Lecture 23:

Diodes
Prof. Abhishek Dixit, Prof. Lalan Kumar, Prof. S. D. Joshi, Prof. I.
N. Kar, Prof. M. Veerachary
October 1, 2024 (3:25 PM to 3:35 PM)
• Password: ub1lcq

2
Conduction in Solids
• Conduction occurs if free electrons are available to carry
charge under action of electric field.
• Depending on availability of free electrons, solids can be
categorized into :
• Conductors : large number of mobile charge carriers.
• Insulators : Practically no free charge carriers.
• Semiconductors : Conductivity intermediate of conductors and
insulators.

Tuesday, October 1, 2024

Ability to conduct electricity
Insulator
Material Resistivity Conductivity
Glass
Sulphur
Quartzfused

Conductor
Material Resistivity Conductivity
Silver
Copper
Aluminium

4
Ability to conduct electricity
Insulator
Material Resistivity Conductivity
Glass
Sulphur
Quartzfused

Conductor
Material Resistivity Conductivity
Silver
Copper
Aluminium

5
Ability to conduct electricity
Insulator
Material Resistivity Conductivity
Glass
Sulphur
Quartzfused - - - - - -
- - - -
-

Conductor
Material Resistivity Conductivity
-
Silver - -- - - - -- -
- --
Copper - -
- - - -
- - - -
Aluminium

6
Ability to conduct electricity
Semiconductor
Material Resistivity Conductivity
Germanium
- - - - - -
Silicon - - - -
-

7
Ability to conduct electricity
Semiconductor
Material Resistivity Conductivity
Germanium
- - - - - -
Silicon - - - -
-

External energy

8
Ability to conduct electricity
Semiconductor
Material Resistivity Conductivity
Germanium
- - - - - -
Silicon - - - -
-

External energy

9
Semiconductors
• At room temperature, few electrons gain enough thermal
energy to get into conduction band (free electrons).
• Where there was an electron, there is a ‘hole’ left now.
• Region with free electron has net
-ve charge
• Region with hole has net +ve charge
• Both contribute to conduction
• Conductivity : 𝜎 = 𝑛𝜇𝑛 + 𝑝𝜇𝑝 𝑒
• n,p – electron/hole concentrations
• 𝜇𝑛 , 𝜇𝑝 - mobility of the charge carriers

Tuesday, October 1, 2024

Drift current of Intrinsic semiconductor

11
Drift current of Intrinsic semiconductor

12
Drift current of Intrinsic semiconductor

13
Drift current of Intrinsic semiconductor

14
Drift current of Intrinsic semiconductor

15
Doping
• The conductivity of a Si/Ge semiconductor can be altered by
adding impurity element from the third of fifth column of
periodic table.

Tuesday, October 1, 2024

Doping
• The conductivity of a Si/Ge semiconductor can be altered by
adding impurity element from the third of fifth column of
periodic table.
• Typical choices : For Silicon
• Boron, Gallium (trivalent),
• Phosphorus, Arsenic (Pentavalent)
• A semiconductor without doping
Is called intrinsic/pure (𝑛 = 𝑝 = 𝑛𝑖 )

Tuesday, October 1, 2024

Doped Semiconductor

n-type

18
Doped Semiconductor

n-type p-type

19
Doping – Pentavalent (n-type)
• When a pentavalent atom replaces Si atom in crystal.
• There is an excess free electron
which can go into conduction band.
(with little thermal energy).
• Resulting material has negative
Charge carriers in electrically
neutral material
n-type semiconductor (𝑛 ≫ 𝑝, 𝑛𝑝 = 𝑛𝑖2 )

Tuesday, October 1, 2024

Doping – Trivalent (p-type)
• When a trivalent atom replaces Si atom in crystal.
• There only 3 valence electrons are available instead of 4.
• If the remaining unfilled covalent
Is filled from neighbouring atom,
There is a ‘hole’ created.
• Resulting material has positive
Charge carriers in electrically
neutral material (effectively)
p-type semiconductor

