Special Diodes

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Tunnel diode

Tunnel diodes, also known as Esaki diodes, are a type of semiconductor devices, capable

of very fast and high frequency operation, well into the microwave frequency region.

Invented in August 1!" by #eona Esaki $1%!&, when he was with 'ony, these are notable

for their longevity, with devices made in the 1()s still functioning. #obert *oyceindependently came up with the idea of a tunnel diode while working for +illiam 'hockley,

but was discouraged from pursuing it. In 1" Esaki, along with Ivar -iaever and rian

/avid 0osephson, was awarded the *obel ri2e for research he had conducted around 1!3

regarding electron tunneling in solids.

They employ quantum mechanical effect called tunneling and hence the name tunnel diodes.

4irst these diodes were manufactured by 'ony in 1!" and from about 1() onwards by

-eneral Electric and other companies. +ell known for their dynamic negative resistance

property, they can be viewed as 5ener diodes, however, with breakdown voltage reduced to

2ero volts. They are usually made from germanium, but can also be made from gallium

arsenide and silicon materials. The circuit symbol and characteristics are shown in 4igure 6.

Figure 4.3 Tunnel diode$a& circuit symbol $b& circuit model in negative resistance region and

$c& volt7amper characteristic.

These diodes are characteri2ed by higher levels of impurity concentration and

consequently lower levels barrier widths. The impurity concentration is more than 1)1cm7

or one part in 1) and the width of 8unction barrier is less than 1)))A or 1)73m.

The equivalent circuit of tunnel diode consists of a parallel combination negative

resistance of 9 Rn with 8unction capacitance C in series with a series combination of lead

ohmic resistance R and lead inductance L as shown in 4igure 6.$b&.

As shown in 4igure 6.$c&, for small forward bias voltages, the forward current keeps

on increasing with voltage and attains a peak value known as the peak current I P at somespecific forward bias voltage, V P known as peak voltage. It corresponds to ma:imum forward

tunneling. 4or further increase in the voltage the current starts dropping and reaches a

minimum value I V known as valley current at another specific voltage known as valley

voltage V V . +ith further increase in voltage the current starts increasing setting in the region

of positive differential resistance. The voltage V F known as forward peak point current

voltage is the voltage corresponding to that point on the characteristic where the current is

equal to the ma:imum specified peak point current.

The diode e:hibits infinite differential resistance at peak and valley points. In between

peak and valley points the diode is e:hibiting negative differential resistance and in the

remaining portion of the characteristic the differential resistance is positive. It has been

estimated that minimum value of negative resistance occurs at the inflection point of the

negative resistance characteristic and it is given, appro:imately, by



min

% p

p

V R

I = −

. $6.3&

;nder reverse bias the reverse current increases almost linearly with the voltage. Thus the

device acts as a resistor in this region.

Figure 4.4 The two components of tunnel diode current. $a& Tunneling component, $b&

ordinary diode component and $c& total current.

The ratio of the peak current to valley current, I P < I V , is known as the figure of merit of

the diode. Its typical value is appro:imately equal to .! for Si, 3 for -e and 1! for -aAs

tunnel diodes. =ost commercially available tunnel diodes are made from -e or -aAs. It is

difficult to manufacture silicon tunnel diode with a high ratio of peak to valley current I P < I V .

-aAs has the highest ratio, I P < I V and largest voltage swing is nearly one volt i.e. V F 9 V P >

1.)?.

The current in the V-I characteristic can be considered as consisting of two

components@ $a& a tunneling component and $b& ordinary diode current given by 'hockleycurrent component as shown in 4igure 6.6

The peak point current of the diode is determined by an etching process and can be

held within %.!B or better on a production basis. Cowever, the peak point voltage, valley

point voltage and forward point voltage are determined by the semiconductor material and

are largely fi:ed. 4or -ermanium these voltages are !!, !) and !))m? respectively at room

temperatures. The magnitude of the negative resistance is equal to the slope dv<di of the

voltage current characteristic. 4or a 1milliamp -ermanium diode, the negative resistance lies

in between 1)) and 1()D.

Figure 4.5 Energy band diagrams of $a& tunnel diode and $b& ordinary diode.



The energy band diagram of tunnel diodes shows some marked difference with that of

ordinary diodes, as shown in 4igure 6.!. The 4ermi level in tunnel diodes always lies outside

the forbidden energy gap. As a result of the heavy doping concentrations, on the n-side, it lies

in the conduction band and on the p-side it lies in the valence band. The contact difference of

potential energy E o is more than the forbidden gap energy E - in tunnel diodes, i.e. E o E - .

