radiation detectors and instrumentation - electronics

8/12/2019 Radiation Detectors and Instrumentation - Electronics

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Dr. ISMAYIL

Assistant Professor

Department of PhysicsManipal Institute of Technology

RADIATION PHYSICS(Open Elective for B.E - VI Semester)

(PHY 322)

1


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Dr. ISMAYIL

Assistant ProfessorDepartment of Physics

Manipal Institute of Technology

Manipal University

Academic Block-2 Basement,

Near AC Seminar Hall

E-mail:[email protected]

Mobile Number: 98454 975462


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yllabus


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Radiation Detectors and

Instrumentation [16 hours]

Semiconductors diodes JFET

MOSFET

Integrated Circuits

OPAMP and their characteristics

Differential Amplifier

Operational amplifier systems

Pulse Amplifiers.

Ref:

Robert L Boylestad, Electronic Devices and Circuit theory

Radiation Detection and Measurement Glenn F Knoll

Measurement and Detection of radiation - NicholasTsoulfanidis

Proportional counters

GM counters Scintillation detectors

Semiconductor detectors

Thermo luminescent

Dosimeters

Radiation spectroscopy with

scintillators

Gamma spectroscopy

Multichannel pulse analyzer

Slow neutron detectionmethods

Reactor instrumentation

Principles of radiation detection

and measurements

Gas filled detectors

Ionization chambers

Theory and design

Gas multiplication


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Radiation Detectors and Instrumentation

Semiconductors diodes :

pn-junction is formed when a

p-type semiconductor is joinedto an n-type semiconductor.

A pn-junction has three distinct

regions: a p-region, an n-region,

and a depletion region at the

junction [Figure (a)].

The depletion region has no

movable charges (conduction

electrons and holes) because the

conduction electrons on the n-side

have crossed the junction due to

diffusion and neutralized the

holes on the p-side.


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Because of the fixed ion cores in the depletion region, an

electric field exists [Figure (b)], due to which there is a

potential difference Vo across the

junction [Figure (c)].

In the forward bias, the

p-side of the junction is made

positive with respect to the n-side, by application of an

external voltage V. Then the

internal potential difference Vo

across the junction decreases.

This gives rise to a current I

which increases exponentially

with the increase in forward

bias V.

CHARACTERISTIC

CURVE OF A pn-JUNCTION

REVERSE

BIAS

FORWARD

BIAS


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FORWARD BIAS :

REVERSE BIAS :


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Problem 3.1: The current in a diode under forward bias of

100 mV is 200 mA at a temperature of 300 K. What is the

current in the diode if it is under reverse bias of 100 mV ?

Solution:


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Bipolar Junction Transistors (BJT) :

The transistor is a three-layer semiconductor device

consisting of either two n-type and one p-type layers ofmaterial (npn transistor) OR two p-type and one n-type

layers of material (pnp transistor).

There are three terminal namely Emitter, Base and

Collector.

The emitter layer is heavily doped, the base lightly doped,

and the collector also lightly doped. The outer layers have

widths much greater than the sandwichedp-type or n-type

material.

Bipolar junction transistors are so named because their

operation involves both electrons and holes.


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In 1947 three American scientists named William

Shockley, John Bardeen and Walter Brattain at Bell Labs,

announced the creation of the first transistor.

The name transistor is a combination of the words

transfer and resistor - a transfer resistor - a transistor.

When it was announced the name was explained;

"because it is a resistor or semiconductor device whichcan amplify electrical signals as they are transferred

through it from input to output terminals."

This, the very first transistor was called a point-contacttransistor.

Shockley, Bardeen and Brattain received the Nobel Prize

in Physics 1956 "for their researches on semiconductors

and their discovery of the transistor effect."


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Input Characteristics Output Characteristics


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16BJTis a current-controlled device


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Field-Effect Transistors (FET) :

The field-effect transistor (FET) is a three-terminal device

used for a variety of applications that match, to a largeextent, those of the Bipolar Junction Transistors (BJT).

The primary difference between the two types of

transistors is the fact that the BJT is a current-controlled

device, while the FETis a voltage-controlled device.

Just as there are npn andpnp bipolar transistors, there are

n-channel andp-channelfield-effect transistors.

The BJT is a bipolar device i.e., the conduction level is afunction of two charge carriers, electrons and holes.

The FET is a unipolar device depending solely on either

electron (n-channel) or hole (p-channel) conduction.


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Current Controlled vsVoltage Controlled Devices


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Two types of FETs are:

Junction Field-Effect Transistor (JFET)

Metal-Oxide-Semiconductor Field-Effect Transistor

(MOSFET)

The MOSFET transistor has become one of the most

important devices used in the design and construction of

integrated circuits for digital computers.

Its thermal stability and other general characteristics

make it extremely popular in computer circuit design.


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Junction Field-Effect Transistor (JFET)

JFET is a three-terminal device

with one terminal capable of

controlling the current between

the other two.

