This document provides information about PN junction diodes and their characteristics: 1) It describes how a PN junction is formed by combining P-type and N-type semiconductors, forming a depletion region. 2) It explains the I-V characteristics of a diode under forward and reverse bias, including how the depletion region changes with bias. 3) Additional topics covered include drift and diffusion currents, temperature effects, capacitance effects, and recovery time characteristics important for switching applications. Special diodes like Zener diodes are also introduced.

1. UNIT I
PN JUNCTION AND SPECIAL DIODE
ANALOG ELECTRONICS

2. Introduction
 Types of extrinsic semiconductors
 N-type - Electrons are majority charge carriers and
while holes are minority charge carriers
 P-type – Holes are majority charge carriers and
while electrons are minority charge carriers
 These two types of materials are chemically
combined with a special fabrication technique to
form a p-n junction. Such a p-n junction form an
electronics device called diode.

3. PN JUNCTION DIODE
 Join a piece of P-type semiconductor to a piece of N-
type semiconductor such that the crystal remains
contionues at the boundary.
 PN junction forms very useful device and is called a
seiconductor diode or PN junction diode as shown
Holes and Electrons – mobile charge carriers
Positive and negative ions – immobile charges

4. Formation of Depletion Layer in a
PN junction
 In P-region has holes(Majority carrier) and negatively
charged impurity atoms, called negative ions (acceptor
ions)
 In N-region has free electrons(Majority carrier) and
positively charged impurity atoms, called negative ions
(donor ions)
 Holes and Electrons are the mobile charge carriers.
Positive and Negative ions are immobile charges and it
do not in conduction.
 As soon as the PN junction is formed, some of the holes
in P-region and free electrons in N-region diffuse each
other and disappear due to recombination.

5. Formation of Depletion Layer in a PN
junction

6. V-I characteristics under forward
biased condition
Cut in voltage (V0) 0.3 v for Germanium
0.7 v for silicon
At cut in voltage, potential barrier is overcome and current through
the junction starts increases rapidly.

7.  When positive terminal of the battery is connected to
the P-type and negative terminal to the N-type of the PN
junction diode, the bias is known as forward bias.
 Applied positive potential repels the holes in P-type
region so that holes move towards the junction and the
applied negative potential repels the electrons in the N-
type region and the electrons moves towards the
junction.
 When applied potential is more then disappear
depletion region.
 When VF<V0 then forward current almost zero.
 When VF>V0 then large forward current flows. Here
potential barrier or depletion layer is disappear.

8. V-I characteristics under reverse
biased condition

9.  When negative terminal of the battery is connected to the P-
type and positive terminal to the N-type of the PN junction
diode, the bias is known as reverse bias.
 Under reverse bias, holes of majority carrier in P-type region
move towards negative terminal of the battery and electrons
of majority carrier in N-type region move towards positive
terminal of the battery.
 Which is increases the depletion layer or potential barrier.
 Ideally there is no current flows. Practically very small current
of order of few microampere flows.
 Minority carrier in P-region and N-region trying to flows
across junction and give rise to small reverse current. This
current known as Reverse saturation current.
 When large reverse voltage applied then sufficient energy to
dislodge valance electron. Then conduction takes place and
which voltage called breakdown voltage

10. Ideal Diode
 The first electronic device to be introduced is called the
diode. It is the simplest of semiconductor devices but
plays a very vital role in electronic systems, having
characteristics that closely match those of a simple
switch. It will appear in a range of applications,
extending from the simple to the very complex.
 In addition to the details of its construction and
characteristics, the very important data and graphs to
be found on specification sheets will also be covered to
ensure an understanding of the terminology employed
and to demonstrate the wealth of information typically
available from manufacturers.