Figure 4.5 #elative positions of various energy bands in tunnel diode at various points on

volt7ampere characteristic.

The manifestitation of negative resistnace in tunnel diodes can be e:plained with the

help of energy band diagrams, as shown in 4igure. Fonsider various points marked on the

volt7ampere characteristic. at At point 1, electrons are at the same level on both sides of

8unction, resulting in no net current. At point %, electrons on right side are raised until they

are opposite to empty states on left side, resulting in strong current from right to left.At point, electrons on right raised still farther. 'ome are opposite to forbidden gap and some are

opposite to empty states, resulting in decreased current.At point 6, electrons all are opposite

to forbiddedn gap, resulting in very small current.At point !, electrons are raised until they

spill over barrier, resulting in increased current.

The 1*% -ermanium tunnel diode, capable of carrying a ma:imum forward

current of ! mA, is designed for low level switching and small signal operations. It has

frequency capabilities upto %.%-C2., with a closely controlled peak point current, good

temperature stability and e:treme resistance to nuclear radiation. Its peak point current I P is

1.)mA, valley point current I V is ).1)mA and hence, the ratio, I P < I V is 1).). The peak point

voltage V P is ()m?, and the valley point voltage V V is !)m?, total capacitance is !p4,

series inductance is ).()nC, series resistance is 1.!)D, negative conductance is (.( G1) 7

mhos and the ma:imum operating temp is 1))) F.



There is absolutely no minority carrier storage and hence with no diffusion

capacitance in these devices and thus they are capable of e:tremely high frequency operation

often in the -C2 range.

Applications@ Applications for tunnel diodes includes local oscillators for ;C4 television

tuners, trigger circuits in oscilloscopes, high7speed counter circuits, and very fast7rise time pulse generator circuits. How7noise microwave amplifiers can be designed with these diodes.

They are used in frequency converters and detectors in microwave amplifiers and also as high

speed switchs. They are also used in the design of oscillators, amplifiers, and in switching

circuits using hysteresis.

Limitations@ The tunnel diode showed great promise as an oscillator and high7frequency

trigger device since it operated at frequencies far greater frequencies well into the microwave

bands. Cowever,

• 'ince its discovery, more conventional semiconductor devices have surpassed its

performance using conventional oscillator techniques.

• eing a two terminal device is a serious disadvantate with tunnel diode. 4or many

purposes, a three7terminal device, such as a field7effect transistor, is more fle:ible than

a device with only two terminals.

• ractical tunnel diodes operate at a few milliamperes and a few tenths of a volt,

making them low7power devices. The -unn diode has similar high frequency

capability and can handle more power.

• These diodes are susceptible to damage by overheating, and thus special care is needed

when soldering them.

Advantages@ The main advantages of these devices are, low cost, low noise, low power and

high speed. Also it has been found that tunnel diodes are less sensitive to nuclear radiation

when compared to transistors. This makes them well suited to higher radiation environments

such as those found in space. Their disadvantages include low output swing and being two

terminal devices.

4.4.Light Emitting Diodes

The light emitting diodes or HE/s in short, are two terminal semi7conductor devices, that

convert the electrical energy into light. These are widely used in display systems. The symbol

of this device is shown in 4igure 6."

Figure 4.7 Fircuit symbol of Hight emitting diode, HE/



Principle@ The working of HE/s is based on the principle that when an electron recombines

with a hole, an amount of energy equal to the band gap energy E - gets released in the form of

radiation. The frequency, f of radiation is given by lancks law

G E hf =

$6.a&

where h is known as lanckJs constant, given by,,6(.(%( 1) 0<sech −

= ×

$6.b&

The wavelength of the radiation can be obtained by converting frequency into

velocity of radiation and wavelength and then substituting the values of the constants, as

follows

G

G

hc hc E hf

E λ

λ = = → =

Cence, the wavelength in meters, is

,6 3(.(%( 1) 07sec , 1) m<sec

in 0G

m E

λ −× × ×

=

The wavelength in Km, becomes,6 3 ((.(%( 1) 07sec , 1) m<sec 1)in m

in 0G E λ µ

−

× × × ×=

,6 3 (

1

(.(%( 1) 07sec , 1) m<sec 1 ) 1.%6

in e? 1.( 1) $e?&G G

E E

−

−

× × × ×= =

× ×

Cence, the wavelength of the emitted radiation in Km and gap7energy in e?, are related by( )

1.%6Km

$e?&G E λ =

. $6.1)&

It can be observed from the Eq. $6.1)& that for the emission to lie in the visible

spectrum which is typically between 3) nm and "!) nm, the band gap energy of the material

should lie in between 1.(!e?

and %.%(e?

