resistivity.
Intrinsic semiconductors Germanium (Ge) has a band gap of 0.67eV. How do you expect the electrical resistivity of intrinsic Ge to change as you increase its temperature from room temp to 100?
We would expect the resistivity to decrease quite rapidly as we increase the temperature in this range because the increase in thermal energy allows more electrons to jump from the valence band across the band gap into the conduction band, hence creating two charge carriers (electrons and holes)
Why are the dominate carriers in intrinsic semiconductors?
Electrons in the conduction band and holes in the valence band
If we had a small amount of Antimony to the Ge we can create extrinsic n doped semiconductor Ge.
What are the dominant charge carriers in n doped extrinsic Ge within the extrinsic regime?
Electrons in the conduction band
If we had a small amount of Antimony to the Ge we can create extrinsic n doped semiconductor Ge.
What are the dominant charge carriers in n doped extrinsic Ge within the freeze out regime?
Electrons in the conduction band
If we had a small amount of Antimony to the Ge we can create extrinsic n doped semiconductor Ge.
What are the dominant charge carriers in n doped extrinsic Ge within the so called intrinsic regime at high temperature?
Both electrons in the conduction band as well as holes in the valence band
If we had a small amount of Antimony to the Ge we can create extrinsic n doped semiconductor Ge.
How do you expect the electrical resistivity of n doped Ge to change as you increase its temperature from absolute 0K to 800K?
“Freeze out”- Decreasing resistivity because of the increase in number of conductor electrons
“Extrinsic” - The slight increase with increase in temperature because of decrease mobility
“Intrinsic” - Increase in number of electrons and holes dominates over decrease in mobility
There are three approaches that can be used to create a white light LED. Explain these three approaches.
In one device use colour mixing by including a red,green and blue led with suitable sizes to have the right amount of each colour to give the impression of white light
Use a red and blue LED and instead of using the green LED which suffers from the so called green gap use another blue LED coated with a green phosphor. This approach also uses colour mixing but has a more efficient generation of green light.
Use a blue LED and cover it with a white phosphor. White phosphors are a mix of several phosphors and using this in combination with the blue LED light, some of which may make its way through the phosphor creates the ability to generate light that looks white.
In a remote phosphor LED, blue LEDs are usually used, blue LEDs are more expensive than red LEDs. What cannot red LEDs be used for this purpose?
Light that is absorbed by the phosphor can only create light of a lower energy, this is conversation of energy.
If we use red light which has the lowest energy of the visible spectrum, the red light may be able to activate the phosphor to create other light, but the other light would be in the infrared spectrum. Blue LEDs have the highest energy of typical LEDs in the visible spectrum and hence they have he possibility of creating light of the other colours from a suitable phosphor.
What is the difference between intrinsic and extrinsic semiconductors?
Intrinsic semiconductors are pure semi conducting materials with no impurity atoms needed. The number of free electrons in the conduction band and the number of holes in valence band is exactly equal and very small. It’s electrical conductivity is a function of temp alone.
Extrinsic semiconductors are prepared by doping a small quantity of impurity atoms to the material. The number of electrons and holes are never equal. Excess of electrons in n type and excess holes in p type.
What does p type and n type mean?
P type- you dope the material with Boron and an acceptor state is made just above the valence band
N type - You dope Silicon with Phosphor and an extra electron and donor state is created just below the conduction band.
Why does so much care need to be taken when controlling the dopant concentrations added to semiconductors? What happens if you add too much?
The conductivity of extrinsic semiconductors is controlled by the concentration of dopants to the Si. The more dopants added, then the greater the number of charge carriers. However, the greater the number of dopants, the greater the reduction in the electron and hole mobility.
Explain how a LED works.
When a diode is subjected to a forward bias, the motion of holes and electrons an lead to recombination. Recombination represents the relaxation of an excited electron from the conduction band into a hole in the valence band. In certain semiconductors, this relaxation is accompanied by the emission of a photon of light with energy equal to or less than the band gap. This form of electricity stimulated luminescence is known as electroluminescene.