Tuesday, October 1, 2024

Doped Semiconductor
Recombination
• In a semiconductor,
the mobile electrons and holes tend to recombine and disappear
• The rate of recombination

• For the doped and intrinsic semiconductors,

22
Doped Semiconductor
Recombination
• In a semiconductor,
the mobile electrons and holes tend to recombine and disappear
• The rate of recombination

• For the doped and intrinsic semiconductors,

23
Doped Semiconductor
Recombination
• In a semiconductor,
the mobile electrons and holes tend to recombine and disappear
• The rate of recombination

• For the doped and intrinsic semiconductors, we have

24
Doped Semiconductor
Conductivity of dopped semiconductor
• In a typical n-type material, donor atoms provide a mobile electron concentration

• Using

• Increasing reduces

Conductivity of the doped semiconductor is determined by the doping concentration

25
Doped Semiconductor
Conductivity of dopped semiconductor
• In a typical n-type material, donor atoms provide a mobile electron concentration

• Using

• Increasing reduces

Conductivity of the doped semiconductor is determined by the doping concentration

26
Doped Semiconductor
Conductivity of dopped semiconductor
• In a typical n-type material, donor atoms provide a mobile electron concentration

• Using

• Increasing reduces

Conductivity of the doped semiconductor

is determined by the doping concentration

27
Doped Semiconductor
Diffusion current
• Non-uniform concentration of electric charges
enables the charges to move from a high concentrated region to a low one.

28
Doped Semiconductor
Diffusion current
• Non-uniform concentration of electric charges
enables the charges to move from a high concentrated region to a low one.

29
Doped Semiconductor
Diffusion current
• Non-uniform concentration of electric charges
enables the charges to move from a high concentrated region to a low one.

• The diffusion current crossing a unit:

30
Doped Semiconductor
Diffusion current Drift current
Diffusion current movement caused by Drift current movement caused by electric
variation in the carrier (hole or carrier) fields.
concentration
Direction of the diffusion depends on the slope Direction of the drift current is always in the
of the carrier concentration direction of the electric field.

• Total current in a semiconductor

31
Doped Semiconductor
Diffusion current Drift current
Diffusion current movement caused by Drift current movement caused by electric
variation in the carrier (hole or carrier) fields.
concentration
Direction of the diffusion depends on the slope Direction of the drift current is always in the
of the carrier concentration direction of the electric field.

• Total current in a semiconductor

32
Doped Semiconductor
Diffusion current Drift current
Diffusion current movement caused by Drift current movement caused by electric
variation in the carrier (hole or carrier) fields.
concentration
Direction of the diffusion depends on the slope Direction of the drift current is always in the
of the carrier concentration direction of the electric field.

• Total current in a semiconductor

33
Doped Semiconductor
Diffusion current Drift current
Diffusion current movement caused by Drift current movement caused by electric
variation in the carrier (hole or carrier) fields.
concentration
Direction of the diffusion depends on the slope Direction of the drift current is always in the
of the carrier concentration direction of the electric field.

• Total current in a semiconductor

34
Doped Semiconductor

p-type n-type

35
Junction Diodes
• All semiconductor doped/undoped are bilateral.
• But, if a p-type region placed close to an n-type region, there
is difference in carrier concentration.
• Current flows preferentially
In one direction.
• This device is a
Semiconductor diode.

Tuesday, October 1, 2024

pn-Junction Behaviour
• Majority carriers : Main cause of flow of current in a region.
(hole in p, electron in n)
• Because of the concentration
gradient, the majority carriers
diffuse across the junction and
recombine.