Table 6.1 LED materials and their properties.

'.*o

Folor 'emiconductor Type

eak

wavelength$Km&

eak bandgapenergy $e?&

1.

#ed

-a@5n7L Indirect ).")) 1.""

-aAs).().6 /irect ).(!) 1.1

Al-aAs

6. Lrange7

#ed-aAs).!).(!@* Indirect ).() 1."

!.Lrange AlIn-a /irect ).(%) %.))

(.Mellow

-aAs).16).3(@* Indirect ).!3! %.1%

AlIn-a /irect3. -reen AlIn-a /irect ).!") %.13



-a@ * Indirect ).!(! %.1

1).lue

'iF Indirect ).6") %.(6

-a* /irect ).6!) %."(

Lne way to achieve HE/ action is by forming a pn 8unction with a direct band7gap

semiconductor, and then forward bias it. In a forward biased p-n 8unction, electrons and holes

are in8ected into the same region of space i.e., 8unction region, where they can recombine andemit photons . Therefore, the emission in HE/s is concentrated near the 8unction.

HE/s are fabricated with III7? semiconductors due to their desirable optical

properties. A list of various semiconductors commonly used for HE/ fabrication to produce

different colors along with their radiation wavelengths and bandgap energies are given in

Table 6.1. /irect gap semiconductors are highly conducive for radiative recombination and

because of this reason they are widely used for HE/ fabrication. Cowever, it is possible to

have efficient radiative recombination from indirect gap semiconductors with the help of

certain trap levels and hence they are also used for HE/ fabrication.

4or visible blue light emission 'ilicon Farbide, -allium *itride are used. 4or other

colors, the materials universally used for HE/ fabrication are -allium hosphide $-a&,

-allium Arsenic hosphide $-aAs&, 5inc Telluride $5nTe& and 5inc 'elenide $5n'e&.

*ote that 'ilicon is not suitable for the fabrication of HE/s because of two reasons@

one it is an indirect band gap material in which the recombination takes place only with the

aid of traps& and other is its band gap energy is only 1.1%e?

less than that which is required

to emit light.

The semi7conductor, -aAs, is a direct band gap material in which the recombination

is direct without the aid of traps. Cowever, its band gap energy is only 1.6%e?

and hence

not sufficient for visible light emission. In action, its energy release is in the form of radiation

that fall in the infra7red region.

?oltage and current@ /ue to the higher value of the band gap, HE/s typically show ahigher forward voltage drop of the order of 1.%7%.! ?, and often carry current of the order of

tens of mA

An important figure of merit for HE/s is their optical conversion efficiency, which is

defined as the optical power output divided by the electrical power input. The efficiency of

radiation increases with the in8ected current as well with temperature.

Applications@ ?isible light emitting diodes find applications in calculators, watches

whereas infrared emitters are widely used in security systems, for optical coupling and safety

controls etc.

+hite HE/s are made recently and they are e:pected to replace fluorescent lamps and

incandescent bulbs soon. These are made with either one of the two methods@ one, the outputfrom red, green and blue HE/s are suitably combined to give while light or light from blue or

;? HE/ is used to e:cite electrons in phosphor material to high energy levels which then

come down to lower energy levels reradiating several wavelengths, all combining to give

white light.

LASER



The term HA'E# is an acronym for Hight Amplification by 'timulation Emission of

#adiaiton. It is a source of coherent electromagnetic waves at infrared and light frequencies.

As the light frequencies are high, the capacity to carry information is immense for lasers.

The first laser was a pulsed on using ruby. The continuously operating laser developed after

three years used helium and neon gases.

'emiconductor lasers are diodes with a p7n 8unction inside a slab of semiconductor that istypically less than a millimeter in any dimension. -allium Arsenide, -aAs, has been used in

the fabrication of lasers widely. It was discovered in 1(% that a forward biased -aAs diode

is capable of producing laser action. /epending on its precise chemical composition the

-aAs diode can give radiation within the range of "!) to ))nm

i.e. in the infrared region.

$Hight occupies the ) to "")nm

range&. In these lasers, the necessary e:citation is provided

by electrical current flow through the device and the cleaved ends of the diode provide the

feedback mirrors.

