This document provides information about an electronic devices course, including its objectives, outcomes, and content. The objectives are to study semiconductor physics, diode characteristics, bipolar junction transistors, field effect transistors, and integrated circuit fabrication. The outcomes include interpreting diode characteristics, designing rectifier circuits, analyzing transistor configurations, and distinguishing different transistor types. The content will cover semiconductor materials and doping, energy bands, carrier transport, generation and recombination, and continuity equations. It will also examine p-n junction diodes, bipolar junction transistors, and field effect transistors.

1. MATRUSRI ENGINEERING COLLEGE
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
SUBJECT NAME: ELECTRONIC DEVICES (PC202EC)
FACULTY NAME: Mrs. P.SRAVANI
MATRUSRI
ENGINEERING COLLEGE

2. ELECTRONIC DEVICES
COURSE OBJECTIVES:
Study semiconductor physics and analyze the behavior of
Semiconductor diodes in Forward and Reverse bias.
Develop half wave and Full wave rectifiers with L, C Filters.
Explain V-I characteristics of Bipolar Junction Transistor in CB, CE & CC
configurations.
Design DC Biasing techniques and evaluate A.C parameters for BJT in
Amplifier Applications.
Explore V-I characteristics of FETs, MOSFETs and study IC fabrication
techniques.
MATRUSRI
ENGINEERING COLLEGE

3. ELECTRONIC DEVICES
COURSE OUTCOMES:
Interpret the characteristics and apply diode models to analyze various
applications of diodes.
Identify the merits and demerits of various filters, formulate and
design rectifier circuits with filters Calculate ripple factor, efficiency
and percentage regulation of rectifier circuits.
Discriminate the BJT configurations to recognize appropriate transistor
configuration for any given application and design the biasing circuits
with good stability.
Analyze, Compare and design of BJT amplifiers with various biasing
circuits.
Distinguish the working principles of BJT and FET also between FET &
MOSFET.
MATRUSRI
ENGINEERING COLLEGE

4. UNIT-I
OUTCOMES:
To understand the semiconductor materials that are
suitable for electronic devices
To study the properties of materials for electronic devices
To understand the biasing of diode
Able to design Diode circuit
Able to apply some types of diodes
MATRUSRI
ENGINEERING COLLEGE
INTRODUCTION:
 Semiconductors are materials with electrical conductivity between
conductors and insulators.
 The most commonly used semiconductor materials are silicon and
germanium.
 Some compounds, such as GaAs, SiC and SiGe.
 Most important property is its conductivity can be controlled by adding certain
impurities in the process called doping.

5. OUTCOMES:
To understand the semiconductor materials that are
suitable for electronic devices
To study the properties of materials for electronic devices
MODULE-I : Basics of Semiconductors
MATRUSRI
ENGINEERING COLLEGE
CONTENTS:
Energy bands in intrinsic and extrinsic silicon.
Carrier transport: Diffusion current, Drift current,
Mobility and Resistivity;
Generation and Recombination of carriers,
Poisson and Continuity equation,
Hall effect

6. Energy bands in intrinsic and extrinsic silicon
MATRUSRI
ENGINEERING COLLEGE

7. MATRUSRI
ENGINEERING COLLEGE

8. MATRUSRI
ENGINEERING COLLEGE

9. MATRUSRI
ENGINEERING COLLEGE

10. MATRUSRI
ENGINEERING COLLEGE

11. MATRUSRI
ENGINEERING COLLEGE
(a) Energy band diagram.
(b) Density of states (number of states per unit energy per unit volume).
(c) Fermi-Dirac probability function (probability of occupancy of a state).
(d) The product of g(E) and f (E) is the energy density of electrons in the CB (number of electrons per unit
energy per unit volume). The area under nE(E) versus E is the electron concentration.
Semiconductor Statistics

12. MATRUSRI
ENGINEERING COLLEGE
vde = Drift velocity of the electrons, e = Electron drift mobility, Ex = Applied
electric field, vdh = Drift velocity of the holes, h = Hole drift mobility
vdh = hEx
Conductivity of a Semiconductor
 = Conductivity, e = Electronic charge, n = Electron concentration in the CB, e =
Electron drift mobility, p = Hole concentration in the VB, h = Hole drift mobility
 = ene + eph
vde = eEx
Electron and hole drift velocities