Note that the major part of the

structure is the n-type material

that forms the channel between

the embedded layers of p-type

material.BJT FET

Emitter SourceBase GateCollector Drain


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The top of the n-type channel is

connected through an ohmic contact

to a terminal referred to as the drain

(D), while the lower end of the samematerial is connected through an

ohmic contact to a terminal referred

to as the source (S).

The two p-type materials are

connected together and to the gate

(G)terminal.

In principle the drain and source areconnected to the ends of the n-type

channel and the gate to the two

layers ofp-type material.


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In the absence of any applied

potentials the JFET has two p-n

junctions under no-bias

conditions.

The result is a depletion region

at each junction as shown in Fig.

that resembles the same regionof a diode under no-bias

conditions.

A depletion region is that region

void of free carriers and

therefore unable to support

conduction through the region.


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A positive voltage VDS has been

applied across the channel.

Gate has been connected directlyto the source to establish the

condition VGS=0 volt.

The instant voltage VDS is

increased, the electrons will be

drawn to the drain terminal,

establishing the conventional

current ID with the defined

direction.

VGS= 0, VDS= + ve

BJT FET

Emitter SourceBase GateCollector Drain


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Water analogy for the JFET control

mechanism:

The source of water pressure can be

likened to the applied voltage from

drain to source that will establish a

flow of water (electrons) from the

tap (source).The gate, through an applied

signal (potential), controls the flow

of water (charge) to the drain.

The drain and source terminals areat opposite ends of the n-channel as

introduced in Fig. because the

terminology is defined for electron

flow.


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The path of charge flow clearly

reveals that the drain and

source currents are equivalent

(ID= IS).

The flow of charge is limited by

the resistance of the n-channel

between drain and source.


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The nonconductive depletion region becomes thicker with increased reverse bias.

(Note:The two gate regions of each FET are connected to each other.)

JFET is a voltage controlled device.

N-Channel JFET Operation


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JFET Symbol :

JFET Ch t i ti


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JFET Characteristics

As the voltage VDSis increased from 0 to a few volts, the current will

increase as determined by Ohms law and the plot of IDversus VDS

will appear as shown below.

IDSSis the maximum drain currentfor a JFET


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The relative straightness of the plot reveals that for the

region of low values of VDS, the resistance is essentially

constant.

As VDSincreases the depletion region will widen, causing a

noticeable reduction in the channel width.

The reduced path of conduction causes the resistanceto

increase and the curve in the graph to occur.

The more horizontal the curve, the higher the resistance,

suggesting that the resistance is approaching infinite

ohms in the horizontal region.


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If VGS is increased to a level

where it appears that the

two depletion regions would

touch,a condition referred

to as pinch-offwill result.

At the pinch-offpoint:

Any further increase in VGSdoes not produce any increase in ID.

VGSat pinch-off is denoted as Vp.


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As VGSbecomes more negative:

the JFET will pinch-off at a lower voltage (Vp).

IDdecreases (ID< IDSS) even though VDSis increased.

Eventually IDwill reach 0A. VGSat this point is called Vpor VGS(off).

Also note that at high levels of VDS the JFET reaches a breakdown

situation. IDwill increase uncontrollably if VDS > VDSmax.


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FET as a Voltage-Controlled Resistor

The region to the left of the pinch-off point is called the ohmic region.

The JFET can be used as a variable resistor, where VGScontrols the

drain-source resistance (rd).

As VGSbecomes more negative, the resistance (rd) increases.


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Transfer (Transconductance) Curve

From this graph it is easy to determine the value of IDfor a given value of VGS.

It is also possible to determine IDSSand VPby looking at the knee where VGSis 0

Shockleys equation =


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p-Channel JFET:

p-Channel JFET operates in a similar manner as the n-channel JFET

except the voltage polarities and current directions are reversed


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P-Channel JFET Characteristics

As VGS

increases more positively,

the depletion zone increases

IDdecreases (ID< IDSS)

eventually ID= 0A

Also note that at high levels of VDS the JFET reaches a breakdown

situation. IDincreases uncontrollably if VDS> VDSmax.


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Problem 3 2 : Sketch the transfer curve for a n channel JFET


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Problem 3.2 : Sketch the transfer curve for a n-channel JFET

defined by IDSS= 12 mA and VP= -6 V.

Solution : Two plot points are defined by

At VGS =VP/2 = -6/2 =- 3 Vthe drain current will be determined by

ID= IDSS/4 = 12 /4 = 3 mA

At ID=I

DSS/2 = 12/2 = 6 mA the gate-to-

source voltage is determined by

VGS =0.3VP= 0.3 -6 = - 1.8 V

Using these four points the

complete transfer curve can be plotted.

=


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Problem 3.3: Sketch the transfer curve for ap-channel JFET

with IDSS = 4 mA and VP= 3 V.

