Zener Diode
Tunnel Diode
LED
Zener Diode and its
Characteristics
Introduction:
A
Zener Diode is a highly doped PN junction, which is specially designed to
operate in reverse direction. Generally
Zener Diode acting as voltage regulator used in power supply sections for
maintaining constant voltage. The
zener diode as shown in figure 1.24a,b,c is specially designed for optimizing
the breakdown region.
Figure 1.24a Zener diode circuit connections in bread board.
Then take Digital Multimeter as shown in 1.24b, verify and check the voltage across the zener Diode, it is displays breakdown voltage.
Figure 1.24b Zener
diode testing using Mulitmeter
This Diode can be used in all power supply sections interface with mother boards like TV, Washing Machine, ATM Machine and Electronic Panel Boards as shown in figure 1.24c.
Figure 1.24c Zener diode used power
supply boards
Figure 1.24d Heavily doped Zener diode
The symbolic representation as shown in figure 1.24e of Zener Diode is similar to the P – N junction, but with bend edges on the vertical bar.
Figure 1.24e Symbol of Zener diode
Operation:
Zener Diodes acts like normal p-n junction Diodes under forward biased condition. When forward biased voltage is applied to the zener Diode it allows electric current in forward direction like a normal Diode as shown in Figure 1.25a.
Figure 1.25a Forward bias Characteristics of silicon
diode and Zener diode
But,
the uniqueness lies in the fact is that it also operates in reverse break down
region.
Breakdown in zener Diode:
There are
two types of reverse breakdown regions in a zener Diode.
·
zener breakdown and
·
Avalanche break down
Zener break
down:
When
reverse biased voltage applied to the Diode is increased, the narrow depletion
region generates strong electric field. When the voltage reaches
close to zener voltage as shown in Figure 1.25b (less than 6V for zener break
down). This electric field in the depletion region is strong enough to pull
electrons from their valence band. The valence electrons which gains sufficient
energy from the strong electric field of depletion region will breaks bonding
with the parent atom. The valance electrons which break bonding with
parent atom will become free electrons.
This
free electron carries electric current from one place to another place. At zener
breakdown region, a small increase in voltage will rapidly increases the
electric current.
Figure 1.25b Reverse bias connection in Zener Diode
with Zener breakdown Characteristics
Avalanche
breakdown:
The
avalanche breakdown occurs in both normal Diodes and zener Diodes at high
reverse voltage. When high reverse voltage is applied, the free electrons (minority
carriers) gains large amount of energy and accelerated to greater
velocities.
Figure 1.25c Reverse bias connection in Zener Diode with Avalanche breakdown Characteristics
The
free electrons moving at high speed will collides with the atoms and
knock off more electrons. These electrons are again accelerated and collide
with other atoms. Because of this continuous collision with the atoms, a large
number of free electrons are generated. As a result, electric current in the Diode
increases rapidly. This sudden increase in electric current may permanently
destroys the normal Diode. However, avalanche Diodes may not be destroyed
because they are carefully designed to operate in avalanche breakdown
region.Avalanche breakdown occurs in zener Diodes with zener voltage (Vz)
greater than 6V as shown in Figure 1.25c.
V-I Characteristics:
This curve shows that the Zener Diode, when connected in forwarding bias, behaves like an ordinary Diode. when the reverse voltage applies across itis less than 6v, the zener break down occurs in the Diode. When it rises beyond 6V the Avalanche break down occurs as shown in Figure 1.25d.
Figure 1.25d zener diode Characteristics
Applications
of Zener Diode :
1. Zener Diode as a voltage
In a DC circuit, Zener Diode can be used as a voltage regulator or to provide voltage reference. The main use of zener Diode lies in the fact that the voltage across a Zener Diode remains constant for a larger change in current. This makes it possible to use a Zener Diode as a constant voltage device or a voltage regulator.
In any power supply circuit, a regulator is used to provide a constant output (load) voltage irrespective of variation in input voltage or variation in load current. The variation in input voltage is called line regulation, whereas the variation in load current is called load regulation.
Figure 1.26a Circuit diagram of Zener diode as a
voltage regulator
2. Zener Diode as a voltage reference
In power supplies and many other circuits, Zener Diode finds its application as a constant voltage provider or a voltage reference. The only conditions are that the input voltage should be greater than zener voltage and the series resistor should have a minimum value such that the maximum current flows through the device.
Figure 1.26b Circuit diagram of Zener diode as a
voltage reference
3. Zener Diode as a voltage clamper
In a circuit involving AC input source, different from the normal PN Diode clamping circuit, a Zener Diode can also be used. The Diode can be used to limit the peak of the output voltage to zener voltage at one side and to about 0V at other side of the sinusoidal waveform.