13. MATRUSRI
ENGINEERING COLLEGE
1. Fermi energy is a convenient way to represent free carrier concentrations (n
in the CB and p in the VB) on the energy band diagram.
2. However, the most useful property of EF is in terms of a change in EF.
3. Any change DEF across a material system represents electrical work input or
output per electron.
1. For a semiconductor system in equilibrium, in the dark, and with no applied
voltage or no emf generated, DEF = 0 and EF must be uniform across the system.
2. For readers familiar with thermodynamics, its rigorous definition is that EF is the
chemical potential of the electron, that is Gibbs free energy per electron. The
definition of EF anove is in terms of a change in EF.
DEF = eV
Fermi Energy

14. Extrinsic Semiconductors: n-Type
MATRUSRI
ENGINEERING COLLEGE
(a) The four valence electrons of As allow it to bond just like Si but the fifth
electron is left orbiting the As site. The energy required to release to free fifth-
electron into the CB is very small. (b) Energy band diagram for an n-type Si
doped with 1 ppm As. There are donor energy levels just below Ec around As+
sites.

15. MATRUSRI
ENGINEERING COLLEGE
e
d
h
d
i
e
d eN
N
n
e
eN 


 










2
 Nd >> ni, then at room temperature, the electron concentration in
the CB will nearly be equal to Nd, i.e. n ≈ Nd
 A small fraction of the large number of electrons in the CB
recombine with holes in the VB so as to maintain np = ni
2
n = Nd and p = ni
2/Nd
np = ni
2
Extrinsic Semiconductors: n-Type

16. MATRUSRI
ENGINEERING COLLEGE
(a) Boron doped Si crystal. B has only three valence electrons. When it
substitutes for a Si atom one of its bonds has an electron missing and therefore
a hole. (b) Energy band diagram for a p-type Si doped with 1 ppm B. There are
acceptor energy levels just above Ev around B- sites. These acceptor levels
accept electrons from the VB and therefore create holes in the VB.
Extrinsic Semiconductors: P-Type

17. MATRUSRI
ENGINEERING COLLEGE
h
a
e
a
i
h
a eN
N
n
e
eN 


 










2
 Na >> ni, then at room temperature, the hole concentration in the VB
will nearly be equal to Na, i.e. p ≈ Nd
 A small fraction of the large number of holes in the VB recombine with
electrons in the CB so as to maintain np = ni
2
p = Na and n = ni
2/Na
np = ni
2
Extrinsic Semiconductors: P-Type

18. MATRUSRI
ENGINEERING COLLEGE
Intrinsic, i-Si
n = p = ni
n-type
n = Nd
p = ni
2/Nd
np = ni
2
p-type
p = Na
n = ni
2/Na
np = ni
2
Semiconductor energy band diagrams

19. MATRUSRI
ENGINEERING COLLEGE
Energy band diagrams for (a) intrinsic (b) n-type and (c) p-type semiconductors. In
all cases, np = ni
2. Note that donor and acceptor energy levels are not shown. CB =
Conduction band, VB = Valence band, Ec = CB edge, Ev = VB edge, EFi = Fermi
level in intrinsic semiconductor, EFn = Fermi level in n-type semiconductor, EFp =
Fermi level in p-type semiconductor. c is the electron affinity. F, Fn and Fp are the
work functions for the intrinsic, n-type and p-type semiconductors
Semiconductor energy band diagrams

20. MATRUSRI
ENGINEERING COLLEGE
Electrons and holes
When temperature is not 0, there are
electrons in the CB and holes in the VB
Have discussed creation of electrons and
holes by doping with impurities
In this section we’ll discuss transitions
between the bands
Carrier Generation & Recombination

