MSE 2090 Exam 2 Questions with
Complete Solutions
electron energy band - ANSWER-when atoms are in close proximity are perturbed and
form a band of closely spaced states
formation of electron energy band - ANSWER-sodium example)) valence electrons in a
isolated Na atom --> hypothetical Na4 molecule - the 3s band splits in 4 distinct states --
> Na solid - a continuum of closely spaced discrete states and 3s valence "band" is half-
filled
the splitting of an energy state is dependent on the - ANSWER-interatomic spacing
facts about valence and energy band - ANSWER--Fans out because they do not want
to occupy the same energy spaces
-Along the equilibrium interatomic spacing, there is an energy band
-Others are not as wide --> because they are not the valence electrons, they are
splitting as much
-Band structures are based on quantum numbers
-Band gaps do not have energy states in them, so we can not have electrons in them
a "continuum" - ANSWER-energy band structure at equilibrium spacing
band gaps are WHAT energy ranges - ANSWER-forbidden
why do we care about energy band structure? - ANSWER-the nature of this energy
band structure determines if a material is a conductor, semi-conductor, or insulator
fermi energy - ANSWER-is the highest filled state at 0K; related to valence structure
Metals have energy states adjacent to highest filled state at Ef. What are two ways this
can happen? - ANSWER-1. partially filled band
2. a filled band overlaps with an empty band
for what and what, the valence band is full and separated from the empty conduction
band - ANSWER-semiconductors and insulators
insulator - ANSWER-- valence band is separated by a larger energy gap (>2 eV)
- ionic or strongly covalent
- ions, electrons, and holes
, semiconductor - ANSWER-- valence band is separated by a smaller energy gap (< 2
eV)
- weakly covalent
- electrons and holes
conduction in metals due to - ANSWER-free electrons
free electrons - ANSWER-electrons with energies > fermi energy (Ef)
conduction in metals - ANSWER-1. energy states within a band are VERY close
together (the lines & dots)
2. energy from electric field excites electron into conduction band
3. metals have high density of free carriers (electrons) --> HIGH CONDUCTIVITY
conduction in insulators and semiconductors due to - ANSWER-free electrons and
holes
the position of a missing electron; is treated as a positively charged particle
- charge is the same as electron but opposite sign (+1.6 e-19 C) - ANSWER-hole
larger band gap - ANSWER-insulators - more energy required to create "free charge
carriers" --> LOWER CONDUCTIVITY
true or false? higher temperature = more energy to excite electrons - ANSWER-true
are there holes in metals? - ANSWER-NO because do not jump a band-gap isolated to
a single or overlapping band
How do free charge carriers move and what affects their movement? - ANSWER-An
electric field exerts a FORCE on the FREE CHARGE CARRIERS
- electrons will accelerate in opposite direction
- holes will accelerate in the field direction
True or False? In a perfect crystal, electrons will continue to accelerate as long as an
electric field is applied, thus current would continually increase with time. - ANSWER-
False. Current reaches a constant value. Crystal is not perfect!! Imperfections lead to
FRICTIONAL FORCES!!!
drift velocity - ANSWER-when electrons reach a terminal velocity due to scattering
scattering - ANSWER-- electrical resistance comes from scattering
- causes: atom vibrations, impurities, structural defects (point, line, and planar)
- lower amount of scattering --> fewer impurities/defects --> more mobility
Complete Solutions
electron energy band - ANSWER-when atoms are in close proximity are perturbed and
form a band of closely spaced states
formation of electron energy band - ANSWER-sodium example)) valence electrons in a
isolated Na atom --> hypothetical Na4 molecule - the 3s band splits in 4 distinct states --
> Na solid - a continuum of closely spaced discrete states and 3s valence "band" is half-
filled
the splitting of an energy state is dependent on the - ANSWER-interatomic spacing
facts about valence and energy band - ANSWER--Fans out because they do not want
to occupy the same energy spaces
-Along the equilibrium interatomic spacing, there is an energy band
-Others are not as wide --> because they are not the valence electrons, they are
splitting as much
-Band structures are based on quantum numbers
-Band gaps do not have energy states in them, so we can not have electrons in them
a "continuum" - ANSWER-energy band structure at equilibrium spacing
band gaps are WHAT energy ranges - ANSWER-forbidden
why do we care about energy band structure? - ANSWER-the nature of this energy
band structure determines if a material is a conductor, semi-conductor, or insulator
fermi energy - ANSWER-is the highest filled state at 0K; related to valence structure
Metals have energy states adjacent to highest filled state at Ef. What are two ways this
can happen? - ANSWER-1. partially filled band
2. a filled band overlaps with an empty band
for what and what, the valence band is full and separated from the empty conduction
band - ANSWER-semiconductors and insulators
insulator - ANSWER-- valence band is separated by a larger energy gap (>2 eV)
- ionic or strongly covalent
- ions, electrons, and holes
, semiconductor - ANSWER-- valence band is separated by a smaller energy gap (< 2
eV)
- weakly covalent
- electrons and holes
conduction in metals due to - ANSWER-free electrons
free electrons - ANSWER-electrons with energies > fermi energy (Ef)
conduction in metals - ANSWER-1. energy states within a band are VERY close
together (the lines & dots)
2. energy from electric field excites electron into conduction band
3. metals have high density of free carriers (electrons) --> HIGH CONDUCTIVITY
conduction in insulators and semiconductors due to - ANSWER-free electrons and
holes
the position of a missing electron; is treated as a positively charged particle
- charge is the same as electron but opposite sign (+1.6 e-19 C) - ANSWER-hole
larger band gap - ANSWER-insulators - more energy required to create "free charge
carriers" --> LOWER CONDUCTIVITY
true or false? higher temperature = more energy to excite electrons - ANSWER-true
are there holes in metals? - ANSWER-NO because do not jump a band-gap isolated to
a single or overlapping band
How do free charge carriers move and what affects their movement? - ANSWER-An
electric field exerts a FORCE on the FREE CHARGE CARRIERS
- electrons will accelerate in opposite direction
- holes will accelerate in the field direction
True or False? In a perfect crystal, electrons will continue to accelerate as long as an
electric field is applied, thus current would continually increase with time. - ANSWER-
False. Current reaches a constant value. Crystal is not perfect!! Imperfections lead to
FRICTIONAL FORCES!!!
drift velocity - ANSWER-when electrons reach a terminal velocity due to scattering
scattering - ANSWER-- electrical resistance comes from scattering
- causes: atom vibrations, impurities, structural defects (point, line, and planar)
- lower amount of scattering --> fewer impurities/defects --> more mobility