11.  The term ideal will be used frequently in this text as
new devices are introduced. It refers to any device or
system that has ideal characteristics—perfect in every
way.
 It provides a basis for comparison, and it reveals
where improvements can still be made. The ideal
diode is a two-terminal device having the symbol and
characteristics shown in figures

12.  Ideally, a diode will conduct current in the direction
defined by the arrow in the symbol and act like an open
circuit to any attempt to establish current in the
opposite direction.
 The characteristics of an ideal diode are those of a
switch that can conduct current in only one direction.
 One of the important parameters for the diode is the
resistance at the point or region of operation.
 If we consider the conduction region defined by the
direction of ID and polarity of VD in figure. (upper-right
quadrant of figure.), we will find that the value of the
forward resistance, RF, as defined by Ohm’s law is

13.  Consider the region of negatively applied potential (third
quadrant) of figure.

14. DIODE CURRENT EQUATION
 The diode current equation relating the voltage V and
current I is given by
 Where
 I = diode current
 Io = diode reverse saturation current at room temperature
 V = external voltage applied to the diode
 ղ = a constant, 1 for germanium and 2 for silicon
 VT = kT/q = T/11600, volt-equivalent of temperature, i.e
thermal voltage
 Where
 k = Boltzmann’s constant (1.38066 * 10-23) J/ K
 q = charge of electron (1.60219 * 10-19 C)
 T = temperature of the diode junction (K) = (C+ 273 )

15.  At room temperature, (T= 300K ), VT= 26mV.
Substituting this value in the current equation, we get
 If the value of applied voltage is greater than unity, then
the equation of diode current for germanium,
and for silicon
• when the diode is reverse biased, its current may be
obtained by changing the sign of the applied voltage V.
thus, the diode current with reverse bias is
• If V>>VT, then the term, therefore I ≈ IO,
termed as reverse saturation current, which is valid as
long as external voltage is below the breakdown voltage.

16. Drift Currents
 When an electric field is applied across the
semiconductor material, the charge carriers attain a
certain drift velocity vd, which is equal to the product of
the mobility of the charge carriers and the applied
electric field intensity, E.
 The holes move towards the negative terminal of the
battery and electron move towards the positive terminal.
 This combined effect of movement of the charge carriers
constitutes a current known as the drift current.
 Thus the drift current is defined as the flow of electric
current due to the motion of the charge carriers under
the influence of an external electric field.

17.  The equation for the drift current density, Jn, due to free
electron is given by
Jn= qnμnE A/cm2
and the drift current density, Jp due to holes is
given by
Jp = qpμpE A/cm2
 Where
n = number of free electrons per cubic centimeter
p = number of holes per cubic centimeter
μn = mobility of electrons in cm2/V-s
μp = mobility of holes in cm2 /V-s
E = applied electric field intensity in V/cm
Q = charge of an electron = 1.6 * 10-19 C

18. Diffusion Currents
 It is possible for an electric current to flow in a
semiconductor even in the absence of the applied
voltage provided a concentration gradient exists in the
material.
 A concentration gradient exists if the number of either
electrons or holes is greater in one region of a
semiconductor as compared to the rest of the region.
 In a semiconductor material, the charge carriers have
the tendency to move from the region of higher
concentration to that of lower concentration of the same
type of charge carriers.
 Thus, the movement of charge carriers takes place
resulting in a current called diffusion current.

19.  As indicated in figure, the holes concentration p(x) in a
semiconductor bar varies from a high value to a low
value along the x-axis and is constant in the y- and z-
directions.
 Diffusion current density due to holes, Jp is given by

20.  Since the hole density p(x) decreases with increasing x
as shown in figure, dp/dx is negative and the minus
sign in the above equation is needed in order that Jp
has a positive sign in the positive x-direction.
 Diffusion current density due to the free electrons, Jn is
given by

21.  Where dn/dx and dp/dx are the concentration gradients
for electrons and holes respectively, in the x-direction
and Dn and Dp are the diffusion coefficients expressed
in cm2/s for electrons and holes, respectively.
 Total Current:
 The total current in a semiconductor is the sum of drift
current and diffusion current. Therefore, for a P-type
semiconductor, the total current per unit area, i.e the total
current density is given by
Jp = qpμpE -
 Similarly, the total current density for an N-type
semiconductor is given by
Jn = qpμnE +