21. MATRUSRI
ENGINEERING COLLEGE
Carrier Recombination
 Generation: an electron acquires extra energy and is
excited from the valence band to the conduction band
• An electron “appears” in the conduction band
• A hole “appears” in the valence band
• The pair is called and electron-hole pair (EHP)
Carrier Generation
 Recombination: an electron in the conduction band loses
energy and goes to the valence band to occupy a hole
• The electron “disappears” from the conduction band
• The hole “disappears”
• The electron-hole pair is annihilated

22. MATRUSRI
ENGINEERING COLLEGE
Equilibrium vs. non-equilibrium
 Generation and recombination occur at equal rates
 Equilibrium concentrations n0 and p0 are constant
 Under non-equilibrium conditions
• Not necessarily true that n=n0 or p=p0
• The quantities n and p are the total electron and
hole concentrations
 There are a number of generation and recombination
processes

23. MATRUSRI
ENGINEERING COLLEGE
Band-to-band generation
This example involves
absorption of a photon and
a phonon
Band-to-band recombination
This example involves
emission of multiple
phonons

24. MATRUSRI
ENGINEERING COLLEGE
1. Electron absorbs phonon,
goes to acceptor state
2. Electron absorbs photon,
goes to conduction band
Two-step generation via acceptor
Two-step recombination via acceptor state
1. Electron temporarily trapped by
acceptor state, emits photon
2. Electron returns to valence
band, emitting phonon, and
annihilating hole

25. MATRUSRI
ENGINEERING COLLEGE
Two-step generation via trap
Trap states are energy states
deep in the forbidden band,
usually result of crystal
defects or unintentional
impurities
Two-step recombination via trap

26. Continuity equation
• Says particles are conserved
• Tremendously useful for understanding electrical
and electro-optical operation of devices
• We’ll consider 1-D case
• In a wire, electrons flow in one end and out the
other
• In semiconductor, electrons and holes flow in and
out
– And they can be generated or they can recombine
MATRUSRI
ENGINEERING COLLEGE

27. Consider this 1D semiconductor
• Consider electrons here
• Electron fluxes in and out: Fn(in) and Fn(out)
– In general, these two not equal
• Electrons generated in the region dx at rate Gn
• Electrons disappear in region dx at rate Rn
MATRUSRI
ENGINEERING COLLEGE

28. Difference between Fn(in) and Fn(out)
• Is made up by generation and
recombination
• Carriers may also pile up in the region or be
depleted
• In the differential length dx, rate of increase
in n is ¶n
¶t
dx = -
¶ Fn
¶x
dx + (Gn
- Rn
)dx
MATRUSRI
ENGINEERING COLLEGE

29. Multiple generation mechanisms
• Carriers generated thermally or optically
• Concentration could also be decreased by
trapping of electrons in the volume (or
increased by de-trapping)
– Call these other mechanism “other”
Gn
= Gn(th)
+Gn(op)
+Gn(other)
MATRUSRI
ENGINEERING COLLEGE

30. Flux density is related to current density
• Thus
• Becomes and similarly
Fn
=
-Jn
q
¶n
¶t
dx = -
¶ Fn
¶x
dx + (Gn
- Rn
)dx
¶n
¶t
=
1
q
¶ Jn
¶x
+ Gn
- Rn
( )
Continuity
equation for
electrons
¶ p
¶t
= -
1
q
¶ Jp
¶x
+ Gp
- Rp
( )
Continuity
equation for holes
MATRUSRI
ENGINEERING COLLEGE

31. Recall carrier concentrations
• The total concentration is
• And the equilibrium concentration n0 is a
constant, so
And
• And, let’s ignore the “other” generation
processes and have
n = n0
+ Dn
¶n0
¶t
= 0
¶n
¶t
=
¶n0
¶t
+
¶Dn
¶t
=
¶Dn
¶t
Gn
= Gth
+ Gop
MATRUSRI
ENGINEERING COLLEGE

32. Now consider recombination
• An electron at some elevated state has
some average time associated with how
long it takes (on average) to recombine
– All things seek lower energies
• Called the “carrier lifetime”
– We’ll discuss in much more detail later
• For a well-defined minority carrier lifetime
τn, recombination rate is proportional to
number of electrons available (in
conduction band), so Rn
=
n
tn
=
n0
tn
+
Dn
tn
MATRUSRI
ENGINEERING COLLEGE

