Fundamentals Flashcards
What is a hole?
A ‘gap’ left behind when an electron escapes it’s bonds and jumps from the valence band to the conduction band.
What are the gaps left behind by an escaping electron called?
A hole
How does a hole move?
A hole is restricted to only move within the bonds
What is the charge of a hole?
Positive
What is the importance of holes, in regard to conductivity?
The importance of a hole is that it may serve as a carrier of electricity in the same manner as a free electron but in the opposite direction.
What are the two energy domains in which current can flow?
The conduction band and the valence band
What is the conduction band?
The range of energies that electrons must possess if they are to be free to conduct electricity (rather than held in a bond).
What is the valence band?
The range of electron energies that electrons are said to possess when they take part in a hole conduction process.
How does mobility vary between an electron and a hole?
The holes in the valance band are less mobile than electrons in the conduction band by a factor of up to 10.
What is mobility?
The speed at which electrons or holes move under the influence of unit electric field.
(speed with which carriers move is proportional to the applied electric field)
Define: hole-electron pair generation
This refers to the excitation of electrons out of crystal bonds leaving holes behind
What is electron density?
The number of free electrons per unit volume in the semiconductor
What is hole density?
The number of holes per unit volume in the semiconductor
What is a band/energy gap?
The minimum energy needed by an electron to escape its bonds, and move from the valence band to the conduction band
What is a valence shell of an atom?
The outermost shell. It represents a band of energy levels so is also known as the valence band
What happens if we supply enough energy to an electron in the valence band?
The electron will escape from the valence band and go to the conduction band
How does the band gap of a material relate to it’s conductivity?
If the band gap is very large, then electrons will require a lot more energy in order to jump from the valence band to the conduction band so the material will be a poor conductor.
If the band gap is smaller the electrons can jump to the conduction band more readily and therefore the material is a better conductor.
What is recombination?
Recombination is the fall of electrons into the gaps (holes) in the chemical bonds in the crystal with a release of energy Eg(electron-volts) per electron.
What does the intrinsic carrier density represent?
The number of holes or electrons in a pure semiconductor
What is Boltzmann’s constant?
k = 1.38 x 10^(-23) JK^(-1)
What is the intrinsic carrier density of silicon at 300K?
ni = 1.5 x 10^(16) m^(-3)
What is the intrinsic carrier density of germanium at 300K?
ni = 2.5 x 10^(19) m^(-3)
Describe qualitatively how the recombination rate is found.
The probability of an electron falling into a hole depends upon the density p0 of holes available. This probability must be multiplied by the density n0 of the electrons waiting to recombine.
This allows us to obtain the net average recombination rate.
