diode as a temperature sensor

8/10/2019 DIODE AS A TEMPERATURE SENSOR

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DIODE S TEMPER TURE

SENSOR

Submitted by

BEFIN SKARIA

DEPARTMENT OF PHYSICS

St. Peters University


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CONTENTS

Chapter 1

Diode Characteristics

1.1 Introduction

1.2 Intrinsic Semiconductors

1.3 Extrinsic Semiconductors

1.3 (a) N-type Semiconductors

1.3 (b) P-type Semiconductors

1.4 Majority & Minority Carriers

1.5 P-N Junction Diodes

1.6 Forward Biasing

1.7 Reverse Biasing

1.8 Types of Diodes

1.9 Diode Equation

Chapter 2 Diode as a Temperature Sensor

2.1 Introduction

2.2 Theory

2.3 Constant Current Source

Chapter 3 Diffusion Capacitance

3.1 Junction Capacitance

3.2 Theory

3.3 Diffusion Capacitance

Observations

Conclusion


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Bibliography

CHAPTER 1

DIODE CHARACTERISTIC


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1.1 INTRODUCTION

In electronics, a Diode is a two-terminal electroniccomponent with asymmetric conductance; it has low (ideally zero),

resistance to current flow in one- direction and high (ideally infinite)

resistance in the other. Diodes were the first semiconductor electronic

devices. The first semiconductor diodes called cats whisker diodes,

developed around 1906 where made of mineral crystals such as galena.

Today most diodes are made of silicon but other

semiconductors such as germanium or selenium are sometimes used. A

semiconductor diode, the most common type today is a crystalline

piece of semiconductor material with a P-N junction connected to two

electrical terminals. The most common function of a diode is to allow

an electric current to pass in one direction called forward direction.

While blocking current in the opposite direction (Reverse direction).

By explaining the concepts of semiconductors, the band

structure consists of valance band and conduction bond. The energy

band occupied by the valance electrons is called valance band. It is the

highest occupied band. If valance electrons acquire sufficient energy to

overcome their binding energy, they will leave the valance bond. Such

electrons are called free electron. The energy band occupied by these

free electrons are called conduction band. Depending upon electrical

properties material can be classified as insulators, conductors and

semiconductors.


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Semiconductors are materials whose electrical conductivity

lies between those of insulators and conductors. The conductivity of

semi conductor increases with temperature. At room temperature they

have partially filled valance and conduction bands with a narrow

energy gap between them. Eg. Germanium and silicon.

Semiconductors are two types:

1.2

INTRINSIC SEMICONDUCTORS

A semiconductor in an extremely pure form is known as

intrinsic semiconductor. Germanium (32G73) and Silicon (14Si28) are

commonly used intrinsic semiconductors. At sufficiently low

temperature there is no free electron in a semiconductor are not free to

wonder about as they are in metals, but rather are trapped in a bond

between two adjacent ions.

Electron hole pairs

As the atmospheric temperature, the ambient temperature

increases some of the electrons acquire enough heat energy to break

away from the valance bond and move to the conduction band. When

an electron breaks away from the covalent bond and becomes free, a

vacancy is left behind in the valance bond. This vacancy is termed as

hole. Thus in a semiconductor electrons and holes are produced in pairs

by thermal agitation.


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1.3EXTRINSIC SEMICONDUCTOR

The conductivity of an intrinsic semiconductor can beincreased by adding a little amount of suitable impurities. The process of

adding impurity to a semiconductor is known as doping. The amount of

impurity added is extremely small; say 1 to 2 atoms of impurity for 106

atoms of semi conductor. Depending upon the type of impurity added

extrinsic semiconductors are of two types.

1.3 (a) N-type Semiconductor

When pentavalent impurity atoms like arsenic, antimony,

phosphorous, or bismuth having five valance electrons are added to

Germanium or Silicon, covalent bonds are established between the

atoms of germanium or Silicon and the impurity. Four of the five

electrons of each impurity atom from covalent bonds with four of the

Germanium or Silicon atoms. The 5thelectron is free to wander within

the crystal. The presence of these five electrons increases the

conductivity of the crystal. Such crystals with excess free electrons is

called N-type semiconductors.