33. Substituting
• Gives
Rn
=
n
tn
=
n0
tn
+
Dn
tn
¶n
¶t
=
1
q
¶ Jn
¶x
+ Gn
- Rn
( )
into
¶n
¶t
=
¶Dn
¶t
=
1
q
¶Jn
dx
+ Gth + Gopt -
n0
tn
-
Dn
tn
æ
è
ç
ö
ø
÷
MATRUSRI
ENGINEERING COLLEGE

34. Consider special case: equilibrium
• No excess carriers, no current, and no
optical generation
• Then
• Becomes
Dn = 0, Jn
= 0 and Gop
= 0
¶n
¶t
=
¶Dn
¶t
=
1
q
¶Jn
dx
+ Gth + Gopt -
n0
tn
-
Dn
tn
æ
è
ç
ö
ø
÷
Gth
=
n0
tn
MATRUSRI
ENGINEERING COLLEGE

35. Combine results
• To obtain
• And similarly
• Note that in general, Gopt, Δn and Δp are
functions of position
Gth
=
n0
tn
¶n
¶t
=
¶Dn
¶t
=
1
q
¶Jn
dx
+ Gth + Gopt -
n0
tn
-
Dn
tn
æ
è
ç
ö
ø
÷
¶n
¶t
=
¶Dn
¶t
=
1
q
¶Jn
¶x
æ
è
ç
ö
ø
÷ + Gop
-
Dn
tn
æ
è
ç
ö
ø
÷
¶p
¶t
=
¶Dp
¶t
= -
1
q
¶Jp
¶x
æ
è
ç
ö
ø
÷ + Gop
-
Dp
t p
æ
è
ç
ö
ø
÷
MATRUSRI
ENGINEERING COLLEGE

36. Next, consider the current density
• Consists of drift and diffusion
• Continuity equations become
Jn
= Jn(drift)
+ Jn(diffusion)
= qnmn
E + qDn
dn
dx
Jp
= Jp(drift)
+ Jp(diffusion)
= qpmp
E - qDp
dp
dx
¶n
¶t
=
¶Dn
¶t
= nmn
¶E
¶x
+ mn
E
¶n
¶x
+ Dn
¶2
n
¶x2
+ Gop
-
Dn
tn
¶p
¶t
=
¶Dp
¶t
= - pmp
¶E
¶x
- mp
E
¶p
¶x
+ Dp
¶2
p
¶x2
+ Gop
-
Dp
t p
MATRUSRI
ENGINEERING COLLEGE

37. In three dimensions
¶n
¶t
=
1
q
Ñ× Jn
+ Gn
- Rn
( )
¶ p
¶t
= -
1
q
Ñ× Jp
+ Gp
- Rp
( )
MATRUSRI
ENGINEERING COLLEGE

38. Key points
• Have to account for movement of electrons and
holes as well as generation and recombination
• Carriers can be generated or recombine thermally
or optically, or by “other” mechanisms
• Excess carrier concentration may vary with time and
with position
• We derived the continuity equations for electrons
and holes
MATRUSRI
ENGINEERING COLLEGE

39. Poisson’s Equation
MATRUSRI
ENGINEERING COLLEGE

40. Mobility μ
MATRUSRI
ENGINEERING COLLEGE
• At low fields, carrier drift velocity v (cm/s) is proportional
to electric field E (V/cm). The constant of proportionality
is the mobility, :
• vn = - nE and vp = pE, where
• vn and vp = electron and hole velocity (cm/s),
 n and p = electron and hole mobility (cm2/Vs)
• Hole mobility is less than electron since hole current is the
result of multiple covalent bond disruptions, while
electrons can move freely about the crystal.

