Rec101 unit 1 (part ii) pn junction diode

PN junction diode

REC 101: Basic Electronics Unit 1

PN junction diode: Introduction of Semiconductor Materials Semiconductor Diode: Depletionlayer, V-I characteristics, ideal and practical, diode resistance, capacitance, Diode EquivalentCircuits, Transition and Diffusion Capacitance, Zener Diodes breakdown mechanism (Zenerand avalanche) Diode Application: Series , Parallel and Series, Parallel Diode Configuration,Half and Full Wave rectification, Clippers, Clampers, Zener diode as shunt regulator, Voltage-Multiplier Circuits Special Purpose two terminal Devices :Light-Emitting Diodes, Varactor(Varicap) Diodes, Tunnel Diodes, Liquid-Crystal Displays.

9/5/2017 1REC 101 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

PN Junction diode: Working


p-type n-type• A p-type material is represented by acceptor ions, holes as majority

and electrons as minority carriers

• A n-type material is represented by donor ions, electrons as

majority and holes as minority carriers

• As the name depicts, pn junction diode working is due to the

junction effect between p-type and n-type

PN Junction diode: No bias


VD = 0 V

Depletion region

E

IphInemajority

minorityInhIpe

n-typep-type

PN Junction diode: No bias


• As the p-type and n-type are “joined”, the electrons & holes diffuses

across the junction and combine, resulting in a region with +ve charge

in n-region and –ve charge in p-region thus forming a barrier potential.

• This action continues until the barrier potential repels further diffusion

• In the region near junction only donor and acceptor atoms remain

• The region of positive and negative ions is called the depletion region

due to the “depletion” of free carriers in the region.

• In depletion region, Electric field (from +ve to –ve ions) oppose

majority carrier crossover and facilitate minority carrier crossover.

• At equilibrium number of majority carrier crossing the junction is

equal to number of minority carriers, thus no current flows (Direction

of minority current component is opposite to majority carrier current

component)

PN Junction diode: forward bias


Depletion region

E

majority

minorityInhIpe

IphIne

n-typep-type

V

PN Junction diode: forward bias


• A forward-bias or “on” condition is made by applying theforward bias potential (+ve potential to the p -type and the -vepotential to the n -type material)

• Forward-bias potential will “pressure” electrons in the n -typeand holes in the p-type material to recombine with the ions nearthe boundary and reduce the width of the depletion region.

• The minority-carrier flow does change in magnitude, but thereduction in the width of the depletion region results in a heavymajority flow across the junction.

• As the forward bias increases in magnitude, the depletion regionwill continue to decrease in width until a flood of electrons canpass through the junction, resulting in an exponential rise incurrent as shown in forward-bias region of the characteristics

PN Junction diode: Reverse bias


Depletion region

E

majority

minorityInhIpe

IphIne

n-typep-type

V

PN Junction diode: Reverse bias


• A reverse-bias or “off” condition is made by applying the reversebias potential (-ve potential to p -type and +ve potential to the n -type material)

• Reverse-bias potential will result in widening of width of thedepletion region.

• The minority-carrier flow does change in magnitude, but thewidening of the depletion region results in great increase ofpotential barrier to majority carrier flow across the junction to thepoint that majority carrier flow across the junction stops.

• The current that exists under reverse-bias is called the reversesaturation current (only due to minority carriers)

• As reverse bias increases, the depletion region will continue towiden until a breakdown occurs.

PN Junction diode: V-I Characteristics


Forward bias

reverse bias VK: Cut-in or knee voltage

VD

ID (mA)

VBV: Breakdown voltage

Practical

+ VD -

ID

1T

D

V

V

SD eII

mV 26 C), (27 re temperaturoomAt

C 10 x 1.6electron of Charge

Kin eTemperatur

J/K 10 x 1.38constant Boltzman

, voltage;Thermal

factor on various depending 2or 1 factor;Ideality

diode across volagebias Forward

current saturation Reverese

current, Diode

0

19-

0

23-

T

k

kTT

D

S

D

V

q

T

K

q

KTVV

V

I

I

Shockley’s equation of diode

V-I Characteristics of commercial DiodesGe, Si, GaAs


Forward bias

reverse biasVK(Ge) 0.3 V

VD

ID (mA)

VK(GaAs) 1.2 V

VK(Si) 0.7 V

GaAsSiGe

IS(Ge) 1 A

IS(GaAs): 1 pA

IS(Si) 100 pA

VBV(GaAs)

VBV(Si)

VBV(Ge)

