Band theory of solid

1. BBAANNDD TTHHEEOORRYY
OOFF
SSOOLLIIDD

2.  In isolated atoms the energy levels of
electrons are discrete.
 When N number of electron interact with
each other, the discrete energy levels of
electrons in the solid are split up.

3.  The last completely filled (at least at T =
0 K) band is called the Valence Band
 The next band with higher energy is the
Conduction Band
• The Conduction Band can be empty or
partially filed
 The energy difference between the
bottom of the CB and the top of the VB is
called the Band Gap (or Forbidden Gap)

4. Conductor
Insulator
Semiconductor
Conduction Band
Valence Band
Conduction Band
Valence Band
EEgg
Conduction Band
Valence Band
EEgg

5.  Semiconductor is a material that has a
resistivity value between that of a
conductor and an insulator.
 There are two types of semiconductors
• Intrinsic semiconductors
• Extrinsic semiconductor
 P - type semiconductor
 N - type semiconductor

6.  An intrinsic semiconductor is a pure
semiconductor
 The number of excited electrons and the
number of holes are equal: n = p.
CB
VB
Ec
Ef
Ev

7. An extrinsic semiconductor is a semiconductor
that has been doped
P - TYPE SEMICONDUCTOR N - TYPE SEMICONDUCTOR
 p-type semiconductors have a
larger hole concentration than
electron concentration
 n-type semiconductors have a
larger electron concentration
than hole concentration
CB
EC
Ef
VB Ev
CB
EC
Ef
VB Ev

8.  P–N junction is a junction formed by combining
P-type and N-type semiconductors
P-type N-type
EC
Ev
Ef
Ev
CB
CB
EC
VB
VB
Depletion
Region

9. EECC
EEvv
EEff
EEvv
CCBB
CCBB
EECC
DDeepplleettiioonn
RReeggiioonn
VVBB VVBB
FFoorrwwaarrdd bbiiaass

10. VVBB
VVBB
VVBB
EECC
EEvv
EECC
EEff
EEvv
VVBB
DDeepplleettiioonn
RReeggiioonn
Reverse bias

11. Anode Cathode
 Conducting in one
direction and not in the
other is the I-V
characteristic of the diode.
 Forward biasing voltage
makes it turn on.
 Reverse biasing voltage
makes it turn off
Forward bias
Reverse bias

12.  The Zener diode is made to operate under
reverse bias
 Brackdown voltage varies from 2 – 200 V
Forward bias
Reverse bias
Zener diode as voltage regulator
Breakdown
voltage
A CC

13.  Variable reactance diode
 Voltage-controlled capacitance of a p n
junction can be used in tuning stage of a
radio or TV receiver.
Anode Cathode
Symbol
CT
VR
CT ∞ 1/√VR
w ∞ √VR

14.  Many of these diodes involve direct
bandgap semiconductors.
 Devices to convert optical energy to
electrical energy
• photodetectors: generate electrical signal
• Solar cells: generate electrical power
 Devices to convert electrical energy to
optical energy
• light emitting diodes (LEDs)
• laser diodes

15.  movement of carriers across their depletion
region emits photons of light energy
 For visible light output, the bandgap should
be between 1.8 and 3.1 eV.
P type
N type
Ohmic Contact
AA CC
Symbol

16.  Photodiodes are sensitive to light.
 In Photoconductive mode the saturation
current increases in proportion to the
intensity of the received light.
• An important characteristic of any
photo-detector. Measures how the
photocurrent, IL varies with the
wavelength of incident light.
AA CC
Symbol
V
I
Increasing
light intensity

17.  In Photovoltaic mode, when the pn junction
is exposed to a certain wavelength of light,
the diode generates voltage and can be
used as an energy source.
 This type of diode is used in the production
of solar power.
 Also known as
Solar cells

18.  Solar cells are large area pn-junction
diodes designed specifically to avoid
energy losses.
• Voc= the open circuit voltage
• Isc = current when device is
short circuited
h = power conversion
efficiency
= (Im Vm)/Pin
I
Voc
VA
Vm
– Isc
–Im

19. Thanks……….