41. Intrinsic Silicon Resistivity
• Given drift current and mobility, we can calculate resistivity:
jn
drift = Qnvn = (-qn)(- nE) = qn nE A/cm2
jp
drift = Qpvp = (qp)( pE) = qp pE A/cm2
jT
drift = jn + jp = q(n n + p p)E =  E
• This defines electrical conductivity:
 = q(n n + p p) (cm)-1
• Resistivity  is the reciprocal of conductivity:
 = 1/ (cm)



E
j
T
drift

V
/
cm
A
/
cm
2

cm






MATRUSRI
ENGINEERING COLLEGE

42. Mobility and Resistivity in Doped Semiconductors
MATRUSRI
ENGINEERING COLLEGE

43. Two kinds of current in semiconductors
• Drift: electrons and holes move under
influence of an applied electric field
• Diffusion: electrons and holes move to
regions of lower concentration (no force at
work)
Jn
= qmn
nE + qDn
dn
dx
= qmn
nE +
kT
q
dn
dx
é
ë
ê
ù
û
ú
Jp
= qmp
pE - qDp
dp
dx
= qmp
pE -
kT
q
dp
dx
é
ë
ê
ù
û
ú
MATRUSRI
ENGINEERING COLLEGE

44. MATRUSRI
ENGINEERING COLLEGE
Drift Current
• Electrical resistivity  and its reciprocal, conductivity , characterize
current flow in a material when an electric field is applied.
• Charged particles move or drift under the influence of the applied
field.
• The resulting current is called drift current.
• Drift current density is
j = Qv (C/cm3)(cm/s) = A/cm2
j = current density, (Coulomb charge moving through a unit area)
Q = charge density, (Charge in a unit volume)
v = velocity of charge in an electric field.
Note that “density” may mean area or volumetric density, depending on
the context.

45. Diffusion Current
• In practical semiconductors, it is quite useful to create
carrier concentration gradients by varying the dopant
concentration and/or the dopant type across a region of
semiconductor.
• This gives rise to a diffusion current resulting from the
natural tendency of carriers to move from high
concentration regions to low concentration regions.
• Diffusion current is analogous to a gas moving across a
room to evenly distribute itself across the volume.
MATRUSRI
ENGINEERING COLLEGE

46. Diffusion Current (cont.)
• Carriers move toward regions of lower
concentration, so diffusion current densities
are proportional to the negative of the
carrier gradient.
Diffusion currents in the
presence of a concentration
gradient.
2
2
A/cm
)
(
A/cm
)
(
x
n
qD
x
n
D
q
j
x
p
qD
x
p
D
q
j
n
n
diff
n
p
p
diff
p
















-
-

-







-


Diffusion current density equations
MATRUSRI
ENGINEERING COLLEGE

47. Diffusion Current (cont.)
• The proportionality constants Dp and Dn are the
hole and electron diffusivities with units cm2/s.
Diffusivity and mobility are related by Einstein's
relationship:
• The thermal voltage, VT = kT/q, is approximately
25 mV at room temperature. We will encounter
VT throughout this book.

D
n

n

kT
q

D
p

p

V
T
Thermal voltage
D
n

n
V
T , D
p

p
V
T
MATRUSRI
ENGINEERING COLLEGE

48. Total Current in a Semiconductor
Total current is the sum of drift and diffusion currents:
Rewriting using Einstein’s relationship (Dn = nVT),

jn
T
qnnEqD
n
n
x
jp
T
qppE-qD
p
p
x
1
1








-









x
p
p
V
E
p
q
j
x
n
n
V
E
n
q
j
T
p
T
p
T
n
T
n






MATRUSRI
ENGINEERING COLLEGE

49. HALL EFFECT
MATRUSRI
ENGINEERING COLLEGE
Moving electrons experience a force
due to a perpendicular B field
An electric field develops in response
to this force.
•The sign of this field perpendicular
to the flow of current determines the
carrier type.
•Density and mobility can also be
calculated.