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1.3(b) P-type semiconductors

In P-type semiconductor, if trivalent impurity atoms like

Boron, Indium, aluminum, or Gallium have three valance electrons are

introduced into Germanium or Silicon; covalent bonds are established

between the atoms of Germanium or Silicon and the impurity. The three

valance electrons of the impurity atoms form covalent bonds with three

of neighboring Germanium or Silicon atoms. This deficiency is known as

hole. Due to thermal agitations the covalent bonds of an adjacent atom

may break and an electron thus released may fill the hole. Thus a new

hole is formed. So the hole moves from one place to another.

1.4 MAJORITY CARRIERS AND MINORITY CARRIERS

In an N-type semiconductor since the free electrons outnumber the holes, electrons are called majority carriers and the holes

minority carriers. But in the case of P-type semiconductor, holes

outnumber free electrons, Hence here holes are called the majority

carriers and free electrons the minority carriers.


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1.5 P-N JUNCTION DIODES

Junction Diode Symbol

If one side of a semiconductor crystal is doped with acceptor

impurity atoms and the other side of the same crystal is doped with

donor impurity atoms a P-N junction is formed. A P-N junction is

known as a semiconductor diode or crystal diodes. When a P type

semiconductor is properly joined to an N type semiconductor. We get P-

N junction diode. The contact surface of two types of extrinsic

semiconductors is called PN junction. When P-N junction is formed.

Majority carrier from the P region and that of N region will immediately

cross each other across the junction by diffusion. At the junction each

electron will recombine with a hole releasing a certain amount of

energy. This process is called electron hole pair recombination. The

recombination process will now create a thin layer of immobile ions of

the junction. This layer is called depletion layer.

The width of the depletion layer depends on the doping level,the heavier the doping the thinner is depletion layer.

1.6 FORWARD BIASING

When a P-N junction is forward based, the positive

potential drives, the holes from the P-region and negative potential

P N


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drives the electrons from the N-region towards the junction. Thus they

recombine and stimulate current flow from P to N side across the

junction. This process reduces the width of the depletion layer and

causes a reduction in electrical resistance of the crystal. The current

continues as long as the forward potential is maintained.

1.7 REVERSE BIASING

When the PN junction is reversed biased the holes in the P-

region migrates towards the negative terminal of the external power

source and the electrons in the N-region migrate to the positive

terminal. This creates a widening of the depletion layer and an increase

of resistance prevents the electron hole recombination at the junction.

Hence no current will flow across the junction as long as it reverse

biased.


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1.8 Types of Diodes

There are several types of P_N-junction diodes which

emphasize, either a different physical aspect of diode often bygeometric scaling, doping level, choosing the right electrodes.

1. Silicon Diodes

A Silicon diode is a semiconductor that has positive and

negative polarity, and can allow electrical current to flow in one

direction while restricting it in another. The element Silicon, in its

pure form acts as an electrical insulator. To enable it to conduct

electricity, minute amount of other elements in a process known as

doping-are added to it. When a Silicon diode is made it has both

positive and negative side and a connection between the two known

as the P-N junction. The two differently charged sides are a result of

different elements being added to the Silicon.

2. Germanium Diodes

Germanium diodes are part of an electrical circuit and

conduct electrical signal through the diode travelling in one

direction only. Diodes such as the germanium diode are constructed

out of a semiconductor material and impurities are added to the

Germanium so it will allow the right amount of current to passthrough, Though not as popular as the Silicon diode, a germanium

diode does have certain advantages over Silicon, less energy lost in a

Germanium diode as the current passes through as compared to the

loss in a Silicon diode. This makes it an ideal choice for dealing with

signals caused by small currents where a large loss of energy could

disrupt the signal.


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1.9 DIODE EQUATION

As the mechanisms of diffusion and mobility of the carriers are affected

by the lattice vibrations in crystals, they are related to the temperature

in the form of Einsteinsrelation,

---(1)

Where, is the diffusion constant for holesDe is the diffusion constant for electrons

Is the charge mobilityK is the Boltzman constant

T is the temperature in Kelvin

VT=

The net hole current density across the junction

Jn = nEeDh (2)

Where P hole density

- Conc. of hole w.r to distance x

- Conc. of electrons w.r to distance x

Similarly the net electron current density

Je = neE + eDe

Where n = electron density


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For an unbiased PN junction the net hole current across the junction is

zero.

