Unit I -Special Diodes

7/30/2019 Unit I -Special Diodes

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Light Emitting Diode (LED)

Principle of Operation:

The LED is a specialized form of PN junction that uses a compound junction. The commonly used semiconductor materials including silicon and germanium are simple elements and junction

made from these materials do not emit light.

The semiconductor material used for the junction must be a compound semiconductor.

Junctions made from compound semiconductors emit light.

e.g. gallium arsenide, gallium phosphide and indium phosphide are compound semiconductors and junctions made

from these materials emit light.

These compound semiconductors are classified by their valence bands. For gallium arsenide, gallium has a valency of three and arsenic a valency of five and this is what is termed a

group III-V semiconductor.

When a voltage is applied across a general purpose diode to make it forward biased, current flows in the junction.

Holes from the p-type region and electrons from the n-type region enter the junction and recombine to enable the

current to flow.

When this occurs energy is released, in the form of heat.

LED emits light when it is forward biased.

When a voltage is applied across the junction to make it forward biased, current flows as in the case of any pn-jn.

Holes from the p-type region and electrons from the n-type region enter the junction and recombine like a normal

diode to enable the current to flow.

When this occurs energy is released in the form of light photons.


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To produce light which can be seen the correct materials must be chosen.

Pure gallium arsenide releases energy in the infra read portion of the spectrum (invisible light).

To bring the light emission into the visible red end of the spectrum aluminium is added to the semiconductor to

give aluminium gallium arsenide (AlGaAs).

LEDs made from a combination of the elements gallium, arsenic, and phosphorus (calledgallium-arsenide-

phosphide =GaAsP) glow bright red, and are some of the most common LEDs manufactured.

For other colours other materials are used to altering the chemical constituency of the PN junction.

Phosphorus can also be added to give red light.

For example :

gallium-phosphide (GaP) gives red / green light and

aluminium indium gallium phosphide give yellow / orange light.

Construction and Operation:

LEDs operate at voltage levels from about 1.5V to about 3.3V with currents from 5mA to about 30mA range.

Response time = nanoseconds

Power requirement from 10 to 150mW.

The positive power is applied to one side of the LED semiconductor through a lead (1 anode) and a whisker (4).

The other side of the semiconductor is attached to the top of the anvil (7) i.e. the negative power lead (2 cathode).

It is the chemical makeup of the LED semiconductor (6) that determines the color of the light the LED produces.

The epoxy resin enclosure (3 and 5) has three functions:

It is designed to allow the most light to escape from the semiconductor,

it focuses the light (view angle), and

it protects the LED semiconductor from the elements.

The entire unit is totally embedded in epoxy. This is what make LEDs virtually indestructible.

There are no loose or moving parts within the solid epoxy enclosure.


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Eye Protection

LEDs are very bright. DO NOT look directly into the LED light!!

The light can be intense enough to injure human eyes.

Advantages:Operating Life: Long = approx 20 years

Because LEDs are solid-state devices they are not subject to sudden failure whenoperated within design parameters.

DDP LEDs are designed to operate upwards of 50,000 hours at 25C ambient

temperature. Operating life is characterized by the degradation of LED intensity over

time.

When the LED degrades to half of its original intensity after 50,000 hours it is at the

end of its useful life although the LED will continue to operate as output diminishes.

Unlike standard incandescent bulbs, DDP LEDs resist shock and vibration and can

be cycled on and off without excessive degradation.

Other colors may be obtained by combining two or more primary-color (red, green, and blue) LEDs together

in the same package, sharing the same optical lens.

This allowed for multicolor LEDs, such as tricolor LEDs (commercially available in the 1980's) using red and

green (which can create yellow) and later RGB LEDs (red, green, and blue), which cover the entire color

spectrum.

Multicolor LEDs:


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Photo Diode Aphotodiode is a diode optimized to produce an electron current flow in response to

irradiation by ultraviolet, visible, or infrared light. A photodiode is a type of photo-detector capable of converting light into either current

or voltage, depending upon the mode of operation.

It is based on a principle that if a reversed biased diode is illuminated then the current

flow varies linearly with the light flux.

Silicon is most often used to fabricate photodiodes; though, germanium and gallium

arsenide can also be used.

