Chapter 3 -carriers ( Lec 7-10)

Question 1

What is a charge carrier

Answer 1

Particles that carry electric charges to contribute to current

Question 2

How do you find the total charge transported by the charge carriers

Answer 2

number of charge carriers multplied by the electron charge (1e-19C)

Question 3

What are the two types of charge carriers

Answer 3

1) Postive CC - Holes
2) Negative CC - Electrons

Question 4

What are the two semiconductor classifications in terms of impurity contents

Answer 4

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

Question 5

Describe the three energy levels associalted with INTRINSIC semiconductors

Answer 5

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

Question 6

What is EHP generation and its properties

Answer 6

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)

Question 7

What is EHP recombination and its properties

Answer 7

When electron looses energy (due to collision) and falls back to the valance band it unites with a hole
- and EHP is annihilated

Question 8

What is true for an intrinsic semiconductor in regards to number of electrons and number of holes

Answer 8

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

Question 9

Why would you increase the amount of impure atoms in an extrinsic semiconductor

Answer 9

To increase the concentration of one type of charge carrier (electron or hole) over the other

Question 10

Whate are N and P type semiconductors

Answer 10

N - higher concentration of electrons thean holes (n for negative)
P- higher concentration of holes than electrons (p for positive)

Question 11

What is Doping

Answer 11

The process of adding impurity atooms to a semiconductor

Question 12

What happens when you dope with V group elements. give two examples of v elements

Answer 12

Increases concentration of electrons in an intrinisic semiconductor
Phosphorous arsenic

Question 13

What happens when you dope with group III materials. give two examples of III elements

Answer 13

Increases concentration of holes in an intrinisic semiconductor
Boron Indium

Question 14

Why is N type doping refered to ass Donor

Answer 14

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

Question 15

Why is P type doping refered to ass ACCEPTOR

Answer 15

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.

Question 16

What is the majority and minortiy carriers for both n and p type materials

Answer 16

N type- Electrons are majority and holes are minority
P type- holes are majority and electrons are minority

Question 17

What is the location of Ef for an intrinsic semiconductor

Answer 17

Ef in in the middle of the bandgap between Ec and Ev

Question 18

In regards to firmi function, how does temperature affect the probability of occupancy of an electron

Answer 18

As the temperature increases the porbability of occupancy of an energy level above the fermi level increases
Inverse is also true

Question 19

what is a non degenerate semicondictor

Answer 19

WHen the fermi level Ef is at least 3kt above Ev or 3kt below Ec

Question 20

Explain the electron and hole concentration for non degenrate semiconductors

Answer 20

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.

Question 21

Describe the process of Complete Ionization

Answer 21

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.

Question 22

Explain the different regions of electron concentrations of Si as a function of temperature

Answer 22

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

Question 23

What does the random motion of electrons lead to in terms of displacement

Answer 23

Zero net displacement

Question 24

what id the mean free path and mean free time

Answer 24

The average distance between 2 collisions (lambda)
The average time between 2 collisions ( tau c )

Question 25

How does doping effect mobility

Answer 25

Higher doping resutls in higher ionized impurity scattering and mobility goes down - consequently current goes down

Question 26

Explain Lattice Scattering

Answer 26

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

Question 27

What is the relationship between vibrationa and collision in regards to lattice scattering

Answer 27

As vibration increases mu decreases meaning collison increases the opposite is also true

Question 27

what does invariance of the fermi level imply

Answer 27

No discontinuity or gradient can arise in the equilibrium Fermi level when two materials are brought into intimate contact to form a metallurgical junction

Question 28

What is the Lorentz Force

Answer 28

The force that causes the particle to move in a direction perpendicular to both the electric field and magnetic field

Question 29

What is the charge of the hall voltage or P and N type

Answer 29

P - postive hall voltage
N - Negative hall voltage