22. Effect of temperature on PN junction
diodes
 The rise in temperature increases the generation of
electro- hole pairs in semiconductors and increases
their conductivity.
 As a result, the current through the PN junction diode
increases with temperature as given by the diode
current equation,
 At room temperature, i.e at 300K, the value of barrier
voltage or cut-in voltage is about 0.3V for germanium
and 0.7V for silicon.
 The barrier voltage is temperature dependant and it
decreases by 2 mV/ for both germanium and silicon.
 This fact may be expressed in mathematical form,

23.  Where I01= saturation current of diode at temperature
(T1) and I02= saturation current of diode at temperature
(T2).
 The figure shows the effect of increased temperature on
the characteristics curve of a PN junction diode. A
germanium diode can be used up to a maximum of 75
deg and silicon diode to a maximum of 175 deg.

24. TRANSITION AND DIFFUSION
CAPACITANCE
 Electronic devices are inherently sensitive to very high
frequencies. Most shunt capacitive effects that can be
ignored at lower frequencies because the reactance
XC= ½ pi fC is very large (open-circuit equivalent).
 This, however, cannot be ignored at very high
frequencies. XC will become sufficiently small due to the
high value of f to introduce a low-reactance “shorting”
path.
 In the p-n semiconductor diode, there are two capacitive
effects to be considered.
 Both types of capacitance are present in the forward-
and reverse-bias regions, but one so outweighs the
other in each region that we consider the effects of only
one in each region.

25.  In the reverse-bias region we have the transition- or
depletion-region capacitance (CT), while in the forward-
bias region we have the diffusion (CD) or storage
capacitance.
 Recall that the basic equation for the capacitance of a
parallel-plate capacitor is defined by C= εA/d, where ε
is the permittivity of the dielectric (insulator) between
the plates of area A separated by a distance d.
 In the reverse-bias region there is a depletion region
(free of carriers) that behaves essentially like an
insulator between the layers of opposite charge. Since
the depletion width (d) will increase with increased
reverse-bias potential, the resulting transition
capacitance will decrease, as shown in figure.

26.  Transition and diffusion capacitance versus
applied bias for a silicon diode

27.  Including the effect of the transition or diffusion
capacitance on the semiconductor diode
 The capacitive effects described above are represented
by a capacitor in parallel with the ideal diode, as shown
in figure.
 For low or mid-frequency applications (except in the
power area), however, the capacitor is normally not
included in the diode symbol.

28. JUNCTION DIODE
CHARACTERISTICS
 Diodes are often used in a switching mode. When the
applied bias voltage to the PN junction diode is suddenly
reversed in the opposite direction, the diode response
reaches a steady state after an interval of time, called
the recovery time.
 The forward recovery time, tf is defined as the time
required for forward voltage or current to reach a
specified value (time interval between the instant of 10%
diode voltage to the instant this voltage reaches within
10% of its final value) after switching diode from its
reverse to forward biased state.
 Fortunately, the forward recovery time posses no series
problem. Therefore, only the reverse recovery time, trr
has to be considered in practical applications.

29.  When the PN junction is forward biased, the minority
electron concentration in the P-region is approximately
linear. If the junction is suddenly reverse biased, at t1,
then because of this stored electronic charges, the
reverse current (IR) is initially of the same magnitude as
the forward current (IF).
 The diode will continue to conduct until the injected or
excess carrier minority density (p=-po) or (n-no) has
dropped to zero.
 However, as the stored electrons are removed into the
N-region and the contact, the available charge quickly
drops to an equilibrium level and a steady current
eventually flows corresponding to the reverse bias
voltage as shown in figure (c).
 As shown in figure (b), the applied voltage Vi=VF for the
time up to t1 is in the direction to forward bias the diode.

30.  Then the current is I= . Then at time t= t1, the input
voltage is suddenly reversed to the value of –VR.
 Due to the reason, the current does not become zero
and has the value I= until the time t=t2.
 At t= t2, when the excess minority carriers have reached
the equilibrium state, the magnitude of the diode current
starts to decreases as shown in figure (d).
 During the time interval from t1 to t2, the injected minority
carriers have remained stored and hence this interval is
called the storage time (ts).
 After the instant t= t2 the diode gradually recovers and
ultimately reaches the steady state.