50. MATRUSRI
ENGINEERING COLLEGE
OUTCOMES:
To understand the biasing of diode
Able to design Diode circuit
Able to apply some types of diodes
MODULE-II : Junction Diode
MATRUSRI
ENGINEERING COLLEGE
CONTENTS:
PN Junction formation
Characteristics
Biasing–band diagram and current flow
Diode current equation
Breakdown in diodes
Diode as a circuit element
Small signal diode models
Diode switching characteristics
Zener Diode
Zener voltage regulator and its limitation
Schottky diode

51. Boron and gallium each have only three outer electrons. When mixed into the
silicon lattice, they form "holes" in the lattice where a silicon electron has
nothing to bond to. The absence of an electron creates the effect of a positive
charge, hence the name P-type. Holes can conduct current. A hole happily
accepts an electron from a neighbor, moving the hole over a space. P-type
silicon is a good conductor.
MATRUSRI
ENGINEERING COLLEGE
PN Junction formation
P-Type Doping

52. MATRUSRI
ENGINEERING COLLEGE
Phosphorus and arsenic each have five outer electrons, so they're out of place
when they get into the silicon lattice. The fifth electron has nothing to bond
to, so it's free to move around. It takes only a very small quantity of the
impurity to create enough free electrons to allow an electric current to flow
through the silicon. N-type silicon is a good conductor. Electrons have a
negative charge, hence the name N-type.
PN Junction formation
N-Type Doping

53. MATRUSRI
ENGINEERING COLLEGE
PN Junction
We create a p-n junction by joining together two pieces of semiconductor,
one doped n-type, the other p-type. In the n-type region there are extra
electrons and in the p-type region, there are holes from the acceptor
impurities .

54. MATRUSRI
ENGINEERING COLLEGE
PN Junction
In the p-type region there are holes from the acceptor impurities and in the n-
type region there are extra electrons. When a p-n junction is formed, some of
the electrons from the n-region which have reached the conduction band are
free to diffuse across the junction and combine with holes. Filling a hole makes a
negative ion and leaves behind a positive ion on the n-side. A space charge
builds up, creating a depletion region.

55. This causes a depletion zone to form around the junction (the join)
between the two materials. This zone controls the behavior of the diode.
MATRUSRI
ENGINEERING COLLEGE
PN Junction

56. Forward biasing the p-n junction drives holes to the junction from the p-
type material and electrons to the junction from the n-type material. At
the junction the electrons and holes combine so that a continuous current
can be maintained.
MATRUSRI
ENGINEERING COLLEGE
Forward Biasing

57. The application of a reverse voltage to the p-n junction will cause a
transient current to flow as both electrons and holes are pulled away
from the junction. When the potential formed by the widened depletion
layer equals the applied voltage, the current will cease except for the
small thermal current.
MATRUSRI
ENGINEERING COLLEGE
Reverse Biasing

58. MATRUSRI
ENGINEERING COLLEGE
V-I Characteristics
When forward-biased, there is a small amount of voltage necessary to get the
diode going. In silicon, this voltage is about 0.7 volts. This voltage is needed to
start the hole-electron combination process at the junction. When reverse-biased,
an ideal diode would block all current. A real diode lets perhaps 10 microamps
through -- not a lot, but still not perfect. Usually, the breakdown voltage is a lot
more voltage than the circuit will ever see, so it is irrelevant.

59. where
IS = reverse saturation current (A)
vD = voltage applied to diode (V)
q = electronic charge (1.60 x 10-19 C)
k = Boltzmann’s constant (1.38 x 10-23 J/K)
T = absolute temperature
n = non ideality factor (dimensionless)
VT = kT/q = thermal voltage (V) (25 mV at room temp.)
IS is typically between 10-18 and 10-9 A, and is strongly temperature
dependent due to its dependence on ni
2. The non ideality factor is
typically close to 1, but approaches 2 for devices with high current
densities. It is assumed to be 1 in this text.
Diode current equation
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60. IS, the reverse bias saturation current for an ideal p-n diode is given as
where
• IS is the reverse bias saturation current,
• e is elementary charge
• A is the cross-sectional area
• Dp,n are the diffusion coefficients of holes and electrons, respectively,
• ND,A are the donor and acceptor concentrations at the n side and p
side,respectively,
• ni is the intrinsic carrier concentration in the semiconductor material,
• τp,n are the carrier lifetimes of holes and electrons, respectively.
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Diode current equation(contd…)

