Chapter 3 -carriers ( Lec 7-10) Flashcards
What is a charge carrier
Particles that carry electric charges to contribute to current
How do you find the total charge transported by the charge carriers
number of charge carriers multplied by the electron charge (1e-19C)
What are the two types of charge carriers
1) Postive CC - Holes
2) Negative CC - Electrons
What are the two semiconductor classifications in terms of impurity contents
Intrinsic semiconductor - Pure and defect-free
Extrinsic semi - Impurity atoms are artificially introduced to increase or decrease the number of charge carriers. This either increases other number of electrons or increases the number of holes
Describe the three energy levels associalted with INTRINSIC semiconductors
Ec - E level at the lower edge of the conduction band (minimum conduction band energy)
Ev - E level at the upper edge of the valence band ( Max valence band energy)
Eg- Bandgap energy ( Energy difference between minimum conduction band energy and the maximum valence band energy
Ec = Ev + Eg
What is EHP generation and its properties
When an electron jumps to the conduction band from the valance band a hole is created in the valance band
For every free electron in the conduction band there is a hole in the valance band
- Electorn holes are made in pairs hence electrn hole pair
as a whole the semiconductor remains natural
positive charge (hole) = negative charge (electron)
What is EHP recombination and its properties
When electron looses energy (due to collision) and falls back to the valance band it unites with a hole
- and EHP is annihilated
What is true for an intrinsic semiconductor in regards to number of electrons and number of holes
The equilibrium concentration of electrons (no) in the conduction band must be equal to the equilibrium concentration of the holes (po) in the valance band. This is referred to as ni
no = po =ni
Why would you increase the amount of impure atoms in an extrinsic semiconductor
To increase the concentration of one type of charge carrier (electron or hole) over the other
Whate are N and P type semiconductors
N - higher concentration of electrons thean holes (n for negative)
P- higher concentration of holes than electrons (p for positive)
What is Doping
The process of adding impurity atooms to a semiconductor
What happens when you dope with V group elements. give two examples of v elements
Increases concentration of electrons in an intrinisic semiconductor
Phosphorous arsenic
What happens when you dope with group III materials. give two examples of III elements
Increases concentration of holes in an intrinisic semiconductor
Boron Indium
Why is N type doping refered to ass Donor
Becasue when an element is dopen with a n type element if forms the 4 nessicary tetrehedreal bonds but there is an extra valance electron left over (5 total valance - 4 used in bonds) so the extra electron is DONATED and moves to the conduction band
Why is P type doping refered to ass ACCEPTOR
In the sames sense as N type. the p type doping atom has 3 valance electrons and needs 4 to create the 4 tetrahedral bonds to dope the lattice structed so it “accepts” an electron that is thermally generated to form the 4th bond.
What is the majority and minortiy carriers for both n and p type materials
N type- Electrons are majority and holes are minority
P type- holes are majority and electrons are minority
What is the location of Ef for an intrinsic semiconductor
Ef in in the middle of the bandgap between Ec and Ev
In regards to firmi function, how does temperature affect the probability of occupancy of an electron
As the temperature increases the porbability of occupancy of an energy level above the fermi level increases
Inverse is also true
what is a non degenerate semicondictor
WHen the fermi level Ef is at least 3kt above Ev or 3kt below Ec
Explain the electron and hole concentration for non degenrate semiconductors
the electron or hole
concentration is much lower than the effective density of states in the
conduction band (NC) or the valence band (Nv), respectively.
Describe the process of Complete Ionization
For shallow donors in n-Type silicon and gallium arsenide there is usually enough thermal energy to supply the energy Ed to ionize all donor impurities at room temp and provide the right number of electrons in the conduction band.
Explain the different regions of electron concentrations of Si as a function of temperature
Low temp - thermal energy in the crystal isn’t enough to ionize so some electrons are frozen in the donor level
Mid temp - as temp increases complete ionization is reached as the temp is further increases the concentration remains the same over a wide temp range
High temp - Beyond this point the semi conductor becomes intrinsic after the point where the intrinsic carrier concentration ni increases with temp
What does the random motion of electrons lead to in terms of displacement
Zero net displacement
what id the mean free path and mean free time
The average distance between 2 collisions (lambda)
The average time between 2 collisions ( tau c )
How does doping effect mobility
Higher doping resutls in higher ionized impurity scattering and mobility goes down - consequently current goes down
Explain Lattice Scattering
Due to thermal energy ations in a lattice vibrate
As the atoms are bonded together they cannont vibrate indipentantly
Each bond acts like stiffness
the vibrations take the form of the collective modes which propagate throughout the material
What is the relationship between vibrationa and collision in regards to lattice scattering
As vibration increases mu decreases meaning collison increases the opposite is also true
what does invariance of the fermi level imply
No discontinuity or gradient can arise in the equilibrium Fermi level when two materials are brought into intimate contact to form a metallurgical junction
What is the Lorentz Force
The force that causes the particle to move in a direction perpendicular to both the electric field and magnetic field
What is the charge of the hall voltage or P and N type
P - postive hall voltage
N - Negative hall voltage