Figure 1.26c Circuit diagram of Zener diode as a voltage clamper
In the above circuit, during positive half cycle, once the input voltage is such that the zener Diode is reverse biased, the output voltage is constant for a certain amount of time till the voltage starts decreasing.
Now during the negative half cycle, the zener Diode is in forward biased connection. As the negative voltage increases till forward threshold voltage, the Diode starts conducting and the negative side of the output voltage is limited to the threshold voltage.
Note that to get an output voltage in positive range only, use two oppositely biased Zener Diodes in series.
Working Applications of Zener Diode
With growing popularity of smart phones, android based projects are being preferred these days. These projects involve use of Bluetooth technology based device. These Bluetooth devices require about 3V voltage for operation. In such cases, a zener Diode is used to provide a 3V reference to the Bluetooth device.
Advantages
of zener diode
- Power dissipation capacity is very high
- High accuracy
- Small size
- Low cost
Tunnel
Diodes :
Tunnel
diode definition
A Tunnel diode is a heavily doped p-n junction diode in which the electric current decreases as the voltage increases. In tunnel diode, electric current is caused by “Tunneling”. The tunnel diode is used as a very fast switching device in computers. It is also used in high-frequency oscillators and amplifiers.
The tunnel diode was first introduced by Leo Esaki in 1958. Its characteristics, shown in Figure 1.27, are different from any diode discussed thus far in that it has a negative-resistance region. In this region, an increase in terminal voltage results in a reduction in diode current.
The tunnel diode
is fabricated by doping the semiconductor materials that will form the p
– n junction at a level 100 to several thousand times 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 this thin depletion
region, through which many carriers can “tunnel” rather than attempt to surmount,
at low forward-bias potentials that accounts for the peak in the curve of Figure 1.27 For comparison purposes, a
typical semiconductor diode characteristic is superimposed on the tunnel-diode
characteristic of Figure 1.27. This
reduced depletion region results in carriers “punching through” at velocities
that far exceed those available with conventional diodes. The tunnel diode can
therefore be used in high-speed applications such as in computers, where
switching times in the order of nanoseconds or picoseconds are desirable. Recall
from Section 1.15 that an increase in the doping level
reduces the Zener potential. Note the effect of a very high doping level on
this region in Figure 1.27. The semiconductor materials most frequently used in
the manufacture of tunnel diodes are germanium and gallium arsenide. The ratio IP>Iv
is very important for computer applications. For germanium, it is
typically 10:1, and for gallium arsenide, it is closer to 20:1. The peak
current IP of a tunnel diode can vary from a few microamperes
to several hundred amperes. The peak voltage, however, is limited to about 600
mV. For this reason, a simple VOM with an internal dc battery potential of 1.5
V can severely damage a tunnel diode if applied improperly. The tunnel-diode
equivalent circuit in the negative-resistance region is provided in Figure 1.28,
with the symbols most frequently employed for tunnel diodes. The values for the
parameters are typical for today’s commercial units.
The inductor Ls is due mainly to the terminal leads. The resistor R is due to the leads, the ohmic contact at the lead– semiconductor junction, and the semiconductor materials themselves. The capacitance is the junction diffusion capacitance, and the R S is the negative resistance of the region. The negative resistance finds application in oscillators to be described later.
In Figure 1.28,
the chosen supply voltage and load resistance define a load line that
intersects the tunnel diode characteristics at three points. Keep in mind that
the load line is determined solely by the network and the characteristics of
the device. The intersections at a and b are referred to as stable
operating points, due to the positive-resistance characteristic.
That is, at either of these operating points, a slight disturbance in the network will not set the network into oscillations or result in a significant change in the location of the Q-point. For instance, if the defined operating point is at b, a slight increase in supply voltage E will move the operating point up the curve since the voltage across the diode will increase. Once the disturbance has passed, the voltage across the diode and the associated diode current will return to the levels defined by the Q-point at b. The operating point defined by c is an unstable one because a slight Figure 1.29 change in the voltage across or current through the diode will result in the Q-point as shown in moving to either a or b. For instance, the slightest increase in E will cause the voltage across the tunnel diode to increase above its level at c. In this region, however, an increase in VT will cause a decrease in IT and a further increase in VT
Tunnel Diode Energy Band Diagram:
Figure 1.30(a) shows energy level diagrams of the tunnel diode for three interesting bias levels. The shaded area show the energy states occupied by electrons in the valence band, whereas the cross hatched regions represent energy states are occupied by electrons either side of the junctions are shown by dotted lines. When the bias is zero, these linear are at the same height. Unless energy is imparted to the electrons from external source, the energy possessed by the electrons on the N-Side of the junction is insufficient permit over the junction barrier to reach the P-Side. However, quantum mechanics show that there is finite probability for the electrons to tunnel though the junction to reach the other side, provided there are allowed empty energy states in the P-side of the junction at the same energy level. Hence, the forward current is Zero.