Tuesday, October 1, 2024

pn-Junction Behaviour
• Diffusion uncovers bound –ve charges in p region (and +ve
charge in n region)
• This region where the bound
charges are uncovered is
depletion region.
(depleted of majority carriers)
An electric field ε is created at
depletion region

Tuesday, October 1, 2024

pn-Junction Behaviour
• Diffusion uncovers bound –ve charges in p region (and +ve
charge in n region)
• This region where the bound
charges are uncovered is
depletion region.
(depleted of majority carriers)
An electric field ε is created at
depletion region
• The minority carriers drift due
to thermal energy
Tuesday, October 1, 2024
pn-Junction – Open Circuit
• There is a charge build up only in the
transition region.
• Creates an electric field and then a
“potential hill” V0.
• Potential hill OPPOSES diffusion and
ENCOURAGES drift.
• This potential (Contact Potential) is the
‘barrier’ required to balance diffusion
and drift.
• Vo = few tenths of a volt.

Tuesday, October 1, 2024

pn-Junction – Forward Bias
• Forward bias : Connecting of external
source (V) with p-type at higher
potential than n-type.
• The external potential effectively
reduces the barrier potential to V0-V.
• The process is very sensitive to barrier
voltage and a large increase in current
occurs for a small decrease in barrier
potential. (Exponential Relation)
• Process is sustained by supply of
electrons in n region and removal
from p region, by ext. battery
Tuesday, October 1, 2024
pn-Junction – Reverse Bias
• Reverse Bias : p junction is connected
to a lower potential than n junction.
• With reverse bias the potential barrier
increases.
• Probability of current by majority carriers
decreases exponentially.
• Small amount of reverse bias current
due to minority carrier drift.
• Ir is independent of V

Tuesday, October 1, 2024

Ideal and Practical Diode
• An ideal diode has a infinite diffusion, and zero drift.
• Presents
• Zero resistance in forward bias Like a switch
• Infinite resistance in reverse bias.

Ideal diode

Tuesday, October 1, 2024

Ideal and Practical Diode
• An ideal diode acts like a switch.
• A practical diode has a nonlinear I-V characteristics.
• Current-voltage relationship is exponential.

Typical barrier
voltage V0
Si  0.7 V
Ge  0.5 V

Ideal diode Practical Diode

Tuesday, October 1, 2024
45
— —
— — + + + +
— — — + + + + + +
— —
—
+ + + + + — — — — —
—
+ + +
— — — + + + + + +
— — — —— + + + + +
+
— — —
—
— — — —
+
+ + +
—

+
—
+ + +
— +
+
+ + + +
— — — — — + + + +
— + + +
— — — — — — + +
+ —
+
+ + — — — —
+
+ + + + — —
+
+
— — +
— — — + + + + + +
— + +
— — — — + + +
+ + + + + — — — — —
— +
+ — —
—

+
—
+
p-n Junction
p-n Junction
• Free moveable charges recombine => Depletion region

+
—

+
—
— — +
+
— — — — — + + + + +
+ + + — — — — — —
+ + + + —
+ + + + — —
+ — —

+
+
— — + + + +
+
— — — — + +
+
— +
+ + + + — — — — — —
+ + + + — — —
+ + + — — —
+ +

+
+

+
—

+ +
—

+
—

+ + +
+
— — —
—
— — — —
+ + + — —— —
+ + + + + + + —
+ + — — —

+ +
—
— — — — — + + + + + +
—
— —
+ + + + + +
+ + — — —
+ + — —
— —

Potential difference = built-in potential

46
p-n Junction
+ + + + + — — — — — —

—+
—
+
—
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+
—
+
—
+
—
+
—
+
— — + + +
— + +
— — —
+ — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

47
p-n Junction
+ + + + + — — — — — —

—+
—
+
—
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+
—
+
—
+
—
+
—
+
— — + + +
— + +
— — —
+ — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

48
p-n Junction
+ + + + + — — — — — —

—+
—
+
—
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+
—
+
—
+
—
+
—
+
— — + + +
— + +
— — —
+ — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

49
p-n Junction
+ + + + + — — — — — —

—+
—
+
—
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+
—
+
—
+
—
+
—
+
— — + + +
— + +
— — —
+ — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