The output of diode lasers differ from that of other lasers in two significant ways@ beam

divergence and monochromatism. ecause of their small si2e, the beam divergence angles of

diode lasers are large, as much as %) ). The output, in the case of diode lasers, is far lessmonochromatic than other laser types. *evertheless, semiconductor lasers have important

applications in communications and range finding.

Figure 4.1 Como8unction -allium Arsenide laser diode.

Homo!nction Laser @ 4igure 6.1) is a diagram of the simplest and earliest type of -aAs

laser, which is essentially in brick7shape. The -aAs cleaves easily along certain crystal

planes, leaving flat parallel surfaces. ;sually, the mirrors for feedback and output coupling

are formed by the cleaved ends of the laser diode, with no further coating. The reflectivity at

the interface between gallium arsenide and air is appro:imately (B. If output is desired

from only one end of the device, or if mirrors of higher reflectivity are desired to reduce the

threshold for laser operation, the reflectivity may be increased by coating the surfaces with

metal films. Lptical standing waves may e:ist between any two of the parallel surfaces of thediode. Two sides are purposely roughened to reduce reflection and prevent lasing NacrossN the

diode cavity.

The output power available from this laser is limited by the loop gain available within

the laser cavity. The amplifier gain of the active medium is dependent on the current density

through the 8unction. Cigher currents produce greater power, but higher currents also increase

heating effects that can damage the device.

Hoss in the laser cavity has two primary contributors. The first of these is diffraction

loss, which may be reduced by making the 8unction wider and by better confining the light to

the active region. The second loss factor is absorption of the laser light by free carriers in the

8unction region. This loss may be brought down by lowering the temperature of the device,

thereby reducing the number of free carriers.



Hetero!nction Laser @ These are -aAs laser diodes with an advanced 8unction design to

reduce diffraction loss, by modifying the laser material to control the inde: of refraction of

the cavity and the width of the 8unction.

The p-n 8unction of the basic -aAs laser described above is called a homo8unction

because only one type of semiconductor material is used in the 8unction with different

dopants to produce the 8unction itself. The inde: of refraction of the material depends uponthe impurity used and the doping level. The 8unction region is actually lightly doped with p7

type material and has the highest inde: of refraction. The n7type material and the more

heavily doped p7type material both have lower indices of refraction. This produces a light

pipe effect that helps to confine the laser light to the active 8unction region. In the

homo8unction, however, this inde: difference is low and much light is lost.

In hetero8unction laser, a fraction of the gallium in the p7type layer has been replaced by

aluminum. This reduces the inde: of refraction of this layer and results in better confinement

of the laser light to the optical cavity.

Figure 4.11 Ceter8unction -allium Arsenide laser diode.

Str!ct!re@ In the middle it is active layer $ p- type -aAs& surrounded by two barrier layers

$on one side p-type and on other side n-type -aAlAs&. elow barrier n-type -aAlAs layer,

lies the substrate of n" 7 type -aAs. Above the barrier p-type -aAlAs layer e:ists a contact

layer of p-type -aAs. Active layer is surrounded by hetero8unctions whereas a metal contact

e:ists on one side of the contact layer and substrate. Fontact layer is positive with respect to

the substrate.

The device is an in8ection laser, in which electrons and holes originating in the -aAlAs

layers cross the hetero8unctions and give off their e:cess recombination energy in the form of

light. The hetero8unctions are opaque and the active region is constrained by them to the p-

layer of -aAs. Two ends of the slice are very highly polished so that reinforcing reflection

takes place between them. This laser is capable of powers in e:cess of 1 +. The indium

gallium arsenide phosphide, In-aAs, laser is capable of giving radiation at frequencies

lower than that of the -aAs diode. The constitution of various layers for both the types of

lasers are given in table 1.

Table 1 /escription of layers in -aAs and In-aAs HA'E#s.

'.*o. Hayer -aAs HA'E# In-aAs HA'E#

1. Fontact layer p7-aAs p7In-aAs%. arrier layer p7-aAlAs p7In



. Active layer p7-aAs p7In-aAs

6. arrier layer n7-aAlAs n7In

!. 'ubstrate n"7-aAs n"7 In

Applications include distance and speed measuring equipment, industrial welding,

etching, surgery, holograms, communications etc. The HE/s and HA'E#s compete with eachother in certain applications. It is essential to have a comparison of their characteristics. The

ma8or difference between the HE/ $or any other conventional light source& and the diode

laser@ in an HE/ light generation is via spontaneous emission, while in the laser it is

stimulated emission which is responsible for the generation of light

The structure of HE/ is simpler, cheaper and more reliable. In addition, HE/s are not

temperature sensitive and doesnt require polished ends.