When a small forward bias is applied to the junction, the energy level of the P-side is lower as compared with the N-Side. As shown in fig. Figure 1.30(b), electrons in the conduction band of the N-side see empty energy level on the P-side. Hence, tunneling from N-side to P-side takes place. Tunneling is other directions are not possible because the valence band electrons on the P-side are now opposite to the forbidden energy gap on the N-side. The energy band diagram shown in Figure 1.30(b) is for the peak of the diode characteristics.
When the forward
bias is raised beyond this point tunneling will decrease as shown in Figure
1.30(c). The energy of the P-Side is now depressed further, with the result
that fewer conduction band electrons on the N-Side are opposite to the
unoccupied P-side energy levels. As the bias is raised, forward current drops.
This corresponds to the negative resistance region of the diode
characteristics. As forward bias is raised still further, tunneling stops
altogether and it behaves as normal P-N Junction diode.
Advantages
of tunnel diodes
- Long life
- High-speed operation
- Low noise
- Low power consumption
Disadvantages
of tunnel diodes
- Tunnel diodes cannot be fabricated in large numbers
- Being a two terminal device, the input and output are
not isolated from one another.
Applications
of tunnel diodes
- Tunnel diodes are used as logic memory storage devices.
- Tunnel diodes are used in relaxation oscillator
circuits.
- Tunnel diode is used as an ultra high-speed switch.
- Tunnel diodes are used in FM receivers.
LED:
Before going into how LED works,
let’s first take a brief look at light self. Since ancient times man has
obtained light from various sources like sunrays, candles and lamps.
In 1879, Thomas Edison invented the incandescent light bulb. In the light bulb, an electric current is passed through a filament inside the bulb. Unlike the light bulb in which electrical energy first converts into heat energy, the electrical energy can also be directly converted into light energy. Light is a type of energy that can be released by an atom. Light is made up of many small particles called photons. Photons have energy and momentum but no mass.
Light
Emitting Diodes (LEDs) as shown in Figure 1.31a are the most widely used
semiconductor diodes among all the different types of semiconductor diodes
available today. Light Emitting Diodes emit either visible light or invisible
infrared light when forward biased. The LEDs which emit invisible infrared
light are used for remote controls.
When
Light Emitting Diode (LED) is forward biased, free electrons in the
conduction band recombines with the holes in the valence band and
releases energy in the form of light.
The
process of emitting light in response to the strong electric
field or flow of electric current is called electroluminescence.
A
normal p-n junction diode allows electric current only in one
direction. It allows electric current when forward biased and does not allow
electric current when reverse biased. Thus, normal p-n junction diode operates
only in forward bias condition. Like the normal p-n junction diodes, LEDs also
operates only in forward bias condition. To create an LED, the n-type material
should be connected to the negative terminal of the battery and p-type material
should be connected to the positive terminal of the battery. In other words,
the n-type material should be negatively charged and the p-type material should
be positively charged. The construction of LED is similar to the normal p-n
junction diode except that gallium, phosphorus and arsenic materials are used
for construction instead of silicon or germanium materials.
In
normal p-n junction diodes, silicon is most widely used because it is less
sensitive to the temperature. Also, it allows electric current efficiently
without any damage. In some cases, germanium is used for constructing diodes. However,
silicon or germanium diodes do not emit energy in the form of light. Instead,
they emit energy in the form of heat. Thus, silicon or germanium is not used
for constructing LEDs.
Layers of LED
A Light Emitting Diode (LED) consists of
three layers: p-type semiconductor, n-type semiconductor and
depletion layer. The p-type semiconductor and the n-type semiconductor are
separated by a depletion region or depletion layer.
P-type semiconductor
When
trivalent impurities are added to the intrinsic or pure semiconductor, a p-type
semiconductor is formed. In p-type semiconductor, holes are the majority charge
carriers and free electrons are the minority charge carriers. Thus, holes carry
most of the electric current in p-type semiconductor.