The junction through which light enters the semiconductor must be thin enough to

pass most of the light on to the active region (depletion region) where light is converted

to electron-hole pairs.

Ref: All about circuits=sp diodes

Photodiode: Schematic symbol and cross section.

Principle of Operation:

K

A A K

Symbol of a Photodiode


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Construction:

In Fig. a shallow P-type diffusion into an N-type wafer produces a PN junction near the surface of the wafer.

The P-type layer needs to be thin to pass as much light as possible.

A heavy N+ diffusion on the back of the wafer makes contact with metallization.

The top metallization may be a fine grid of metallic fingers on the top of the wafer for large cells.

In small photodiodes, the top contact might be a sole bond wire contacting the bare P-type silicon top.

Photodiode consists of this pn-junction is embedded in a clear plastic case.

One side of this plastic case is transparent, on which light is made to fall.

The remaining sides are either painted black or are enclosed in a metallic case.

Entire unit is very small, having dimensions of the order of a few tenths of a volt.

Photodiode: Schematic symbol and

cross section.

K

A


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Photodiodes are used both to detect the presence of light and to measure light intensity.

Switching purposes

Solar cells ( type of semiconductor device)

Burglar alarms

Photodiodes are also used in

consumer electronics devices such as compact disc players,

smoke detectors, and

the receivers for remote controls in VCRs and televisions.

Applications:


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Schottky point contact diodes were subsequently replaced by a technique in which metal was vacuum deposited.

Fig. Deposited metal Schottky barrier

One of the problems with the simple deposited metal diode is that breakdown effects occur around the edge of

the metalised area.

This arises from the high electric fields that are present around the edge of the plate.

Leakage effects are also noticed.

To overcome these problems a guard ring of P+ semiconductor fabricated using a diffusion process is providedalong with an oxide layer around the edge.

In some instances metallic silicides may be used in place of the metal.

Fig. Deposited metal and oxide film Schottky diode

Construction: 2nd method-

Fig. Symbol of Schottky diode


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Reverse recovery time of Schottky:The most important difference between p-n and Schottky diode is reverse recovery time, when the diode

switches from non-conducting to conducting state and vice versa.

Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than

100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from.

The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes.

With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors

brings increased EMI noise.

With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a

concern.

It is often said that the Schottky diode is a "majority carrier" semiconductor device.

This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a

significant role in normal operation of the device.

The majority carriers are quickly injected into the conduction band of the metal contact on the other side of

the diode to become free moving electrons.

Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease

conduction faster than an ordinary p-n rectifier diode.

This property in turn allows a smaller device area, which also makes for a faster transition.

Operation:

When the signal applied to a diode changes from forward to reverse bias, conduction continues for a short time, while carriers are being swept out of the

depletion region. Conduction only ceases after this trreverse recovery time has expired. Schottky diodes have a shorter reverse recovery time.

Regardless of switching speed, the 0.7 V forward voltage drop of silicon diodes causes poor efficiency in low voltage supplies. This is not a problem in,

say, a 10 V supply. In a 1 V supply the 0.7 V drop is a substantial portion of the output. One solution is to use a Schottky power diode which has a lower

forward drop.


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Figure :Linear plot of current versus voltage

for a Schottky diode illustrating the conceptof a diode turn-on voltage.

Characteristics:

The Schottky diode is what is called a majority carrier device.

This gives it tremendous advantages in terms of speed because it does not rely on holes or electrons recombiningwhen they enter the opposite type of region as in the case of a conventional diode.

By making the devices small the normal RC type time constants can be reduced, making these diodes an order of

magnitude faster than the conventional PN diodes.

This factor is the prime reason why they are so popular in radio frequency applications.

The overall I-V characteristic is shown below.

It can be seen that the Schottky diode has the typical forward semiconductor diode characteristic, but with

a much lower turn on voltage.

Schottky diode has higher current density.

At high current levels it levels off and is limited by the series resistance or the maximum level of current

injection.

In the reverse direction breakdown occurs above a certain level.

The mechanism is similar to the impact ionisation breakdown in a PN junction.


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Applications:

High switching speed is the reason why Schottky diodes are useful in switch-mode power converters; the high

speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the

use of small inductors and capacitors with greater efficiency than would be possible with other diode types.

Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz.

