Energy bands and gaps in semiconductor

1. A SEMINAR ON
ENERGY BANDS AND GAPS
IN SEMICONDUCTOR
K. GANAPATHI RAO
(13031D6603)
Presented
By
Presence of
Mr. M. Venkaiah

2.  Introduction
 Energy band
 Classification of materials
 Direct and indirect band gap of SC
 Classification of SC
 References
Out Line

3. INTRODUCTION
 Si= 14 electrons 1s2 2s2 2p6 3s2 3p2
 4 valence electrons
 Electrons present in the outermost orbit are called valence
e- and their behavior decide electrical characteristics of a
materials.
 For Si 2 e- are present in 3s and 2 e- are in 3p orbital's
 Consider n Si atoms, which are for apart from each other so
that the interaction b/w them is negligible.
 In this 2N e- occupy the 3s level and 2N e- in 3p level.
 When the atoms are brought to closer, the interaction b/w
increases and they now form a electronic system.

4.  The electronic system should obey Paulie’s exclusion
principal , which states that no 2 e- are in the system can
have same amount of energy.
 To obey this 3s and 3p level spilt into multiple levels so that
each e- can occupy a district energy level.
 When inter atomic distance is further reduced. The 2N ‘s’
levels and 6N ‘p’ levels combined into a single band.
 At lattice spacing, single band is split into 2 bands with
upper band containing 4N and lower band with 4N levels.
 At 00 k e- possess lowest energy , they fill up valence band
and conduction band remaining empty.

5.  There Important energy bands are,
 Valence Band
 Conduction Band
 Forbidden Band
Energy Bands theory

6. Valence Band
 Band of energy level which are closer to
nucleolus.
 An e- in valence band, expenses strong force
of attraction from nucleolus.
 And it can’t move freely when extrenal
electric field is applied.
 It is called bounded electron.

7. Conduction Band
 Bond of energy levels which are far away from the
nucleolus.
 The conduction band is the band of orbitals that are
high in energy and are generally empty.
 An e- in Conduction band has weak influence of
nucleolus and hence it can move free under the
effect of applied electric field and thus it produces
current, it is called free electrons

8. Forbidden Band / Energy Gap
 In solid-state physics, an energy gap or bandgap,
is an energy range in a solid where
no electron states can exist.
 It generally refers to the energy difference
(in electron volts) between the top of the valence
band and the bottom of the conduction
band in insulators and semiconductors.

9. Types of Materials
 Materials can be divided into 3 types based
on the values of energy gap
 Insulator
 Conductor
 Semi Conductor

11. INSULATORS
 It is a material with large energy gap
 Eg= several ev
 ev= 1.6*10-19 joules
 Due to large energy gap an e- from valence band can’t
move into conduction band remains complete fill.
 Conduction band completly empty.
 Ex: glass, Diamond, Silicon di oxide
 Energy gap of diamond is ~6ev.

12. CONDUCTORS
 It is a material having zero energy gap. The materials in
which conduction and valence bands.
 Valence electrons can move valence to conduction band
without requiring thermal energy.
 The overlapping indicates a large number of electrons
available for conduction.
 Hence the application of a small amount of voltage
results a large amount of current.
 Ex: All metals.
 Best conducting materials are
 Silver is best, cupper is second best

13. SEMICONDCUTOR
 The materials, in which the conduction and valence
bands are separeated by a small energy gap (1eV) are
called semiconductors.
 Silicon and germanium are the commonly used
semiconductors.
 A small energy gap means that a small amount of
energy is required to free the electrons by moving them
from the valence band in to the conduction band.
 The semiconductors behave like insulators at 0K,
because no electrons are available in the conduction
band.
 If the temperature is further increased, more valence
elctrons will acquire energy to jump into the conduction
band.

14.  If a valence receives sufficient thermal energy, it
can move into conduction band leaving a vacancy
in valence band, which is called hole and therefore,
if one e- is sufficiently thermally energized it creates
a pair of free e- and hole, this process is called
carrier generation
 Carrier generation can happen due to
 Thermal excitation
 Photo excitation
 Electrical excitation
 Impact ionization

15. Eg (T)= 1.21-3.6*10-4 T for si
Eg (T)= 0.785-2.23*10-4 T for Ge
 When external electric field is applied free electron
travels with in conduction band producing electron
current.
 Where as hole travels in valence, producing hole
current.
 The total current will be sum of free & hole current.
 I total = I free e- + I hole
 As temp increase free e- & holes will increase and
conductivity increases.

16. Direct & Indirect band gap SC
 The process of free e- returning from conduction
band to valence band which causes disappearance
of a free e- , hole pair is called carrier
recombination.
 SC are divided into 2 types based upon the method
of recombination.
 Direct band gap SC
 Indirect band gap SC

17. Direct band gap SC
 For these SC , Conduction band minima and valence band
maxima occurs at same value of momentum.
 An e- from CB directly return to VB without changing It’s
momentum. And releases energy in the form of light (photon
hv).
 Ex: GaAS, Gap.GaAsP,

18. Indirect band gap SC
 CB minima and VB maxima occurs at different value of
momentum.
 When e- from CB returns VB after changing its momentum
is called indirect band gap sc.
 E- changes its momentum by releasing phonon which is a
heat particle.
 Ex: Si, Ge

19. Intrinsic SC
 IV group is carbon, Si, Ge, Tin, Lead.
 In it sc crystal each atom forms four covalent bonds to
become stable.
 At 00 , all valence e- participate in covalent band formation.
Conduction not possible.
 if temp is increases, greater no of free e- hole pair will be
created. Causes conduction
 n=p=ni
 J=ni(un+up)eE

20. N type SC
 Vth group Phospurs, Arsenic, Antimon.
 P forms 4 covalent bonds with Si and donate 5th electron to
crystal. It is called free e-.
 According to law of Nutrality nn=pn+nD
 Mass action law nn*pn=ni
2
 Majority is e-
minority is holes

21. P type SC
 3rd group B, Al, Ga.
 B forms 3 covalent bonds with Si and forms 4th bond with
hole formation.
 According to law of Nutrality np +nA=pp
 Mass action law np*pp=ni
2
 Majority is holes
minority is e-

22. References
 http://en.wikipedia.org/wiki/Electronic_band_structu
re
 http://ecee.colorado.edu/~bart/book/book/chapter2/
ch2_3.htm
 http://pubs.acs.org/doi/abs/10.1021/jz402165b
 http://iopscience.iop.org/0305-4608/5/11/020