Now Jn = 0, and Je = 0

When Jn = 0, equ (2) becomes

PeE - e = 0 =

=

(-Edx)

Integrating,

= -

But =

= VB, the potential barrier

=

VB

log=

VB

= exp [

VB]

= exp [

VB]

From eq (1)

= VT

Pn = Ppexp[-

] --(3)

Similarly by putting Je = 0 we get


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= exp[-]

By Law of mass action

= 2 where is the electron density in intrinsicsemiconductor

=

much below room temperature conc. percentage ionization of the

donor level can be expected and for the n material

= ND, the donor atom density and

Pn =

Similarly for P- material

Pp = NA, the acceptor atom density

=

Now eqn. (3) becomes

= NAexp [-

]

= 2.303 x log

Let the PN junction be subjected to forward voltage +ve. The barrier

potential becomes (Vb-V). The reduced barrier potential allows

increased rate of diffusion of holes from P-region to N-region. Those


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current across the forward biased junction is due to the minority

carriers.

Hence an expression for the Diode current is obtainable bycalculating the increase in the minority carrier density on the

application of a forward bias. The hole density increase to

+ = Pp exp [ ]

= Pp exp [

] exp [

] -- (4)

Subtracting eq 3 from 4

= Pp exp [ ] exp (

) 1]

Similarly

= exp [ ] exp [ ]

The hole current in flowing from p to n region,

Where, A is the cross sectional area of junction

is the drift velocity of holes

Or, )-1]

Where )

Similarly electron current, )-1]

Where,


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is the drift velocity of electrons

The total current I=

( ) --(5)

For forward bias,

( ) --(6)

For reverse bias,

If V>>,the exponential term becomes negligible small compared to unity. Now

equation (5) becomes,

)-1] --(7)

Where , T is expressed in Kelvin.

Equation (7) gives the relation between current I and voltage V across thepn junction.

)-1]

Where the factor =1 for Ge, and =2 for Si.


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CHAPTER 2

DIODE AS A TEMPERATURE SENSOR


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2.1 Introduction

Temperature sensor is devices used to measure the

temperature of a medium. There are mainly two kinds of temperature

sensors)

1. Contact Sensors

2. Non-contact Sensors

A temperature sensor is a device typically a thermocouple or

RTD that provides for temperature measurement through an electrical

signal.

A thermocouple (T/C) is made from two dissimilar metals

that generate electrical voltage in direct proportion into charges in

temperature. An RTD (Resistance temperature defector) is a variable

resistor that will change its electrical resistance in direct proportion to

charges in temperature in a precise, repeatable and nearly linear

manner.

+ -

+

-

The charge of biasing voltage (forward or reverse)

changes the width of the depletion region and the current through the

diode. As the rise is temperature in capable of bringing more and more

electron from the valance bond to the conduction based of the


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semiconductor material of the diode, the current through th PN junction

charges. All these information are contained in the diode equation.

)-1]

Neglecting one we get,

)]

Where I is the diode current through the forward biased PN

junction at temperature T (in Kelvin scale) and biasing voltage V. , e,k and n denote the reverse saturation current, electronic charge,

Boltzmans constant and ideality factor respectively.

The ordinary semiconductor diode may be used as a

temperature sensor. The diode is the lowest cost temperature sensor

and can produce more than satisfactory results if you are prepared to

undertake a two point calibration and provide a stable excitation

current. Almost any Silicon diode is ok. The forward biased voltage

across a diode has a temperature efficient of about 2.3mv and in

reasonably linear.

One advantage of the diode as a temperature sensor is that it

can be electrically robust to voltage spikes induced by lightning strike.

This is particularly true if power diodes (eg. IN4001) and is used tolimit power dissipation during high peak currents.

To improve the performance of the diode as a temperature

sensor, two diode voltages (v1 and v2) can be measured at different

currents (I1 and I2), typically selected to be about 1:10 ratio. The

absolute temperature can be calculated from the equation:


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T =

The result is in Kelvins (k). This is the method employed by most

integrated circuit temperatures sensors and explain why some output asignal proportional to absolute temperature.