N-type semiconductor
When
pentavalent impurities are added to the intrinsic semiconductor, an n-type
semiconductor is formed. In n-type semiconductor, free electrons are the
majority charge carriers and holes are the minority charge carriers. Thus, free
electrons carry most of the electric current in n-type semiconductor.
Depletion layer or region
Depletion
region is a region present between the p-type and n-type semiconductor where no
mobile charge carriers (free electrons and holes) are present. This region acts
as barrier to the electric current. It opposes flow of electrons from n-type
semiconductor and flow of holes from p-type semiconductor.
To
overcome the barrier of depletion layer, we need to apply voltage which is
greater than the barrier potential of depletion layer. If the applied voltage
is greater than the barrier potential of the depletion layer, the electric
current starts flowing.
Light Emitting Diode (LED) working:
Light Emitting Diode (LED) works
only in forward bias condition. When Light Emitting Diode (LED) is forward
biased, the free electrons from n-side and the holes from p-side are pushed
towards the junction as shown in Figure 1.31b.
When free electrons reach the
junction or depletion region, some of the free electrons recombine with the
holes in the positive ions. We know that positive ions have less number of
electrons than protons. Therefore, they are ready to accept electrons. Thus,
free electrons recombine with holes in the depletion region. In the similar
way, holes from p-side recombine with electrons in the depletion region.
Because
of the recombination of free electrons and holes in the depletion region,
the width of depletion region decreases. As a result, more charge
carriers will cross the p-n junction.
Some
of the charge carriers from p-side and n-side will cross the p-n junction
before they recombine in the depletion region. For example, some free electrons
from n-type semiconductor cross the p-n junction and recombines with holes in
p-type semiconductor. In the similar way, holes from p-type semiconductor cross
the p-n junction and recombines with free electrons in the n-type
semiconductor.
Thus,
recombination takes place in depletion region as well as in p-type and n-type
semiconductor. The free electrons in the conduction band releases energy in the
form of light before they recombine with holes in the valence band. In silicon
and germanium diodes, most of the energy is released in the form of heat and
emitted light is too small. However, in materials like gallium arsenide and
gallium phosphide the emitted photons have sufficient energy to produce intense
visible light.
Operation of LED
When
external voltage is applied to the valence electrons, they gain sufficient
energy and break the bonding with the parent atom. The valence electrons which
break bonding with the parent atom are called free electrons. When the valence
electron left the parent atom, they leave an empty space in the valence shell
at which valence electron left. This empty space in the valence shell is called
a hole. The energy level of all the valence electrons is almost same. Grouping
the range of energy levels of all the valence electrons is called valence band.
In the similar way, energy level of all the free electrons is almost same.
Grouping the range of energy levels of all the free electrons is called
conduction band.
The
energy level of free electrons in the conduction band is high compared to the
energy level of valence electrons or holes in the valence band. Therefore, free
electrons in the conduction band need to lose energy in order to recombine with
the holes in the valence band.
The free electrons in the conduction band do not stay for long period. After a short period, the free electrons lose energy in the form of light and recombine with the holes in the valence band. Each recombination of charge carrier will emit some light energy.
The energy lose of free electrons or
the intensity of emitted light is depends on the forbidden gap or energy gap
between conduction band and valence band. The semiconductor device with large
forbidden gap as shown in Figure 1.31c emits high intensity light whereas
the semiconductor device with small forbidden gap emits low intensity light. In
other words, the brightness of the emitted light is depends on the material
used for constructing LED and forward current flow through the LED.
In normal silicon diodes, the energy
gap between conduction band and valence band is less. Hence, the electrons fall
only a short distance. As a result, low energy photons are released. These low
energy photons have low frequency which is invisible to human eye. In LEDs, the
energy gap between conduction band and valence band is very large so the free
electrons in LEDs have greater energy than the free electrons in silicon
diodes. Hence, the free electrons fall to a large distance. As a result, high
energy photons are released. These high energy photons have high frequency
which is visible to human eye.
The efficiency of generation of
light in LED increases with increase in injected current and with a decrease in
temperature. In light emitting diodes, light is produced due to recombination
process. Recombination of charge carriers takes place only under forward bias
condition. Hence, LEDs operate only in forward bias condition. When light
emitting diode is reverse biased, the free electrons (majority carriers) from
n-side and holes (majority carriers) from p-side moves away from the junction.
As a result, the width of depletion region increases and no recombination of
charge carriers occur. Thus, no light is produced. If the reverse bias voltage
applied to the LED is highly increased, the device may also be damaged. All
diodes emit photons or light but not all diodes emit visible light. The
material in an LED is selected in such a way that the wavelength of the
released photons falls within the visible portion of the light spectrum. Light
emitting diodes can be switched ON and OFF at a very fast speed of 1 ns.