50
p-n Junction
• Reverse bias
+ + + + + + — — — — — —

+ — —
+ + — + — + — + — — + + — + — + — + — + — + —

+
— —
+ + — + — + — + — — + + — + — + — + — + — + —

+ — —
+ + — + — + — + — — + + — + — + — + — + — + —

+ — —
+ + — —
+ + — + — — + + — + — + — + — + — + —

51
p-n Junction
+ + + + + — — — — — —

—+
—
+
—
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

—+
—
+
—
+
—
+
—
+
— — + + +
— + +
— — —
+ — +
—

—+ —
+ —
+ —
+ —
+ — — + + +
— + +
— — — + — +
—

52
p-n Junction
• Forward bias
+ + + + + + + — — — — — — — —

— —
+ + — + —
+ —
+ —
+ —
+ +
— +
— +
— — + — + — + — +
—

— —
+ + — + —
+ —
+ —
+ —
+ +
— +
— +
— — + — + — + — +
—

— —
+ + — + —
+ —
+ —
+ —
+ +
— +
— +
— — + — + — + — +
—

— —
+ + — + —
+ —
+ —
+ —
+ +
— +
— +
— — + — + — + — +
—

53
p-n Junction
The direction of current flow is opposite to electron-flow
• Forward bias
+ + + + + + + — — — — — — — —

— —
+ + —
+ —
+ —+ —+ + — + — +— —+ —+ — + — + — +
—

— —
+ + —
+ —
+ —+ —+ + — + — +— —+ —+ — + — + — +
—

— —
+ + —
+ —
+ —+ —+ + — + — +— —+ —+ — + — + — +
—

— —
+ + —
+ —
+ —
+ —+ + — + —
+— —+ —+ — + — + — +
—

54
Diode

55
Diode

56
Circuit with diode – 1

57
Circuit with diode – 1

Turn-on voltage

58
Circuit with diode – 1

59
Circuit with diode – 1

60
Circuit with diode – 1

61
Circuit with diode – 1

62
Circuit with diode – 1

63
Circuit with diode – 1

64
Circuit with diode – 2

65
Circuit with diode – 2

66
Circuit with diode – 2

67
Circuit with diode – 2

68
Circuit with diode – 3

–
+

69
Circuit with diode – 3

–
+

70
Circuit with diode – 3

–
+

–
+

71
Circuit with diode – 4

+ +
– –

72
Circuit with diode – 4

+ +
– –

73
Circuit with diode – 4

+ +
– –

74
Circuit with diode – 4

+ +
– –

75
Circuit with diode – 4

+ +
– –

76
Circuit with diode – 3 (p. 96)

77
Circuit with diode – 3 (p. 96)

78
Circuit with diode – 3 (p. 96)

79
Diode: Full-wave rectifier

80
Diode: Full-wave rectifier

81
Diode: Full-wave rectifier

82
Diode: Capacitor filter

By initial charges
in the capacitor

• During the positive half cycle,

the source voltage increases and the capacitor discharges

83
Diode: Capacitor filter

By initial charges
in the capacitor

• During the positive half cycle, if the source voltage is greater than the capacitor voltage
the diode will conduct, and the capacitor charges rapidly (C is small)

• As the input starts to go negative, the diode turns off

and the capacitor will slowly discharge through the load

84
Diode: Capacitor filter

By initial charges
in the capacitor

• During the positive half cycle, if the source voltage is greater than the capacitor voltage
the diode will conduct, and the capacitor charges rapidly (C is small)

• When the capacitor voltage is greater than the input voltage, the diode is reverse-bias:
the capacitor will slowly discharge through the load

85
Diode: Digital logic

(Low) (Low)
(High) (Low)
(Low) (High)
(High) (High)

86
Diode: Digital logic

(Low) (Low)
(High) (Low)
(Low) (High)
(High) (High)

87
Diode: Digital logic

(Low) (Low) (Low)

(High) (Low)
(Low) (High)
(High) (High)

88
Diode: Digital logic

(Low) (Low) (Low)