61. Diode Current for Reverse, Zero, and Forward Bias
• Reverse bias:
• Zero bias:
• Forward bias:

i
D

I
Sexp
v
D
nV
T






-
1







I
S0
-
1
 

-
I
S

i
D

I
Sexp
v
D
nV
T






-
1







I
S1
-
1
 

0
i
D

I
Sexp
v
D
nV
T






-
1







I
S
exp
v
D
nV
T






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ENGINEERING COLLEGE

62. Diode as a circuit element
V and R may represent the Thévenin
equivalent of a more complex 2-
terminal network. The objective of
diode circuit analysis is to find the
quiescent operating point for the
diode.
Q-Point = (ID, VD)
The loop equation for the diode circuit is:
This is also called the load line for the
diode. The solution to this equation can
be found by:
• Graphical analysis using the load-line
method.
• Analysis with the diode’s mathematical
model.
• Simplified analysis with the ideal diode
model.
• Simplified analysis using the constant
voltage drop (CVD) model.
D
D V
R
I
V 

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ENGINEERING COLLEGE

63. Load-Line Analysis (Example)
Problem: Find diode Q-point
Given data: V = 10 V, R = 10k.
Analysis:
To define the load line we use,
These points and the resulting
load line are plotted.Q-point is
given by intersection of load line
and diode characteristic:
Q-point = (0.95 mA, 0.6 V)
10
10 4
D
D V
I 


For
V
D

0
,I
D

10
V
10
k

 

1
mA
For
V
D

5
V
,I
D

5
V
10
k

 

0
.
5
mA
MATRUSRI
ENGINEERING COLLEGE

64. MATRUSRI
ENGINEERING COLLEGE
Small signal diode models
Rs represents the inevitable series resistance of a real device structure. The current
controlled current source models the ideal exponential behavior of the diode.
Capacitor C includes depletion-layer capacitance for the reverse-bias region as well
as diffusion capacitance associated with the junction under forward bias.
Typical default values: Saturation current IS = 10 fA, Rs = 0 , transit time TT = 0
seconds, N = 1

65. Breakdown in diodes
• Avalanche Breakdown
Si diodes with VZ greater than about 5.6 volts breakdown according to
an avalanche mechanism. As the electric field increases, accelerated
carriers begin to collide with fixed atoms. As the reverse bias
increases, the energy of the accelerated carriers increases, eventually
leading to ionization of the impacted ions. The new carriers also
accelerate and ionize other atoms. This process feeds on itself and
leads to avalanche breakdown.
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66. Breakdown in diodes (cont.)
• Zener Breakdown
Zener breakdown occurs in heavily doped diodes. The heavy doping
results in a very narrow depletion region at the diode junction.
Reverse bias leads to carriers with sufficient energy to tunnel directly
between conduction and valence bands moving across the junction.
Once the tunneling threshold is reached, additional reverse bias leads
to a rapidly increasing reverse current.
• Breakdown Voltage Temperature Coefficient
Temperature coefficient is a quick way to distinguish breakdown
mechanisms. Avalanche breakdown voltage increases with
temperature, whereas Zener breakdown decreases with temperature.
For silicon diodes, zero temperature coefficient is achieved at
approximately 5.6 V.
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67. MATRUSRI
ENGINEERING COLLEGE
Switching Behavior of Diodes
The non-linear depletion-layer capacitance of the diode prevents the diode
voltage from changing instantaneously and determines turn-on and
recovery times. Both forward and reverse current overshoot the final values
when the diode switches on and off as shown. Storage time is given by:
S
T
ln 1
-
IF
IR













68. 68
A typical V-I curve for a Zener diode.
A Zener diode reference circuit.
The Zener diode is made to operate under reverse bias once a sufficiently high
voltage has been reached. The V-I curve of a Zener diode is shown in Figure.
Notice that under reverse bias and low voltage the current assumes a low
negative value, just as in a normal pn-junction diode. But when a sufficiently
large reverse bias voltage is reached, the current increases at a very high rate.
Zener Diode
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69. Voltage Regulator Using the Zener Diode
The Zener diode keeps the
voltage across load resistor
RL constant. For Zener
breakdown operation, IZ > 0.