While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium diodes 0.3 volts,

Schottky diodes' voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V, which makes

them useful in voltage clamping applications and prevention oftransistor saturation. This is due to the higher

current density in the Schottky diode.

The Schottky barrier diodes are widely used in the electronics industry finding many uses as

diode rectifier.

Its unique properties enable it to be used in a number of applications where other diodes would

not be able to provide the same level of performance.

In particular it is used in areas including:

RF mixer and detector diode

Power rectifier

Power OR circuits

Solar cell applications Clamp diode - especially with its use in LS TTL

Limitations

The most evident limitations of Schottky diodes are the relatively low reverse voltage rating for

silicon-metal Schottky diodes, 50 V and below, and a relatively high reverse leakage current. The

reverse leakage current, increasing with temperature, leads to a thermal instability issue. This often

limits the useful reverse voltage to well below the actual rating, but the diodes are improving. Thevoltage ratings are now at 200 V.

details in Ref: Schottky Diode1


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Tunnel Diode /Esaki Diode

Construction and Operation: The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very

high.

Also the depletion region, the area between the p-type and n-type areas, where there are no carriers is very

narrow.

Typically it is in the region of between five to ten nano-metres - only a few atom widths. The normal junction diode uses semiconductor materials that are lightly doped with one impurity atom for ten-

million semiconductor atoms.

This low doping level results in a relatively wide depletion region.

Conduction occurs in the normal junction diode only if the voltage applied to it is large enough to overcome the

potential barrier of the junction.

In the TUNNEL DIODE, the semiconductor materials used in forming a junction are doped to the extent of one-

thousand impurity atoms for ten-million semiconductor atoms.

This heavy doping produces an extremely narrow depletion zone similar to that in the Zener diode.

Basic principle is that if a semiconductor junction diode is heavily doped with impurities, it will have a region of

negative resistance.

Explanation of why a tunnel diode has a region of negative resistance is best understood by using energy levels:

The theory is known as quantum-mechanical tunneling.

Quantum-mechanical tunneling is an effect due to which an electron can cross a PN- junction without having

sufficient energy to do so otherwise.

Because of the heavy doping the width of the depletion region is only one-millionth of an inch.

The process may be considered to simply be an arc- over between the N- and the P-side across the depletionregion.

In 1958, Leo Esaki, a Japanese scientist, discovered


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(A)Tunnel diode energy diagram

with no bias.

Figure 1A-shows the equilibrium energy level diagram of a tunnel diode with no bias applied.

In view A that the valence band of the P-material overlaps the conduction band of the N-material.

The majority electrons and holes are at the same energy level in the equilibrium state.

If there is any movement of current carriers across the depletion region due to thermal energy,

the net current flow will be zero because equal numbers of current carriers flow in opposite directions. The zero net current flow is marked by a "0" on the current-voltage curve illustrated in view B.

Figure 1


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Figure 2- A, shows the energy diagram of a tunnel diode with a small forward bias (50 millivolts) applied.

The bias causes unequal energy levels between some of the majority carriers at the energy band overlap point,

but not enough of a potential difference to cause the carriers to cross the forbidden gap in the normal manner.

Since the valence band of the P-material and the conduction band of the N-material still overlap, current

carriers tunnel across at the overlap and cause a substantial current flow.

The amount of current flow is marked by point 2 on the curve in view B.

In view A that the amount of overlap between the valence band and the conduction band decreased when

forward bias was applied.

Figure 2A.Tunnel diode energy diagram with 50 millivolts bias. Figure 2B.Tunnel diode energy diagram with 50 mvolts bias.


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Figure 3-A, is the energy diagram of a tunnel diode in which the forward bias has been increased to 450 millivolts.

The valence band and the conduction band no longer overlap at this point, and tunneling can no longer occur.The portion of the curve in view B from point 2 to point 3 shows the decreasing current that occurs as the bias is

increased, and the area of overlap becomes smaller.

As the overlap between the two energy bands becomes smaller, fewer and fewer electrons can tunnel across the

junction.

The portion of the curve between point 2 and point 3 in which current decreases as the voltage increases is the

negative resistance region of the tunnel diode.

Figure 3-A.Tunnel diode energy diagram with 450 millivolts bias. Figure 3-B.Tunnel diode energy diagram with 450 millivolts bias.