31.  The time interval between t2 and the instant t3 when the
diode has recovered nominally is called the transition
time, tr.
 The recovery is said to have completed
 (i) when even the minority carriers remote from the
junction have diffused to the junction and crossed it, and
 (ii) when the junction transition capacitance, CT across the
reverse biased junction has got charged through the
external resistor RL to the voltage -VR.
 The reverse recovery time (or turn off time) of a diode,
trr is the interval from the current reversal at t= t1 until
the diode has recovered to a specified extent in terms
either of the diode current or of the diode resistance i.e
trr= ts + tr
 For a commercial switching type diodes the reverse
recovery time trr, ranges from 1 ns up to as high as 1μs.

32.  If the time period of the input signal is such that T =2.trr,
then the diode conducts as much in reverse as in the
forward direction. Hence it does not behave as a one
way device.
 In order to minimize the effect of the reverse current,
the time period of the operating frequency should be a
minimum of approximately 10 times trr. For example, if a
diode has trr of 2ns, its maximum operating frequency is
 The trr, can be reduced by shortening the length of the
P-region in a PN junction diode.
 The stored charge and consequently the switching time
can also be reduced by introduction of gold impurities
into the junction diode by diffusion.

34. ZENER DIODE
 Zener diode is a reverse biased heavily doped PN
junction diode which operates in breakdown region.
 The reverse breakdown of a PN junction may occurs
either due to zener effect or avalanche effect.
 Zener effect dominates at reverse voltage less than 6V
and avalanche effect dominates above 6V
 For zener diodes, Silicon is preferred to Ge because of
its higher temperature and current capability.
 Symbol of zener diode as shown

35. Forward biasing zener diode
 Anode connected to positive terminal of battery and
cathode connected to negative terminal of battery.
 Its behavior identical to F.B diode
 General zener diode not used in F.B condition

36. Reverse biasing zener diode
 Cathode connected to positive terminal of battery and
Anode connected to negative terminal of battery.
 Its operation is differ from that of diode.
 Zener diode in reverse biased condition is used as a
voltage regulator.

37. V-I characteristics of zener diode
 V-I characteristics of zener diode can be divided into
two parts
 Forward characteristics
 Reverse characteristics
 Forward characteristics
 The characteristics as shown
 It is almost identical to the as a PN junction diode

38.  Reverse characteristics
 Reverse voltage increases, initially small reverse saturation current I0, in order
of μA will flow. This current due to thermally generated minority carriers.
 At particular reverse voltage, reverse current increase sharp and suddenly.
This indication that breakdown occurs.
 This breakdown voltage is called as zener breakdown voltage or zener voltage
and it is denoted by Vz
 After breakdown Vz remains constant and further increase only reverse zener
current.
 For controlling zener current put R and which avoid excess heat.

39. Application
 Zener diode is used as a voltage regulator
 Zener diode is used as a peak clipper in wave
shaping circuits
 Zener diode is used as a fixed reference voltage
in transistor biasing circuits.
 Zener diode is used for meter protection against
damage from accidental application of excessive
voltage.

40. Breakdown mechanism
 If reverse bias voltage applied to a PN junction is
increased, a point will reach when the junction
breakdown and reverse current rises sharply to a value
limited only by the external resistance connected in
series.
 This specific value of reverse bias voltage is called
breakdown voltage
 The breakdown voltage depends on width of depletion
layer. This width of depletion layer depends on doping
level.
 Process of causes junction breakdown due to increase
in reverse bias voltage as
 Zener breakdown
 Avalanche breakdown

41. Zener breakdown
 It observed when Vz<6V. If apply Vz then strong electric
field appear across narrow depletion region.
 Value of electric field as 3*10^5v/cm.
 Due to this electric field pull valance electron into
conduction band to breaking covalent bond.
 So large no of free electron causes to reverse current
through zener diode and breakdown occurs due to
zener effect.