Light emitting diode (LED) symbol
The symbol of LED is similar to the
normal p-n junction diode except that it contains arrows pointing away from the
diode indicating that light is being emitted by the diode.
LEDs are available in different colors. The most common colors of LEDs are orange, yellow, green and red. The schematic symbol of LED does not represent the color of light. The schematic symbol as shown in Figure 1.31d is same for all colors of LEDs. Hence, it is not possible to identify the color of LED by seeing its symbol.
LED Construction
One of the methods used to construct LED is to deposit three semiconductor layers on the substrate. The three semiconductor layers deposited on the substrate are n-type semiconductor, p-type semiconductor and active region. Active region is present in between the n-type and p-type semiconductor layers.
When LED is forward biased, free
electrons from n-type semiconductor and holes from p-type semiconductor are
pushed towards the active region. When free electrons from n-side and holes
from p-side recombine with the opposite charge carriers (free electrons with
holes or holes with free electrons) in active region, an invisible or visible light
is emitted. In LED, most of the charge carriers recombine at active region.
Therefore, most of the light is emitted by the active region. The active region
is also called as depletion region as shown in Figure 1.31e.
Biasing of LED
The safe forward voltage ratings of most LEDs is from 1V to 3 V and forward current ratings is from 200 mA to 100 mA. If the voltage applied to LED is in between 1V to 3V, LED works perfectly because the current flow as shown in Figure 1.31f for the applied voltage is in the operating range. However, if the voltage applied to LED is increased to a value greater than 3 volts. The depletion region in the LED breaks down and the electric current suddenly rises. This sudden rise in current may destroy the device. To avoid this we need to place a resistor (Rs) in series with the LED. The resistor (Rs) must be placed in between voltage source (Vs) and LED.
The resistor placed between LED and
voltage source is called current limiting resistor. This resistor restricts
extra current which may destroy the LED. Thus, current limiting resistor
protects LED from damage.
The current flowing through the LED is mathematically written as
Where
IF=Forward
current
VS=Source
voltage or supply voltage
VD=Voltage
drop across LED
RS=Resistor
or current limiting resistor
Voltage drop is the amount of
voltage wasted to overcome the depletion region barrier (which leads to
electric current flow).
The voltage drop of LED is 2 to 3V
whereas silicon or germanium diode is 0.3 or 0.7 V.
Therefore, to operate LED we need to
apply greater voltage than silicon or germanium diodes.
Light emitting diodes consume more
energy than silicon or germanium diodes to operate.
Output
characteristics of LED
The
amount of output light emitted by the LED is directly proportional to the
amount of forward current flowing through the LED. More the forward current,
the greater is the emitted output light. The graph of forward current vs output
light is shown in the figure.
Figure 1.31g: Characteristics of LED
The material used for constructing LED determines its color. In other words, the wavelength or color of the emitted light depends on the forbidden gap or energy gap of the material. Different colors of LEDs Forward voltage values are mentioned in following table 2.
Ø Different materials emit different
colors of light.
Ø Gallium arsenide LEDs emit red and
infrared light.
Ø Gallium nitride LEDs emit bright
blue light.
Ø Yttrium aluminium garnet LEDs emit
white light.
Ø Gallium phosphide LEDs emit red,
yellow and green light.
Ø Aluminium gallium nitride LEDs emit
ultraviolet light.
Ø Aluminum gallium phosphide LEDs emit
green light.
Advantages of LED
- The brightness of light emitted
by LED is depends on the current flowing through the LED. Hence, the
brightness of LED can be easily controlled by varying the current. This
makes possible to operate LED displays under different ambient lighting
conditions.
- Light emitting diodes consume low
energy.
- LEDs are very cheap and readily
available.
- LEDs are light in weight.
- Smaller size.
- LEDs have longer lifetime.
- LEDs operates very fast. They
can be turned on and off in very less time.
- LEDs do not contain toxic
material like mercury which is used in fluorescent lamps.
- LEDs can emit different colors
of light.
Disadvantages of LED
- LEDs need more power to operate
than normal p-n junction diodes.
- Luminous efficiency of LEDs is
low.
Applications of LED
The various applications of LEDs are
as follows
- Burglar alarms systems
- Calculators
- Picture phones
- Traffic signals
- Digital computers
- Multimeters
- Microprocessors
- Digital watches
- Automotive heat lamps
- Camera flashes
- Aviation lighting