50 V100 V

Knee VoltageGe 0.3 VSi 0.7 VGaAS 1.2 V

Relative Breakdown voltageGe Low (less than 100 V)Si High (50V-1 KV)GaAS Highest (50V-1 KV)

Electron Mobility n(cm2/V.s)

Ge 3900Si 1500GaAS 8500

Ideal and Practical Diode


Practicalideal

VK

VK

ideal

circuit)(open Diode ,0

circuit)(short Diode ,0

Diode Ideal

OFFV

ONV

D

D

Voltage Knee

circuit)(open Diode ,

circuit)(short Diode ,

Diode Practical

K

KD

KD

V

OFFVV

ONVV

Diode ResistanceDC or Static Resistance: Resistance at Q point (quiescent point)

RD = VD / ID

• in forward bias, RD range is 10-80

AC or Dynamic Resistance: ratio of voltage change to current change (co-tangent at Q point)

rd = Vd / Id

By differentiating Schottky equation we get

9/5/2017REC 101 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

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V

I

V

I Q point

V

I

ID Q point

VD

,mV 26

re temperaturoomat mV 26V 1, assuming T

D

d

D

T

SD

T

d

dd

Ir

I

V

II

V

dI

dVr

Transition and Diffusion Capacitance

Diffusion capacitance occurs due to stored carrier charges near thedepletion region. It is proportional to applied voltage. It ispredominant in forward bias.


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Transition capacitance or junction capacitance or depletioncapacitance (due to charges in depletion region). It is predominantin reverse-bias.

,

12

1

0

K

R

T

V

V

CC

voltageKnee V

voltagebias Reverse V

bias nounder eCapacitanc

K

R

0

C

,D

K

TD I

VC

voltageknee

lifetimecarrier Minority

current Diode

K

T

D

V

I

Diode capacitance are put in parallel to the diode

Effect of Temperature on pn junction diode

Increase of temperature results in increase in carrier concentration.

As a result , Knee voltage and reverse breakdown voltage decreases while Reverse saturation current increases

• In the forward-bias region the characteristics of a silicon diodeshift to the left at a rate of 2.5 mV per centigrade degree increasein temperature

• In the reverse-bias region the reverse current of a silicon diodedoubles for every 10°C rise in temperature


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Reverse Recovery Time

• In forward-bias, a large number of majority carriers cross over andestablish as minority carrier on other side. As applied voltage isreversed (reverse-bias), ideally diode should instantly be OFF.

• However, due to large number of minority carriers, diode currentwill reverse and stay at the level for time ts (storage time); timerequired for minority carriers to return to majority-carrier state.


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• After storage phase, the currentwill start reducing to the level ofIs. This second period of time isdenoted by tt (transition time).

• The reverse recovery time is thesum of these two intervals:

trr = ts + tt . trr

ts tt

Is

Iforward

Ireverse

t1

I

t

Change of state(On to OFF) applied at t = t1

Zener Diodes: breakdown mechanismIn reverse bias, very little current flows. As the reverse voltage is increased, a point is reached where there is a dramatic increase in current. This voltage is called the reverse breakdown voltage.

There are two mechanisms for breakdown: Zener and Avalanche

• Avalanche breakdown: occurs in lightly-doped pn-junctions where the depletion region is comparatively long. In reverse bias, electric field in the depletion region can be very high that provides large acceleration to Electron/holes that enter depletion. Accelerated carriers collide with atoms and can knock electrons from their bonds, creating additional electron/ hole pairs and thus additional current. These secondary carriers are swept into the depletion, accelerated and the process repeats itself. This avalanche makes breakdown in reverse bias


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Zener Diodes breakdown mechanism• Zener breakdown: occurs in heavily doped pn-junctions. Heavy

doping results in very thin depletion, so carriers can’t accelerateto cause impact ionization. Very thin depletion & high acceleratingfield of reverse bias allow carriers to tunnel through from valenceto conduction (bond breaks in depletion) causing large current

• Zener mechanism is a contributor only at lower levels of VBV


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• Breakdown due to any mechanism isreferred as Zener breakdown.

• The maximum reverse-bias potential thatcan be applied before breakdown is calledthe peak inverse voltage (PIV rating).

+VZ

-

IZ

Diode Equivalent Circuits


Vk VK

Practical ideal

VK

idealideal

VKRf

Special Purpose two terminal Devices :Light-Emitting Diodes (LED)

• LED is that gives off visible / infrared light when energized.

• In a forward-biased pn junction, holes and electrons close tojunction recombine and emit energy in form of heat and light(photons). This effect is called electroluminescence and the color ofthe light is determined by the energy gap of the semiconductor.

