1. Special-Purpose Diodes
UNIT-1
Mohammad Asif Iqbal
Assistant Professor,
Deptt of ECE,
JETGI, Barabanki

2. Both n -type and p -type materials are formed by
adding a predetermined number of impurity
atoms to a silicon base. An n -type material is
created by introducing impurity elements that
have five valence electrons ( pentavalent ), such
as antimony , arsenic , and phosphorus.
Semiconductor Basics
The p -type material is formed by
doping a pure germanium or silicon
crystal with impurity
atoms having three valence electrons.
Note that there is now an insufficient number of
electrons to complete the covalent bonds of the
newly formed lattice. The resulting vacancy is called
a hole and is represented by a small circle or a plus
sign, indicating the absence of a negative charge.
Since the resulting vacancy will readily accept a free
electron:
The diffused impurities with three valence
electrons are called acceptor atoms.
Note that the four covalent bonds are still present.
There is, however, an additional fifth electron due to
the impurity atom, which is unassociated with any
particular covalent bond. This remaining electron,
loosely bound to its parent (antimony) atom, is
relatively free to move within the newly formed n -
type material. Since the inserted impurity atom has
donated a relatively “free” electron to the structure:
Diffused impurities with five valence electrons are
called donor atoms.

3. Semiconductor Basics
n-type p-type
As we have discussed earlier In an n -type material,
the number of free electrons has changed significantly,
but the number of holes has not changed significantly
from this intrinsic level. The net result, therefore, is
that the number of electrons far outweighs the
number of holes. For this reason: In an n-type
material the electron is called the majority carrier
and the hole the minority carrier.
For the p -type material the number of holes far
outweighs the number of electrons,
Thus In a p-type material the hole is the majority
carrier and the electron is the minority carrier.
When the fifth electron of a
donor atom leaves the parent
atom, the atom remaining
acquires a net positive charge:
hence the plus sign in the
donor-ion representation. For
similar reasons, the minus
sign appears in the acceptor
ion.

4. NP
Now let's join both of them
Depletion Region
At the instant the two
materials are “joined” the
electrons and the holes in the
region of the junction will
combine, resulting in a lack of
free carriers in the region
near the junction, Note the
only particles displayed in
this region are the positive
and the negative ions
remaining after the
absorption of free carriers.
This region of uncovered
positive and negative ions is
called the depletion region
due to the “depletion” of free
carriers in the region.

5. LIGHT-EMITTING DIODES
• As the name implies, the light-emitting diode is a diode that gives off visible or invisible
(infrared) light when energized.
• In any forward-biased p – n junction there is, a recombination of holes and electrons within
the structure and primarily close to the junction.
• This recombination requires that the energy possessed by the unbound free electrons be
transferred to another state.
• In all semiconductor p – n junctions some of this energy is given off in the form of heat and
some in the form of photons.
• In Si and Ge diodes the greater percentage of the energy converted during recombination at
the junction is dissipated in the form of heat within the structure, and the emitted light is
insignificant.
• For this reason, silicon and germanium are not used in the construction of LED devices. On
the other hand
• Diodes constructed of GaAs emit light in the infrared (invisible) zone during the
recombination process at the p–n junction.
Even though the light is not visible,
infrared LEDs have numerous
applications where visible light is not
a desirable effect. These include
security systems, industrial
processing, optical coupling, safety
controls such as on garage door
openers, and in home entertainment
centers, where the infrared light of the
remote control is the controlling
element.
Through other combinations of elements a coherent visible
light can be generated as shown in the table below

6. Process of electroluminescence in the LED
P N
Recombination
Recombination
Recombination
Recombination
Recombination
Emitted
visible
light
Metallic
contact

7. Characteristic and symbol

8. Numerical
Solution:- consider the LED series resistor in figure.
and use the formula for calculation of the unknown
Parameter.

9. NP
Varactor(Variable capacitor) or varicap Diode
Depletion Region
Herewehaveappliedanexternal
potentialofVvoltsacrossthep–n
junction such that the positive
terminal is connected to the n -
type material and the negative
terminal is connected to the p -
typematerialasshowninFig.the
number of uncovered positive
ionsinthedepletionregionofthe
n-type materialwillincreasedue
to the large number of free
electrons drawn to the positive
potential of the applied voltage.
For similar reasons, the number
of uncovered negative ions will
increase in the p -type material.
The net effect, therefore, is a
wideningofthedepletionregion.