2.2 Theory

P-N junction diode have negative co-efficient of resistance ie,

as temperature increases resistance of diode decreases, when a diode is

forward biased the width of depletion region decreases and currentbegan to flow through it. The diode current include contribution from

recombination current and diffusion current. As rise in temperature is

capable of bringing more and more electrons from valance bond to

conduction band of semiconductor material of diode current through

P-N junction changes. The relation between current I passing through

the diode at temperature, T and biasing voltage is given by diode

equation.

)-1] --(1)

Where the reverse saturation current, e is is the electroniccharge, k- Boltzmann constant and s is the ideality factor.

Taking logarithm,

--(2)

--(3)

(since, e=)


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As T varies the variation of lnI is very small. Hence ln I is

taken as constant for variation of T from room temperature to C or

303K to 403K. Therefore if T is kept constant,

Multiplying by T we get,

V= --(4)

i.e., if v is plotted against T we get a straight line. Thus curve is called

calibration graph as they can be used for measurement of unknown

temperature.

2.3 Constant current source

Constant current source can source a current that is fixed by

circuit elements.

In constant source circuit in fig(1) the base emitter junction of the

transistor is stiffly biased by a sensor diode. Since base voltage is a

constant.


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CHAPTER 3

DIFFUSION CAPACITANCE


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3.1 Junction Capacitance

We know that the depletion layer is between p and n

region. When it is reverse biased, the diode act as a capacitor. The p

and n region are then like the plates of a capacitor and depletion layer

like the dielectric. The external circuit can change this capacitor by

removing the valence electrons from the p side and adding free

electrons to n side. The diode capacitance is called the junction

capacitance which refers to the transition from p type to n type

material.

3.2 Theory

As a p-n diode is forward biased, the minority carrier

distribution in the quasi neutral region increases dramatically. In

addition, to preserve quasi- neutrality, the majority carrier density

increases by the same amount. This effect leads to an additional

capacitance called the diffusion capacitance.

The diffusion capacitance is calculated from the change with voltage,

where the charge, DQ is due to the excess carriers. Unlike a parallel

plate capacitor, the positive and negative charge is not partially

separated. Instead, the electrons and holes are separated by the energy

band gap. Nevertheless, these voltage dependent charges yield a

capacitance just as the one associated with a parallel plate capacitor.

The total capacitance of the junction equals the sum of

the junction capacitance and the diffusion capacitance. For reverse

biased voltages and small forward bias voltage, one finds that the


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junction capacitance is dominant. As the forward bias voltage is further

increased the diffusion capacitance increases exponentially and

eventually becomes larger than the junction capacitance.

When p-n junction is forward biased, the depletion

capacitance of the junction increases due to the decrease on the width

of depletion layer. In addition, due to the motion and storage of

minority carriers on either side of the junction, a capacitance known as

the storage or diffusion, capacitance is introduced. The storage or

diffusion capacitance takes into account of the capacitive effects of the

carrier injected into each side of the junction when it is forward biased.

The introduction of the diffusion capacitance leads to the restriction of

the diode at high frequency operations. The diffusion capacitance is

important at low frequencies and forward biased conditions.

In a forward biased p-n junction diode electron density and hole

density co exist in the neutral n region. When the forward bias changes

due to the ac component voltage the minority carrier concentration

also changes. This charge dQ is alternately being charged and

discharged through the junction as the voltage across the junction

changes. This charge is stored minority carriers as a function of the

change in voltage is the diffusion capacitance.

If the change in the number of holes stored per unit area of the n

layer is dQp. When the applied forward bias changes by dV, the

diffusion capacitance due to the scored holes on the n side is

-- (1)


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The diffusion (storage) capacitance per unit area due to electrons on

the p side is,

-- (2)

In a forward biased diode the two capacitors Cp and C.

behave as if they are connected in parallel to each other. Hence the

total diffusion capacitance per unit area in the junction is

-- (3)

3.3 Diffusion Capacitance

For the diode IN4007 try with C = 0.01, 0.02, 0.03 etc.

Capacitance Width of the split trace

Cd

Cd + C

Cd + C + C

Cd + C + C + C

1) Cd =

2) Cd =

3) Cd =


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PROJECT METHOD

The change of biasing voltage changes the width of the

depletion region and the current through the diode. We know that thediode current include contributions from generation recombination

current and diffusion current. As the rise in temperature is capable of

bringing more and more electrons from the valance band to the

conduction bond of the semiconductor material of the diode, the

current through the P-N junction changes.