42. Avalanche Breakdown
 It observed when Vz>6V.
 Reverse bias condition conduction due to only in
minority carrier.
 Reverse voltage increase, then accelerates minority
carrier and causes to increase K.E
 Accelerates minority carrier collide with stationary atom
and K.E causes valance electron present in covalent
bond.
 Now valance electron breakdown covalent bond and
become free for conduction.
 Now increase more no free electrons collide. This
phenomenon is called as avalanche multiplication.

43.  In short time large no of free minority electrons and
holes available for conduction and which causes self
sustained multiplication process called ‘Avalanche
effect’
 Large reverse current starts flowing through zener diode
and occur avalanche breakdown.

44. ZENER REGULATOR
 Fig shows circuit of zener diode shunt regulator.
 Load connected parallel to zener diode and so called
shunt regulator.
 Rs limit the current and V0 taken across RL
 For proper operation Vin>Vz

46. Operation:
 The output voltage is mainly varied due to following two
reason
 Regulation with varying input voltage
 Regulation with varying load current
 Regulation with varying input voltage

47.  Assume RL is fixed and Vin varies
 If Vin↑, I↑. But IL=const. as Vz=constant. Hence Iz↑, to
keep IL=const.
 If Vin↓, I↓. But keep IL=const. Iz↓. As long as Iz is
between Izmax and Izmin, V0 remains const.

48.  Regulation with varying load current
 Assume Vin=const, vary IL and RL
 Vary RL, then current flows through it vary
 Iin and voltage across Rs const.
 When RL↓, then IL↑, causes Iz↓
 VL=const due rise in current equal to drop in resistance
(V=IR)

49. The current limiting resistor (Rs) must be properly
selected to fulfill the following requirements:
1. When the input voltage is minimum and the load
current is maximum, sufficient current must be
supplied to keep the zener diode within its breakdown
region

50. 2. When the input voltage is maximum and the
load current is minimum, the zener current must
not exceed the maximum rated value.

51. Value of load Resistance RL

52. LIGHT EMITTING DIODE
 The increasing use of digital displays in calculators,
watches, and all forms of instrumentation has
contributed to the current extensive interest in
structures that will emit light when properly biased.
 The two types in common use today to perform this
function are the light-emitting diode (LED) and the
liquid-crystal display (LCD).
 As the name implies, the light-emitting diode (LED) is a
diode that will give off visible light when it is energized.
 In any forward-biased p-n junction there is, within the
structure and primarily close to the junction, a
recombination of holes and electrons.
 This recombination requires that the energy possessed

53.  In all semiconductor p-n junctions some of this energy
will be given off as heat and some in the form of photons.
In silicon and germanium the greater percentage is given
up in the form of heat and the emitted light is
insignificant.
 In other materials, such as gallium arsenide phosphide
(GaAsP) or gallium phosphide (GaP), the number of
photons of light energy emitted is sufficient to create a
very visible light source.
 The process of giving off light by applying an electrical
source of energy is called electroluminescence.

54. Process of electroluminescence in
the LED and its Graphic symbol

55.  The conducting surface connected to the p-material is
much smaller, to permit the emergence of the maximum
number of photons of light energy.
 Note in the figure that the recombination of the injected
carriers due to the forward-biased junction results in
emitted light at the site of recombination.
 There may, of course, be some absorption of the
packages of photon energy in the structure itself, but a
very large percentage are able to leave, as shown in the
figure.

56. LIQUID-CRYSTAL DISPLAYS
 The liquid-crystal display (LCD) has the distinct
advantage of having a lower power requirement than
the LED.
 It is typically in the order of microwatts for the display,
as compared to the same order of milliwatts for LEDs.
 It does, however, require an external or internal light
source and is limited to a temperature range of about
0° to 60°C. Lifetime is an area of concern because
LCDs can chemically degrade.
 The types receiving the major interest today are the
field-effect and dynamic-scattering units.
 A liquid crystal is a material (normally organic for LCDs)
that will flow like a liquid but whose molecular structure

57.  For the light-scattering units, the greatest interest is in
the nematic liquid crystal, having the crystal structure
shown in figure
 The individual molecules have a rod like appearance as
shown in the figure.
 The indium oxide conducting surface is transparent, and
under the condition shown in the figure, the incident
light will simply pass through and the liquid-crystal
structure will appear clear.

