PHYS 342/555PHYS 342/555 Introduction to solid state physics · Dai/PHYS 342/555 Spring 2013...

PHYS 342/555PHYS 342/555Introduction to solid state physics

Instructor: Dr. Pengcheng DaiProfessor of PhysicsProfessor of Physics

The University of Tennessee(Room 407A Nielsen 974 1509)(Room 407A, Nielsen, 974-1509)

Chapter 7: Energy BandsLecture in pdf format will be available at:Lecture in pdf format will be available at:http://www.phys.utk.edu

Bloch’s theoremBloch s theorem

The eigenstates of the one-electron Hamiltonian 2

2 ( ) , where ( ) ( ) for all in 2

H U r U r R U r Rm

a Bravias lattice, can be chosen to have the form of a plane wavetimes a function with the p

eriodicity of the Bravias lattice:p y

( ) ( ), where ( ) ( )

f ll i th B i l tti

ik rnk nk nk nkr e u r u r R u r

for all in the Bravias lattice. or

( ) ( )ik R

r R e r

Dai/PHYS 342/555 Spring 2013 Chapter 7-2

1Also, for small , effectively averages to zero so it still looks like the free electron case where

looks like the free electron case where

( ) and / 2

A h / ( 2 / / ) B

ik rr Ae k m

As approaches / ( 2 / / ), we expect a Bragg reflection or a

k a k a standing wave. We can form two different standing

f h li ( / )iwaves from the two traveling waves exp( / ), or(+) exp( / ) exp( / ) 2cos( / );( ) ( / ) ( / ) 2 i ( / )

i x ai x a i x a x ai i i

( ) exp( / ) exp( / ) 2 sin( / )Both standing waves

i x a i x a i x a are composed of equal parts of right- and left-

di d lidirected traveling waves.

Origin of the energy gapOrigin of the energy gap

The two standing waves (+) and ( ) pile up electrons at different regions, and therefore the two waves have different values of the potential energy. This is the origin of the energy gap. The probability

density of a particle is . For a standing wave (+),

( ) cos /x a

( ) cos / .This function piles up electrons (negative charge) on the positive ions centered at 0 2 where the potential e

nergy is lowestcentered at 0, , 2 ,...where the potential ex a a nergy is lowest.

For the other standing wave ( ) the probability density is

( ) sin / .x a

The potential energy of is lower than that of the traveling wave

The potential energy of is lower than that of the traveling wave,

and that of is higher than the traveling wave. The difference isthe energ gap

the energy gap.

Magnitude of the energy gapMagnitude of the energy gap

Th f i h B ill i b d /kThe wavefunction at the Brillouin zone boundary / are

2 cos / and 2 sin / , normalized over unit length of line.

x a x a

If the potential energy of an electron in the crystal at point is( ) cos 2 /

xU x U x aThe first-order energy difference between the two standing wavestates is

( )[ ( ) ( ) ]

2 cos(2 / )(cos / sin / )

gE dxU x

dxU x a x a x a U

2 cos(2 / )(cos / sin / )dxU x a x a x a U

Kronig-Penney ModelKronig Penney Model

Chapter 7: Energy BandsChapter 7: Energy Bands

The periodic potential

2 2 ( )2

H U rm

Metals and insulatorsMetals and insulators

Consider the free electron energy bands of an fcc crystal lattice in theapproximation of an empty lattice, but in the reduced zone scheme in which all 's are transformed to lie in the first Brillouink zone. Plotroughly in the [111] direction the energies of all bands up to six timesroughly in the [111] direction the energies of all bands up to six timesthe lowest band at the zone boundary at

1 1 1(2 / )( )k a (2 / )( , , ).2 2 2

Pentavalent impuritiesPentavalent impuritiesImpurity atom with 5 valence electrons produce n-type semiconductors by contributing extra electrons.

Trivalent impurities Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a "hole" or electron deficiencyby producing a hole or electron deficiency.

The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous

may be added by diffusion of phosphine gas (PH3).may be added by diffusion of phosphine gas (PH3).

The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor createsaluminum or gallium to an intrinsic semiconductor creates

deficiencies of valence electrons, called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the

silicon materialsilicon material.

Semiconductor Current

Both electrons and holes contribute to current flow in an intrinsic semiconductor.

Depletion RegionDepletion Region When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sitesIn so doing they leave behind positive ions at the donor impurity sites.

P-N Energy Bands

For a p-n junction at equilibrium, the fermi levels match on the two sides of the junctions. Electrons and holes reach an equilibrium at the junction and form a depletion region. The upward direction in the diagram represents increasing electron energy. That implies that you would have to supply enery to get an electron to go up on the diagram, and supply energy to get a hole to go down.

P-N Energy Bands

To reverse-bias the p-n junction, the p side is made more negative, making it "uphill" for electrons moving across the junction. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy.

-N Energy Bands

To forward bias the p-n junction, the p side is made more positive, so that it is "downhill" for electron motion across the junction An electron can move across the junction and fill a vacancy or "hole" nearmotion across the junction. An electron can move across the junction and fill a vacancy or hole near the junction. It can then move from vacancy to vacancy leftward toward the positive terminal, which could be described as the hole moving right. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy.

When the p-n junction is forward biased, the electrons in the n type material which have been elevated to the conductionn-type material which have been elevated to the conduction

band and which have diffused across the junction find themselves at a higher energy than the holes in the p-type

t i l Th dil bi ith th h l kimaterial. They readily combine with those holes, making possible a continuous forward current through the junction.

Forward Biased ConductionForward Biased Conduction The forward current in a p-n junction when it is forward-biased (illustrated below) involves electrons from the n-type material moving leftward across the junction and combining with holes in the p type material Electrons canthe junction and combining with holes in the p-type material. Electrons can then proceed further leftward by jumping from hole to hole, so the holes can be said to be moving to the right in this process.

PHYS 342/555PHYS 342/555 Introduction to solid state physics · Dai/PHYS 342/555 Spring 2013...

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