Solution:

At VGS=VP/2 = 3/2 = 1.5 V,

ID=IDSS/4 = 4/4 = 1 mA.

At ID=IDSS/2 = 4 /2 = 2 mA,

VGS=0.3VP =0.33 = 0.9 V.

Using these points thecomplete transfer curve

can be plotted.

=


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MOSFET:

A MOSFET is Metal - Oxide - Semiconductor Field - Effect

Transistor.

There are two types of MOSFETs based on their basic

mode of operation, namely

a) Depletion type

b) Enhancement type

Depletion-type MOSFET (D-MOSFET) :


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Depletion type MOSFET (D MOSFET) :

There are three metal connections to the MOSFET : source,

drain, and gate. The source and drain are connected to n-

type semiconductor regions. These regions are connected by anarrow channelof n-type material [n-CHANNEL].

The source and drain regions and the n-channel are

embedded in a p-type substrate material.


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There is no direct electrical connection between the gate terminaland the channel of a MOSFET.

It is the insulating layer of SiO2 in the MOSFET construction that

accounts for the very desirable high input impedance of the device.

Hence itsa suitable device for amplification circuits.


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Drain and transfer characteristics for an n-channel

depletion-type MOSFET

If a varying voltage is applied to the gate of the


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If a varying voltage is applied to the gate of the

MOSFET, the source-drain current also varies.

A small variation in gate voltage VSG results in alarge variation in source-drain current, and a

correspondingly large output-voltage across the

resistor. Therefore, the MOSFET acts as a voltage

amplifier.

If a negative potential is applied to the gate, the n-

channel decreases in size when VSG increases. This

reduces the source-drain current and stops the

current when VSG is large. Thus MOSFET can be

used as on-off switch by changing the polarity of

VSG.


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D-MOSFET Symbol :

Problem 3 4 Sketch the transfer characteristics for an


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Problem 3.4: Sketch the transfer characteristics for an

n-channel depletion-type MOSFET with IDSS = 10 mA and

VP= - 4 V.

Solution :

At VGS

= +1 Volt,

=

Enhancement-type MOSFET (E-MOSFET) :


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Enhancement type MOSFET (E MOSFET) :

There is NO Channel !


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Both VDSand VGShave been set at some positive voltage

greater than 0 V, establishing the drain and gate at a

positive potential with respect to the source.

The positive potential at the gate will pressure the holes

(since like charges repel) in the p-substrate along the

edge of the SiO2layer to leave the area and enter deeper

regions of thep-substrate.

The result is a depletion region near the SiO2 insulating

layer void of holes.

As VGS increases in magnitude, the concentration ofelectrons near the SiO2surface increases until eventually

the induced n-type region can support a measurable flow

between drain and source.


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Summary Table


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Summary Table

JFET D-MOSFET E-MOSFET

Integrated Circuits:


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g

An integrated circuit (IC) is a collection of interconnected

transistors, diodes, resistors, and capacitors fabricated on a

single piece of silicon known as a chip.

ICs were invented partly to solve the interconnection

problem spawned by the transistor. In addition to solving

the interconnection problem, ICs possess the advantages ofminiaturization and fast response.


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Circuit layout of op-amp type 741


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Circuit layout of op amp type 741

The op-amp is a chip, a small black box with 8 connectors or pins (only 5 are usually used).


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Pin Configuration Symbol

Op-amp Equivalent


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Op-amp is equivalent to an electronic circuit consist of high

input resistor, a voltage source and a low output resistor.

p p q


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Powering the Op-Amp


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Powering the Op-Amp

Since op-amps are used as amplifiers, they need an

external source of (constant DC) power.Typically, this source will supply +15V at +V and -15V at -V.

The op-amp will give output voltage range of somewhat

less because of internal losses.

Op-Amp Intrinsic Gain


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Op Amp Intrinsic Gain

Amplifiers increase the magnitude of a signal by

multiplier called a gain A. The internal gain of an op-amp is very high. The exact

gain is often unpredictable.

This gain is called open-loop gainor intrinsic gain.

The output of the op-amp is this gain multiplied by the

input.

5 6outopen loop

in

VA 10 10

V

idout VAVVAV 21Gain A 2,00,000 for Op-amp IC 741Vid = V1-V2 is the input difference

Op Amp Saturation


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Op-Amp Saturation

The huge gain causes the output to change

dramatically when (V1-V2) changes sign.

However, the op-amp output is limited by the voltage

that you provide to it.

When the op-amp is at the maximum or minimumextreme, it is said to be saturated.

saturationnegativeVVthenVVif

saturationpositiveVVthenVVif

VVV

out

out

out

21

21


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The Non-Inverting Amplifier


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g

f

in

g

f

out

R

RAV

R

RV

11

The Non Inverting Amplifier

The Inverting Amplifier


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in

f

in

in

f

outR

RAV

R

RV

The Inverting Amplifier


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Subject Code : PHY 322


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