58.  If a voltage (for commercial units the threshold level is
usually between 6 and 20 V) is applied across the
conducting surfaces, as shown in figure.
 The molecular arrangement is disturbed, with the result
that regions will be established with different indices of
refraction.
 The incident light is therefore reflected in different
directions at the interface between regions of different
indices of refraction with the result that the scattered

59.  A digit on an LCD display may have the segment
appearance shown in figure
 If the number 2 were required, the terminals 8,7, 3, 4,
and 5 would be energized, and only those regions
would be frosted while the other areas would remain
clear.

60.  The field-effect or twisted nematic LCD has the same
segment appearance and thin layer of encapsulated
liquid crystal, but its mode of operation is very different.
 Similar to the dynamic-scattering LCD, the field-effect
LCD can be operated in the reflective or transmissive
mode with an internal source. The transmissive display
appears in figure.

61.  The reflective-type field-effect LCD is shown in figure.
 In this case, the horizontally polarized light at the far left
encounters a horizontally polarized filter and passes
through to the reflector, where it is reflected back into
the liquid crystal, bent back to the other vertical
polarization, and returned to the observer. If there is no
applied voltage, there is a uniformly lit display. The
application of a voltage results in a vertically incident
light encountering a horizontally polarized filter at the

62. SCHOTTKY BARRIER (HOT-
CARRIER) DIODES
 In recent years, there has been increasing interest in a
two-terminal device referred to as a Schottky-barrier,
surface-barrier, or hot-carrier diode.
 Its areas of application were first limited to the very high
frequency range due to its quick response time
(especially important at high frequencies) and a lower
noise figure (a quantity of real importance in high-
frequency applications).
 In recent years, however, it is appearing more and more
in low-voltage/high-current power supplies and ac-to-dc
converters. Other areas of application of the device
include radar systems,
 Schottky TTL logic for computers, mixers and detectors

63.  Its construction is quite different from the conventional p-
n junction in that a metal semiconductor junction is
created such as shown in figure.
 The semiconductor is normally n-type silicon (although
p-type silicon is sometimes used), while a host of
different metals, such as molybdenum, platinum,
chrome, or tungsten, are used.

64.  Different construction techniques will result in a different
set of characteristics for the device, such as increased
frequency range, lower forward bias, and so on.
 Priorities do not permit an examination of each
technique here, but information will usually be provided
by the manufacturer.
 In general, however, Schottky diode construction results
in a more uniform junction region and a high level of
ruggedness.
 In both materials, the electron is the majority carrier. In
the metal, the level of minority carriers (holes) is
insignificant.
 When the materials are joined, the electrons in the n-
type silicon semiconductor material immediately flow into
the adjoining metal, establishing a heavy flow of majority
carriers.

65.  Since the injected carriers have a very high kinetic
energy level compared to the electrons of the metal, they
are commonly called “hot carriers.”
 In the conventional p-n junction, there was the injection
of minority carriers into the adjoining region.
 Here the electrons are injected into a region of the same
electron plurality.
 Schottky diodes are therefore unique in that conduction
is entirely by majority carriers.
 The heavy flow of electrons into the metal creates a
region near the junction surface depleted of carriers in
the silicon material— much like the depletion region in
the p-n junction diode.

66.  The additional carriers in the metal establish a “negative
wall” in the metal at the boundary between the two
materials.
 The net result is a “surface barrier” between the two
materials, preventing any further current.
 That is, any electrons (negatively charged) in the silicon
material face a carrier-free region and a “negative wall”
at the surface of the metal.
 The application of a forward bias as shown in the first
quadrant of figure. will reduce the strength of the
negative barrier through the attraction of the applied
positive potential for electrons from this region.
 Comparison of characteristics of hot-carrier and p-n
junction diodes as shown

67.  The result is a return to the heavy flow of electrons
across the boundary, the magnitude of which is
controlled by the level of the applied bias potential.
 The barrier at the junction for a Schottky diode is less
than that of the p-n junction device in both the forward-
and reverse-bias regions.