10. 1-In this region there
is no mobile charge
carriers, we may
consider it as an
insulator
2-Positive
charge/plate
3-Negative
charge/plate
Observe, we have got
everything required for a
capacitor

11. • This capacitance is known as transition capacitance CT established by the isolated uncovered
charges and is determined by
𝐶 𝑇 = 𝜖
𝐴
𝑊𝑑
where 𝜖 is the permittivity of the semiconductor materials, A is the p – n junction area, and 𝑊𝑑 is
the depletion width.
• In terms of the applied reverse bias, the transition capacitance is given approximately by
𝐶 𝑇 =
𝐾
(𝑉𝑇+𝑉𝑅) 𝑛
where K constant determined by the semiconductor material and construction technique
𝑉𝑇 = knee potential as defined in Section
𝑉𝑅 = magnitude of the applied reverse-bias potential
n = ½ 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
𝐶 𝑇(𝑉𝑅) =
𝐶(0)
(1 + 𝑉𝑅/𝑉𝑇 ) 𝑛

12. 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 Fig. Note the initial sharp decline in 𝐶 𝑇 with increase in reverse bias.
These are the
common
symbols of
varactor diode
Characteristic and symbol

13. Application
In Fig., the varactor diode is employed in a tuning network. That is, the
resonant frequency of the parallel LC combination is determined by
𝑓𝑝 = 1/2𝜋 𝐿2 𝐶′ 𝑇 , where 𝐶′ 𝑇 = 𝐶 𝑇 + 𝐶𝑐 determined by the applied reverse-
bias potential 𝑉𝐷𝐷.

14. Tunnel or Esaki Diode
• The basic construction of a tunnel diode is same as of normal diode except
the doping concentration which is 100 to several thousand times greater
than that of a typical semiconductor diode.
• This results in a greatly reduced depletion region, of the order of
magnitude of 10−6 cm, or typically about
1
100
the width of this region for a
typical semiconductor diode.
• It is Used as very fast switching device in computers, in high frequency
oscillators and also in amplifiers.
• It can be switched on or off in 1 nano sec, also known as Esaki diode after
the name of Japanese scientist Leo Esaki.
• Its characteristics (will be discussed in next slide), are different from any
diode discussed thus far in that it has a negative-resistance region.

15. NP
Construction
𝐍+𝐏+
Depletion Region
Observe, this
depletion layer
is narrower that
previous ones,
the narrower the
depletion layer
the higher
current will flow.
We have
increase the
impurity
concentrati
on, that will
result in an
increase of
majority
carrier
concentrati
on
We have
increase the
impurity
concentrati
on, that will
result in an
increase of
majority
carrier
concentrati
on

16. Characteristic
Initially current
will increase
with the increase
in the forward
biasing voltage.
At this point, the charge carriers
will gain enough energy, and will
start colliding each other, this
collision will result in an abrupt
decrease in their energy that will
ultimately decreases the current.
Observe, in this
reason current is
decreasing with
increase in
voltage, that is
why this region is
known as
negative
resistance region
(-R)
After a finite amount of time they will
stop colliding and regain their lost
energy, it will again result in an
increase in current. This process will
continue….

17. Symbol
Applications
Using tunnel diode we can design a negative resistor oscillator shown below.

18. A tunnel diode can also be used to generate a sinusoidal voltage using simply a dc supply and a few
passive elements.
Effect of damping
because of resistive
elements in absence of
Tunnel diode
Sinusoidal oscillations with the help of
Tunnel diode.

19. Schottky barrier (hot-carrier) diodes
• The Schottky-barrier diode (SBD) is formed by bringing metal into
contact with a moderately doped n-type semiconductor material.
• the current–voltage characteristic of the SBD is remarkably similar
to that of a pn-junction diode, with two important exceptions:
1.In the SBD, current is conducted by majority carriers (electrons).
Thus the SBD does not exhibit the minority-carrier charge-
storage effects found in forward biased pn junctions. As a result,
Schottky diodes can be switched from on to off, and vice versa,
much faster than is possible with pn-junction diodes.
2.The forward voltage drop of a conducting SBD is lower than that
of a pn-junction diode.