I
S

V
S
-
V
Z
R

(
20
-
5
)
V
5
k


3
mA
I
L

V
Z
R
L

5
V
5
k


1
mA
| I
Z

I
S
-
I
L

2
mA

I
Z

V
S
R
-
V
Z
1
R

1
R
L









0
|R
L

R
V
S
V
Z
-
1













R
min
For proper regulation, Zener current must be
positive. If the Zener current < 0, the Zener
diode no longer controls the voltage across
the load resistor and the voltage regulator is
said to have “dropped out of regulation”.
MATRUSRI
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70. Line and Load Regulation
Line regulation characterizes how sensitive the output voltage is to input
voltage changes. (Circuit on previous page, Figure 3.41)

Line Regulation

dV
L
dV
S
mV/V
For a fixed load curren
Line Regulation
=
R
Z
R
+
R
Z
Load regulation characterizes how sensitive the output voltage is to
changes in load current withdrawn from regulator.
Load regulation is the Thévenin equivalent resistance looking back into
the regulator from the load terminals.

Load Reg
ulation

dV
L
dI
L

For chang
es in load
Load Reg
ula

-
R
Z
R






MATRUSRI
ENGINEERING COLLEGE

71. MATRUSRI
ENGINEERING COLLEGE
Schottky Barrier Diode
One semiconductor region of the PN junction diode can be replaced by a
non-ohmic rectifying metal contact. A Schottky contact is easily formed on
n-type silicon. The metal region becomes the anode. An n+ region is
added to ensure that the cathode contact is ohmic.
Schottky diodes turn on at a lower voltage than PN junction diodes and
have significantly reduced internal charge storage under forward bias.

72. MATRUSRI
ENGINEERING COLLEGE
Additional Topic-CRO(Cathode ray oscilloscope)
The cathode-ray oscilloscope (CRO) is a common laboratory instrument
that provides accurate time and amplitude measurements of voltage
signals over a wide range of frequencies. Its reliability, stability, and
ease of operation make it suitable as a general purpose laboratory
instrument.

73. 1. Define the term cut-in voltage .What is the cut-in voltage value for germanium
diode?
2. A silicon diode has reverse saturation current of 2.5µa at 300ok.Find forward
voltage for a forward current of 10ma?
3. A germanium diode draws 50ma with a forward bias of 0.27v.The junction is at
room temperature of 27oc.Determine the reverse saturation current of the
diode?
4. Determine ac resistance for a semiconductor diode having a forward biased of
200mv and reverse saturation current of 1 µa at room temperature?
5. Describe the hall effect. Derive equation of hall voltage VH and mobility?
Questions
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74. UNIT-II
OUTCOMES:
To understand the operation, performance characteristics
and analysis of different types of rectifier circuits.
To study the properties of filter circuits for electronic devices.
 To study the different types of special purpose diodes
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INTRODUCTION:
A rectifier is an electrical device that converts alternating
current (AC), which periodically reverses direction, to direct current (DC),
which flows in only one direction. The process is known as rectification,
since it "straightens" the direction of current. Rectifiers have many uses,
but are often found serving as components of DC power supplies and high-
voltage direct current power transmission systems... Many applications of
rectifiers, such as power supplies for radio, television and computer
equipment, require a steady constant DC voltage (as would be produced
by a battery). In these applications the output of the rectifier is smoothed
by an electronic filter, which may be a capacitor, choke, or set of
capacitors, chokes and resistors, possibly followed by a voltage
regulator to produce a steady voltage.

75. OUTCOMES:
To understand the operation, performance characteristics
and analysis of different types of rectifier circuits.
To study the properties of filter circuits for electronic devices.
PN Diode Applications
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ENGINEERING COLLEGE
CONTENTS:
Half wave, Full wave and Bridge rectifiers–their operation, performance
characteristics and analysis.
 Filters (L, C filters) used in power supplies and their ripple factor
calculations, design of Rectifiers with and without Filters.