From the diode equation we can see that diode current

depends on absolute temperature. As a project we undertake the work

of studying how the diode voltage varies with temperature for a given

forward biased voltage due to represent it. Graphically for doing this

project we need a DC o/p regulator, diodes (Silicon, Germanium),

resistance of 3.3ohm and a digital multimeter.

Made the forward current at a constant value. Voltage across

the diode at room temperature is found by using multimeter. Then the

temperature of the diode is varied by heating it by beeping it in an oil

bath. The voltage across the diode is noted at each temperature

beeping the forward current constant. Then plot the graph with

temperature T along x-axis and voltage across the diode along y-axis.

We got a straight line. For each value of the applied voltage v, we get a

straight line. These graphs are called calibration graphs as they can be

used for the measurement of unknown temperature say, the melting

point of wax.


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The simplest for the determination of diffusion

capacitance is to use the principle of formation of lissajous figure o a

CRO. By adjusting the rheostat, a fixed voltage is applied to the diode

and resistor. The horizontal and vertical gain controls of the CRO are

adjusted to get the diode pattern on the screen. Here the splitting of

horizontal area is due to diffusion capacitance and vertical area isdue to transition capacitance. When the capacitance value of thecapacitor connected becomes equal to the diffusion capacitance and

the width get doubled. The experiment is repeated with the capacitors

of diffusion capacitance.


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OBSERVATIONS


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V-I Characteristics

Silicon Germanium

Voltage

(Volts)

Current

(mA)

0 0

.1 .2

.2 .5

.3 1.2

.4 2

.5 2.9

.6 3.8

.8 6

Voltage

(Volts)

Current

(mA)

0 0

.1 .1

.2 .2

.3 .3

.4 .5

.5 2

.6 10.7

.8 20.7


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V-I Characteristics (Constant Voltage)

Silicon

V = 6 V= 6.4 V = 7 V = 7.4

Temp

(K)

Current

(mA)

Temp

(K)

Current

(mA)

Temp

(K)

Current

(mA)

Temp

(K)

Curr:

(mA)

30 10.5 30 11.3 30 12 30 12.4

35 10.6 35 11.4 35 12.1 35 12.4

40 10.9 40 11.5 40 12.2 40 12.5

45 11.1 45 11.6 45 12.3 45 12.6

Germanium

V = 6 V = 6.4 V = 7 V = 7.4

Temp

(K)

Current

(mA)

Temp

(K)

Current

(mA)

Temp

(K)

Current

(mA)

Temp

(K)

Curr:

(mA)

30 10.5 30 11.3 30 12 30 12.4

35 10.6 35 11.4 35 12.1 35 12.4

40 10.9 40 11.5 40 12.2 40 12.5

45 11.1 45 11.6 45 12.3 45 12.6


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Temperature Sensor of Diode

Diode-Silicon

Const. Current=6.8mA

Temperature

(Rising Falling

Mean

(mV)

30 678 668 680

35 664 670 666

40 642 646 644

45 628 632 630

50 601 610 608

55 598 602 600

60 580 584 582

65 562 566 564

70 546 550 548

75 536 540 538

80 518 522 520


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Temperature Sensor of Diode

Diode-Germanium

Const. Current=12.1mA

Temperature Rising Falling Mean

30 1296 1300 1298

35 1278 1282 1280

40 1262 1264 1264

45 1240 1244 1242

50 1220 1224 1222

55 1206 1210 1208

60 1188 1192 1190

65 1172 1176 1174

70 1156 1160 1158

75 1136 1140 1138

80 1116 1120 1118


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Conclusion

Diode is used as a temperature sensor to measure

the unknown temperature by keeping voltage constant (or

current) for calibration and temperature measures.


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Bibliography

1.

S. Sankararaman Enjoy Physics through ProjectsA Play withDiodes. Sastra vedi publications 2010.

2. Ittiavirah Kurian Electronis & Practicals

3. V. K Mehta Principles of Electronics S chand & company ltd. 1999

4. www.encyclopedia.com

5. www.wikipedia.com
http://www.encyclopedia.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.encyclopedia.com/


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diode as a temperature sensor

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