10. Semiconductors Flashcards
Why are semiconductors useful materials from an engineering standpoint?
As their conductance can be easily engineered by adding dopants of different valence
What is an intrinsic semiconductor?
A semiconductor that is undoped
What is an extrinsic semiconductor?
A semiconductor that is doped
Explain how N type doping works
N type doing refers to doping with a group V element such as Phosphorous. P has 5 valence e- and since it replaces a Si atom in the lattice, it makes 4 bonds with the surrounding 4 Si atoms. Thus the 5th valence e- of the P atom is extra and only weakly bonded to the nucleus. The group V impurity will generate energy level(s), known as the donor level, below the conduction band edge.
What model can be used to calculate the binding energy of the free electron in N type doping to the group V element’s nucleus, and what does the energy correspond to?
The single electrons attraction to the nucleus can be seen as analogous to the hydrogen atom, so the hydrogenic model of dopant levels can be used to calculate the binding energies of the electron, scaled by the dielectric constant of Si, ε, and th effective mass of the electron in Si, m*.
This energy corresponds to the energy needed to excite the electrons from the donor level to the conduction band states.
What is the equation for the donor or acceptor binding energy?
R = (Ro Z^2 m*) / εr^2 Ro- Rydberg constant (13.6 eV) Z- valence difference m*- effective mass εr- dielectric constant
Name 2 reasons why having a small effective mass m* is important
- Small m* allows easy ionisation of dopants
- Small m* gives high mobility and high conductivity
What is mobility with reference to superconductors?
The carrier velocity per unit of electric field (how fast can the electrons go?)
What are dangling bonds?
Dangling bonds are caused by vacancies in imperfect crystalline lattices. These ‘broken bonds’ give rise to a state in the middle of the ‘forbidden’ band gap, as they are like a state without a bonding partner, as with the bonding level diagrams the unbonded elements have an energy level between the bonding and antibonding levels
Why are dangling bonds not good in semiconductors?
Each dangling bond has one electron, so is looking for one more to make a bond (the DB is half filled). Thus a DB can trap any passing electron from the conduction band and get filled, acting as a recombination centre for excess carriers which is detrimental to both transistors and opto-electronic devices as it stops conduction, so its best to remove and DBs.
What is the purpose of ‘passivation’, oxidation of the free surfaces of the Si?
The free surface consists of a series of dangling bonds, which causes recombination of carriers which is very bad for conduction. The oxidation will convert the DBs to Si-O bonds, which lie outside the Si band gap so are inert.
What are the benefits of amorphous semi-conductors compared to crystalline?
They are attractive for large area semiconductors, due to easy of manufacturability and much lower price
What is the main difference in the band structure between amorphous and crystalline Si, and what is the cause of this?
The band gap for amorphous is not a totally forbidden gap. The valence and conduction states tail into the original band gap, and are called extended states.
This originates from the deviations of bond lengths and angles arising from the long range structural disorder.
How is a-Si made and what are the benefits of this?
Made by plasma deposition from silane SiH4. This still contains some hydrogen and this is able to sit on dangling bonds, removing the DB state from the gap, and since the Si-H bonds are well outside the band gap, the number of defect states in the band gap is greatly reduced, making it a good electronic material.
Name 3 benefits of a-Si:H over c-Si
- Cheaper by a factor of 100,000
- Can be made over large areas
- Can be made at much lower temperatures, 250C rather than 1000C