• Si / Ge diodes, during recombination at the junction mostly heat isgenerated. So Si /Ge are not used for LED.


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LED

Color Material Forward Voltage (V)

Blue GaN 5.0

Green GaP 2.2

Orange GaAsP 2.0

Red GaAsP 1.8

White GaN 4.1

Yellow AlInGaP 2.1LED

Source: Wikipedia

Special Purpose two terminal Devices :Light-Emitting Diodes (LED)

• GaAs diodes emit light in the infrared zone while GaAsP, GaN diodesemit light in visible region.

• The frequency for infrared light extends from about 100-400 THz, with the visible light spectrum extending from about 400-750 THz.

• LED efficacy is, given as ratio of lumens generated per appliedelectrical power (watt)

• A major concern of LED is Low breakdown voltage (typically 3-5 V)

• Earlier only green/yellow/orange/red LED were available (VF=2V)

• In 1990’s blue LED (VF=5 V ) and white LED (VF=4.1 V ) introduced

• Now LED efficacy is 100-135 lumens /watt

• LED find applications in display devices and home lighting


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Special Purpose two terminal Devices : Varactor (Varicap) Diodes,

• A varactor (varicap) diode/ tuning diode/ variable capacitancediode/ variable reactance diode, has a variable capacitance whichis a function of the voltage that is impressed on its terminals.

• Varactor diodes are operated in reverse-bias, so no current flows.However, as thickness of the depletion varies with the appliedbias voltage, the capacitance of the diode varies.

• Capacitance is inversely proportional to the depletion thicknesswhich in turn is proportional to the square root of the voltage.


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Varicap

,

12

1

0

K

R

T

V

V

CC

Special Purpose two terminal Devices : Tunnel Diodes

• Tunnel diode (Esaki diode named after discoverer) is a highlydoped semiconductor device and is used mainly for low voltagehigh frequency switching applications. It works on the principle ofTunnelling effect.

• Tunnel diode is about 1000 times more heavily doped thannormal diode

• Its depletion region is very thin (0.0001 mm)

• Tunnel diode has special characteristics of negative resistance

• Tunnel diodes are used in high speed applications and oscillators.

• Heavy doping reduce Zener breakdown voltage to very low value.


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Special Purpose two terminal Devices : Tunnel Diodes


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VD

ID

Ip

Iv

Vp Vv

-ve resistance zone

Peak

Valley

• As per quantum mechanicsthere exists non zeroprobability that the particlewith energy less than theenergy barrier will cross thebarrier as if it tunnels acrossthe barrier. This is calledas Tunnelling effect. Theprobability increases with thedecreasing barrier energy.

Tunnel

Special Purpose two terminal Devices : Liquid-Crystal Displays.

• LCD has the advantage of having a lower power requirement thanthe LED, typically in W , compared to the mW for LEDs.

• It does, require an external or internal light source, and is limitedto a temperature range of about 0°C to 60°C.

• Lifetime is an area of concern as LCDs can chemically degrade.

• Types of LCD unit are field-effect and dynamic-scattering units.

• A liquid crystal is a material that flows like a liquid but whosemolecular structure has some properties normally associated withsolids. For light scattering units, the greatest interest is in nematicliquid crystal, which has the rodlike crystal structure

• It passes the light, but when voltage is applied, molecularstructure is disturbed, resulting in regions with differentreflections


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25

Glass

Glass

Seal

er

V=0

Incident light

Glass

Glass

Seal

er

+V-

Incident light

Clear region Frosted region

Iridum oxide surface withConductive clear coating

Nematic Liquid crystal with no bias Nematic Liquid crystal with applied bias



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• The field-effect or twisted nematic LCD has the same segmented appearance and thin layer of encapsulated liquid crystal, but its mode of operation is very different.

• Similar to dynamic-scattering LCD, the field-effect LCD can be operated in reflective or transmissive mode with an internal source.

• The internal light source is on the right, and the viewer is on the left. It has an an addition of a light polarizer .

• Only the vertical component of the entering light on the right can pass through the vertical-light polarizer on the right.

• In field-effect LCD, either the clear conducting surface to the right is chemically etched or organic film is applied to orient the molecules in the liquid crystal in the vertical plane, parallel to cell wall.



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Glass

Glass

Seal

er+V-

Incident light

clear Conductive surface

Vertical light polarizers

Rec101 unit 1 (part ii) pn junction diode

Engineering

Transcript of Rec101 unit 1 (part ii) pn junction diode