68.  The equivalent circuit for the device (with typical values)
and a commonly used symbol appear in figure.
 A number of manufacturers prefer to use the standard
diode symbol for the device since its function is
essentially the same.
 The inductance LP and capacitance CP are package
values, and rB is the series resistance, which includes
the contact and bulk resistance.

69. VARACTOR (VARICAP)
DIODES
 Varactor [also called varicap, VVC (voltage-variable
capacitance), or tuning] diodes are semiconductor,
voltage-dependent, variable capacitors.
 Their mode of operation depends on the capacitance
that exists at the p-n junction when the element is
reverse-biased.
 Under reverse-bias conditions, it was established that
there is a region of uncovered charge on either side of
the junction that together the regions make up the
depletion region and define the depletion width Wd.
 The transition capacitance (CT) established by the
isolated uncovered charges is determined by

70.  As the reverse-bias potential increases, the width of the
depletion region increases, which in turn reduces the
transition capacitance.
 The characteristics of a typical commercially available
varicap diode appear in figure.
 Note the initial sharp decline in CT with increase in
reverse bias. The normal range of VR for VVC diodes is
limited to about 20 V.

71.  In terms of the applied reverse bias, the transition
capacitance is given approximately by
 K = constant determined by the semiconductor material
and construction technique
 VT = knee potential
 VR = magnitude of the applied reverse-bias potential
 n =1/ 2 for alloy junctions and 1/ 3 for diffused junctions
 In terms of the capacitance at the zero-bias condition
C(0), the capacitance as a function of VR is given by

72.  The symbols most commonly used for the varicap
diode and a first approximation for its equivalent circuit
in the reverse-bias region are shown in Fig.
 Since we are in the reverse-bias region, the resistance
in the equivalent circuit is very large in magnitude—
typically 1 MΩ or larger—while RS, the geometric
resistance of the diode, is, as indicated in figure very
small.
 The magnitude of CT will vary from about 2 to 100 pF
depending on the varicap considered.

73.  In figure, the varactor diode is employed in a tuning
network.
 That is, the resonant frequency of the parallel L-C
combination is determined by
 The selected frequencies of the tuned network are then
passed on to the high input amplifier for further

74. PHOTODIODES
 The interest in light-sensitive devices has been
increasing at an almost exponential rate in recent years.
 The resulting field of optoelectronics will be receiving a
great deal of research interest as efforts are made to
improve efficiency levels.
 Through the advertising media, the layperson has
become quite aware that light sources offer a unique
source of energy.
 This energy, transmitted as discrete packages called
photons, has a level directly related to the frequency of
the traveling light wave as determined by the following
equation:
W = hf joules

75.  The frequency is, in turn, directly related to the
wavelength (distance between successive peaks) of the
traveling wave by the following equation
where =wavelength, meters
v = velocity of light, 3 * 10^8 m/s
f = frequency of the traveling wave, hertz
 The wavelength is usually measured in angstrom units
(Å) or micrometers (μm),
Where 1 Å= 10-10 m and 1 µm = 10-6 m
 The number of free electrons generated in each material
is proportional to the intensity of the incident light.
 Light intensity is a measure of the amount of luminous
flux falling in a particular surface area.

76.  Luminous flux is normally measured in lumens (lm) or
watts. The two units are related by
1 lm = 1.496 * 10-10 W
 The light intensity is normally measured in lm/ft2
,footcandles (fc), or W/m2 , where
1 lm/ft2= 1 fc = 1.609* 10-9 W / m2
 The photodiode is a semiconductor p-n junction device
whose region of operation is limited to the reverse-bias
region.
 The basic biasing arrangement, construction, and symbol
for the device appear in Fig.

77.  The reverse saturation current is normally limited to a
few microamperes.
 It is due solely to the thermally generated minority
carriers in the n and p type materials.

78.  The application of light to the junction will result in a
transfer of energy from the incident traveling light waves
(in the form of photons) to the atomic structure, resulting
in an increased number of minority carriers and an
increased level of reverse current.
 This is clearly shown in figure. for different intensity
levels.
 The dark current is that current that will exist with no
applied illumination. Note that the current will only return
to zero with a positive applied bias equal to VT.