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Figure 4-A, is the energy diagram of a tunnel diode in which the forward bias has been increased even further.

The energy bands no longer overlap and the diode operates in the same manner as a normal PN junction, asshown by the portion of the curve in view (B) from point 3 to point 4.

Figure 4-A. Tunnel diode energy diagram with 600 millivolts bias. Figure 4-B.Tunnel diode energy diagram with 600 millivolts


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Characteristics of Tunnel Diodes:

When a small forward-bias voltage is applied across a tunnel diode, it begins to conduct current.

As the voltage is increased, the current increases and reaches a peak value called thepeak current (IP).

If the voltage is increased a little more, the current actually begins todecrease until it reaches a low point called

the valley current (IV).

If the voltage is increased further yet, the current begins to increase again, this time without decreasing into

another valley. The forward voltages necessary to drive a tunnel diode to its peak and valley currents are known as peak voltage

(VP) and valley voltage (VV), respectively.

The region on the graph where current is decreasing while applied voltage is increasing (between VP and VV on

the horizontal scale) is known as the region ofnegative resistance.

Tunnel diodes are able to transition between peak and valley current levels very quickly, switching between

high and low states of conduction much faster than even Schottky diodes.

Tunnel diode characteristics are also relatively unaffected by changes in temperature.

Tunnel diode (a) Schematic symbol. (b) Current vs voltage plot.

Tunnel diodes are heavily doped in both the P and N regions, 1000 times the level in a rectifier.


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Negative Resistance Region of Characteristics:

The heavy doping produces an unusually thin depletion region. This produces an unusually low reverse breakdown voltage with high leakage.

The thin depletion region causes high capacitance.

To overcome this, the tunnel diode junction area must be tiny.

The forward diode characteristic consists of two regions:

Normal forward diode characteristic with current rising exponentially beyond VF

, 0.3 V for Ge, 0.7 V for Si.

Between 0 V and VF is an additional negative resistance characteristic peak.

This is due to quantum mechanical tunneling involving the dual particle-wave nature of electrons.

The depletion region is thin enough compared with the equivalent wavelength of the electron that they can

tunnel through.

They do not have to overcome the normal forward diode voltage VF.

The energy level of the conduction band of the N-type material overlaps the level of the valence band in the P-

type region.

With increasing voltage, tunneling begins; the levels overlap; current increases, up to a point.

As current increases further, the energy levels overlap less; current decreases with increasing voltage.

This is the negative resistance portion of the curve.

Ref: Tunnel Diode.doc


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Applications:

Tunnel diodes are not good rectifiers, as they have relatively high leakage current when reverse-biased.

Tunnel diodes find application only in special circuits where their unique tunnel effect has value.

To exploit the tunnel effect, these diodes are maintained at a bias voltage somewhere between the peak and

valley voltage levels, always in a forward-biased polarity (anode positive, and cathode negative).

Perhaps the most common application of a tunnel diode is in simple high-frequency oscillator circuits as in

Figure (c).

A tunnel diode biased to operate in the negative resistance region can be used as either an oscillator or an

amplifier in a wide range of frequencies and applications.

Very high frequency applications using the tunnel diode are possible because the tunneling action occurs so

rapidly that there is no transit time effect and therefore no signal distortion.

Tunnel diodes are also used extensively in high- speed switching circuits because of the speed of the tunneling

action.

(c) Oscillator.

VF

Tunnel diode


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Varactor Diode / Varicap

The VARACTOR, or varicap, as the schematic drawing in Figure 1 suggests, is a diode that behaves like a

variable capacitor, with the PN junction functioning like the dielectric and plates of a common capacitor. A

variable capacitance diode is known as a varicap diode or as a varactor. If a diode is reverse biased, an

insulating depletion region forms between the two semiconductive layers. In many diodes the width of the

depletion region may be changed by varying the reverse bias. This varies the capacitance. This effect is

accentuated in varicap diodes. The varicap thus acts as a variable capacitor under the effect of variable

reverse bias voltage.

Figure 1.Varactor diode schematic symbols.

Figure 2 shows a PN junction. Surrounding the junction of the P and N materials is a

narrow region void of both positively and negatively charged current carriers.