20. Construction and principle of working
Metal 𝐍+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
depletionregioninthep–
njunctiondiode.
The additional carriers in
the metal establish a
“negative wall” in the
metal at the boundary
between the two
materials.Thenetresultis
a “surface barrier”
between the two
materials, preventing any
furthercurrent.

21. Characteristic & symbol
The application of a forward bias as shown in the first quadrant of
Fig. will reduce the strength of the negative barrier through the
attraction of the applied positive potential for electrons from this
region. 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.
Observe this reverse S
for schottky

22. Photodiodes
• The photodiode is a semiconductor p – n junction device whose
region of operation is limited to the reverse-bias region.
• Recall from the previous slides that 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.
• 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.

23. Construction and principle of working
NP
The application of light to the
junctionwillresultinatransferof
energyfromtheincidenttraveling
light waves (in the form of
photons) to the atomic structure,
resultinginanincreasednumber
of minority carriers and an
increasedlevelofreversecurrent.
Lets incident the
light
This negative
basing will result
in an small
amount of
reverse current

24. Characteristic & symbol
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 V T
The almost equal spacing between the curves
for the same increment in luminous flux
reveals that the reverse current and the
luminous flux are almost linearly related. In
other
words, an increase in light intensity will resuent.
The almost equal spacing between the curves
for the same increment in luminous flux
reveals that the reverse current and the
luminous flux are almost linearly related. In
other words, an increase in light intensity will
result in a similar increase in reverse current.

25. Application of photodiode
In Fig, the photodiode is employed in an alarm
system. The reverse current 𝐼𝜆 will continue to
flow as long as the light beam is not broken. If the
beam is interrupted, 𝐼𝜆 drops to the dark current
level and sounds the alarm
In Fig, a photodiode is used to count items on a conveyor
belt. As each item passes, the light beam is broken, 𝐼𝜆 drops
to the dark current level, and the counter is increased by
one.

26. Transistor as a switch
50kΩ
0.7
kΩ
β= 125
𝑉𝐶𝐸 = 𝑉𝐶= 𝑉𝐶𝐶 − 𝑅 𝐶 𝐼 𝐶
𝐼 𝐶𝑀𝑎𝑥 = 𝐼 𝐶𝑠𝑎𝑡 =
𝑉𝐶𝐶
𝑅 𝐶
= 7.1 mA
𝐼 𝐵 =
𝐼 𝐶𝑠𝑎𝑡
β
= 56.8 µA
And condition for saturation is
𝐼 𝐵 >
𝐼 𝐶𝑠𝑎𝑡
β
𝐼 𝐵 > 56.8 µA
So, in order to make this transistor work under saturation region we
have to choose input resistance so as the 𝐼 𝐵 > 56.8 µ𝐴 and it must be
known to you that in saturation 𝑉𝐶𝐸 = 𝑉𝐶= 0
Discussion and calculation of parameter

27. Transistor as a switch
50kΩ
0.7
kΩ
Now lets apply a voltage of 5V at input, we get
𝐼 𝐵 =
𝑉𝑖 − 𝑉𝐵𝐸
𝑅 𝐵
=
5 − 0.7
50
= 86µA > 56.8 µA
It mean when we are applying a 5V potential at input of the transistor it will be
working in saturation region giving an output voltage equal to zero
Now lets take input voltage equals to 0 V It means Base
is at zero
potential
And the emitter is
already at zero
potential, so the base
emitter junction cant
be forward bias and
hence transistor will
be in cutoff region
Thus 𝐼𝑐 = β 𝐼 𝐵 = 0
That will result in 𝐼 𝐵= 0,
Resulting 𝑉𝐶 = 5𝑉
B
E
C
β= 125

28. Lets summarize
Under saturation region
B
E
C
𝑅 𝑠𝑎𝑡 =
𝑉𝐶𝑠𝑎𝑡
𝐼𝑠𝑎𝑡
⇒ 𝑅 𝑠𝑎𝑡=
0
𝐼𝑠𝑎𝑡
⇒ 𝑅 𝑠𝑎𝑡= 0
E
C
It means that
the switch is
on
Under cutoff region
𝑅 𝑠𝑎𝑡 =
𝑉𝐶𝐶
𝐼 𝐶
⇒ 𝑅 𝑠𝑎𝑡=
𝑉𝐶𝐶
0
⇒ 𝑅 𝑠𝑎𝑡= ∞
It means that
the switch is
off

29. THANK YOU!