Cowever the output power of the HE/s is lower to that of lasers and the output is not

monochromatic. In addition the light beam of HE/ is wider. In addition the light output of the

laser can be pulsed at much higher rates than that of HE/s.

4.7.!hotodiodes

The hotodiodes are semiconductor devices whose operation is e:actly inverse to that

of HE/s. They convert the optical energy into electrical power. The symbol, operation and

electrical characteristics are shown in 4igure 6.1%

Principle@ It is a reverse biased pn 8unction diode embedded in a clear plastic and the

radiation is made to fall upon one surface across the 8unction. The remaining sides of the

plastic are either painted black or enclosed in a metallic case.

Its functioning is based on the fact that when illumination falls over the 8unction of a

reverse biased 8unction diode, it results in a current through the diode which varies almost

linearly with the light flu:.

Furrent@ Its volt ampere characteristic is

( )1 # V V

s o I I I e

η

= + −

$6.1%&

where I s is short7circuit current and I o is reverse saturation current. Cere the current I is in

reverse direction.



Figure 4.1" hotodiode $a& Fircuit symbol, $b& operation , $c& volt7amper

characteristic and $d& sensitivity as function of distance of light spot from

8unction.

Photo-c!rrent @ The current, I s in the reverse biased 8unction diode as result of the

incidence of light is called photo7current. The light falling upon the 8unction acts as minority

carrier in8ector and the resulting minority carriers diffuse to the 8unction, cross it resulting in

photocurrent.

$ar% c!rrent @ The current in the absence of illumination which is due to the reversesaturation current of the diode as well as the current due to the carriers generated by the back

ground illumination is called dark current. The dark current corresponds mainly to the reverse

saturation current due to the thermally generated minority carriers.

=a:imum allowed wavelength@ This wavelength corresponds to the minimum

wavelength required for the incident radiation. The energy of the illumination depend upon

its frequency<wavelength. If the wavelength is more than certain value, then the incident

radiation can not generate current because its insufficient energy. The ma:imum allowed

wavelength of the incident radiation depends upon the band7gap energy of the semiconductor

used for the diode. The relation between these two quantities can be found as

( )

1.%6

Km $e?&G E λ =

$6.1&

*ote that it is same as Eq. $6.11&. ;sing the above relation, it can be found that the ma:imum

wavelength allowed for 'ilicon is as 1.11 Km

4requency sensitivity@ The photodiodes are highly frequency sensitive devices. It

means a given intensity of light of one frequency will not generate the same number of

minority carriers as an equal intensity of light of another frequency. In other words the photo

current depends upon the frequency<wavelength of the incident radiation.

The photo current is also a function of the distance from the 8unction at which the

light spot is focused, as shown in 4igure 6.3$d&. The current in the photo diode depends upon

the diffusion of minority carriers to the 8unction. Therefore it depends upon the distance fromthe 8unction at which the light spot is focused because if the focused light spot is far away



from the 8unction, the generated minority carriers can recombine before diffusing to the

8unction resulting in smaller current.

erformance indices@ The three critical parameters associated with performance of

photodiodes are $a& dark current, $b& responsivity and $c& response speed. A good photodiode

should have 2ero dark current, or it should be e:tremely small as compared to the

photocurrent corresponding to the minimum level of illumination.The responsivity of a photodiode is defined as the ratio of the generated photocurrent

and the incident optical power. Ideally it should have a value equal to 1< E G , E G in eV , but the

practical diodes e:hibit less value, recombination being the cause of this reduction.

The response speed of the photo diode is governed by how fast the photo generated

carriers can be collected by the e:ternal circuit. It can be appro:imated with transit time, t rr ,

given by

tr

d

& t

v=

$6.16&

where & is total width of the transition region and vd is the carrier drift velocity. 'o

faster response require small width transition region but it can be shown that larger

photocurrents require wider transition regions.

;seful 'emi7conductors@ At lower frequencies, Head sulfide is generally used as the

photo detector material when the incident wavelength is beyond 1Km all the way to about .!

Km. Even though silicon cannot be used for light emission, in HE/s, it can be effectively

used as photo diode effectively for a wide range incident wavelength from 1)nm to 11))nm.

i.e. at higher end of the spectrum.

Applications@ The pn- photodiode find e:tensive applications in high7speed reading of

computer punched cards and tapes, light detection systems, reading film sound track, light7

operated switches, production7line counting of ob8ects which interrupt a light beam, etc.

Special Diodes

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