This area is called the depletion region.

The size of the depletion region in a varactor diode is directly related to the bias.

Forward biasing makes the region smaller by repelling the current carriers toward

the PN junction.

If the applied voltage is large enough (about .7 volt for silicon material), the negative

particles will cross the junction and join with the positive particles, as shown in

Figure 3.

This forward biasing causes the depletion region to decrease, producing a low

resistance at the PN junction and a large current flow across it.

This is the condition for a forward-biased diode. . Figure 3-Forward-biased PN junction.

Figure 2-PN junction

Construction and Operation:


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By varying the reverse-bias voltage applied to the varactor, the width of the "gap" may be varied.

An increase in reverse bias increases the width of the gap (d) which reduces the capacitance (C) of the PN

junction. Therefore, the capacitance of the varactor is inversely proportional to the applied reverse bias.

The ratio of varactor capacitance to reverse-bias voltage change may be as high as 10 to 1. Figure 5 shows one

example of the voltage-to-capacitance ratio.

View A shows that a reverse bias of 3 volts produces a capacitance of 20 picofarads in the varactor.

If the reverse bias is increased to 6 volts, as shown in view B, the depletion region widens and capacitance drops

to 5 picofarads.

Each 1-volt increase in bias voltage causes a 5-picofarad decrease in the capacitance of the varactor; the ratio of

change is therefore 5 to 1.

Of course any decrease in applied bias voltage would cause a proportionate increase in capacitance, as the

depletion region narrows.

Notice that the value of the capacitance is small in the picofarad range.

Varactor diode Characteristics:

Capacitance range 2pF to 100pF


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A variable capacitance diode is known as a varicap diode or as a varactor.

If a diode is reverse biased, an insulating depletion region forms between the two semiconductive layers.

In many diodes the width of the depletion region may be changed by varying the reverse bias.

This varies the capacitance. This effect is accentuated in varicap diodes.

The schematic symbols is shown in Figure below, one of which is packaged as common cathode dual diode.

In general, varactors are used to replace the old style variable capacitor tuning.

They are used in tuning circuits of more sophisticated communication equipment and in other circuits where

variable capacitance is required.One advantage of the varactor is that it allows a dc voltage to be used to tune a circuit for simple remote control

or automatic tuning functions.

One such application of the varactor is as a variable tuning capacitor in a receiver or transmitter tank circuit like

that shown in Figure 6.

Applications:

Varactor diodes may be used in:

frequency multiplier circuits

Voltage controlled oscillators

Capacitors in IC form


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Zener Diode

With the application of sufficient reverse voltage, a p-n junction will experience a rapid avalanche breakdown

and conduct current in the reverse direction. Valence electrons which break free under the influence of the

applied electric field can be accelerated enough that they can knock loose other electrons and the subsequent

collisions quickly become an avalanche. When this process is taking place, very small changes in voltage can

cause very large changes in current. When the P and N-regions are heavily doped, direct rapture of covalent

bonds takes place because of the strong electric fields, at the junction of the PN diode. The new e--h+ pairs

created increase the reverse current in the reverse biased diode. The increase in current takes place at aconstant value of reverse bias about less than 6volts for heavily doped diodes. The breakdown process depends

upon the applied electric field, so by changing the thickness of the layer to which the voltage is applied, Zener

diodes can be formed which break down at voltages from about 4 volts to several hundred volts.

When breakdown occurs under the reverse bias condition, the voltage across the diode remains constant,

although the current through the diode increases as shown in fig.

Zener Effect:

or

Vz

symbol


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For the Zener diode we are using a modelwith a breakdown voltage Vzand an

effective resistance Rzin the breakdown

as shown on Figure 2 and illustrated onFigure 1.

Zener Diode Model:

Zener Equivalent Voltage

Approximate Model of the Zener diode

under


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Numericals on!!!Zener diode Regulator

Analysis:

1. Determine the state of the Zener diode ON or OFF by

(a) removing it from network=> open circuit

(b) calculate the voltage across the open circuit=> open circuit voltage

Solution: For Zener diode to be ON: Vi Vz

For Zener diode to be OFF: Vi < Vz

Calculate the voltage V applying

the voltage divider rule:


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Numericals on Zener Diode !!!

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Unit I -Special Diodes