(High) (Low)
(Low) (High)
(High) (High)

89
Diode: Digital logic

(Low) (Low) (Low)

(High) (Low) (Low)
(Low) (High) (Low)
(High) (High)

90
Diode: Digital logic

(Low) (Low) (Low)

(High) (Low) (Low)
(Low) (High) (Low)
(High) (High)

91
Diode: Digital logic

(Low) (Low) (Low)

(High) (Low) (Low)
(Low) (High) (Low)
(High) (High) (High)

92
Diode: Clamping circuit
• During negative half-cycle, Diode is ‘ON’

The capacitor charges up to

• During positive half-cycle, Diode is ‘OFF’

93
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

94
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

95
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

96
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

97
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

98
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging. Instead, discharging occurs up to

99
Diode: Clamping circuit

• During positive half-cycle, Diode is ‘OFF’

The capacitor charges up to

No charging.
Instead, discharging occurs
up to

100
Diode: Clamping circuit
• During negative half-cycle, Diode is ‘ON’

The capacitor charges up to

• During positive half-cycle, Diode is ‘OFF’

101
Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Tuesday, October 1, 2024

Example 1
• Consider the Si diode in the circuit. Let V1 = 5 V and
V2 = 0 V. Determine Vo.

Infeasible system

Tuesday, October 1, 2024

Example 2
• Consider the Si diode in the circuit. Let V1 = 5 V and V2 = 0 V
and Vcc =6 V Determine Vo.

Tuesday, October 1, 2024

Example 2
• Consider the Si diode in the circuit. Let V1 = 5 V and V2 = 0 V
and Vcc =6 V Determine Vo.

Tuesday, October 1, 2024

Example 2
• Consider the Si diode in the circuit. Let V1 = 5 V and V2 = 0 V
and Vcc =6 V Determine Vo.

Tuesday, October 1, 2024

Example 2
• Consider the Si diode in the circuit. Let V1 = 5 V and V2 = 0 V
and Vcc =6 V Determine Vo.

Tuesday, October 1, 2024

Example 2
• Consider the Si diode in the circuit. Let V1 = 5 V and V2 = 0 V
and Vcc =6 V Determine Vo.

Tuesday, October 1, 2024

Diode: Clamping circuit
• If , Diode is ‘ON’

The capacitor charges up to

• If , Diode is ‘OFF’

114
Diode: Clamping circuit
• If , Diode is ‘ON’

The capacitor charges up to

• If , Diode is ‘OFF’

115
Diode: Clamping circuit
• If , Diode is ‘ON’

Capacitor discharging, depending on RC The capacitor charges up to

• If , Diode is ‘OFF’

116
Diode: Clamping circuit
Special Diodes : Zener Diode
• As reverse bias voltage increases, depletion region widens .
• At high enough reverse bias, sudden increase in current is
observed, due to strong electric field ε. (~ 100s V in normal)
• Zener Effect.
• Causes sudden increase in
current.

Tuesday, October 1, 2024

Special Diodes : Zener Diode
• If the power dissipated in the diode is within its limits (when
reverse biased beyond breakdown/zener voltage), the diode
is usable.
• Else the diode breaks down permanently.
• Zener Diodes : Heavily doped diodes that
are operable beyond breakdown voltage.
• The breakdown voltage is also tuned
during manufacture.
• Can maintain constant voltage independent
of current (largely).
Tuesday, October 1, 2024
Special Purpose Diodes
• Tunnel Diodes : Very heavily doped diodes exploit
quantum tunneling effect to enable action as high
frequency switch/oscillator.
• Photodiodes : Radiant energy is used to create
electron – hole pairs. Light enhances reverse bias
current by minority carriers. Used in Solar energy
conversion.
• Light Emitting Diodes : If appropriate (III-V) material is
used, the energy released in electron-hole
recombination can be in visible range. Diode emits
light.

Tuesday, October 1, 2024

Thanks for your attention &
Questions ?

121