slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
UNIT - II PowerPoint Presentation
Download Presentation
UNIT - II

Loading in 2 Seconds...

play fullscreen
1 / 73

UNIT - II - PowerPoint PPT Presentation


  • 194 Views
  • Uploaded on

UNIT - II. PHYSICS OF SEMICONDUCTOR DEVICES. P- type semiconductors. Electron (minority carriers). Hole (majority carriers). -. -. -. -. -. -. -. -. Hole (mobile charge). Acceptor ions (immobile charge). -. (p ≈ N A ). -. ≈. N- type semiconductors.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'UNIT - II' - bijan


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

UNIT - II

PHYSICS OF

SEMICONDUCTOR

DEVICES

slide2

P- type semiconductors

Electron (minority carriers)

Hole (majority carriers)

slide3

-

-

-

-

-

-

-

-

Hole (mobile charge)

Acceptor ions (immobile charge)

-

(p ≈ NA)

-

slide4

N- type semiconductors

Electron (majority carriers)

Hole (minority carriers)

slide5

+

+

+

+

+

+

+

+

+

Donor ion(immobile charge)

Electron (mobile charge)

(n ≈ ND )

+

slide6

Formation of a PN-junction

Ionized acceptors

Ionized donors

Junction

P

N

+

+

+

+

+

+

+

+

Space charge region

-

-

-

-

-

-

-

-

(OR)

Depletion region

Potential barrier height(V0)

Potential barrier width

(W)

slide7

Depletion region

  • The diffusing majority carriers from the
  • two regions recombine near the junction
  • and disappear.
  • The uncompensated acceptor and donor
  • ions set up an electric field which halts
  • the further recombination.
slide8

Space charge region

  • The two kinds of majority carriers diffusing across
  • the junction meet each other near the junction and
  • undergo recombination, leaving negative ions on
  • the p-side and positive ions on the n-side of the
  • junction.
  • This distribution of charges is called space charge
  • region.
slide9

_

+

Diode symbol

P

N

Cathode

Anode

slide11

From figure the following points to be noted:

  • Consider that a PN- junction has P-type &
  • N- type materials in close physical contact
  • with each other at the junction.
  • From figure, the Fermi level EF is closer to the conduction band edge Ecn in the N-type while it is closer to the valence band
  • edge Evp in the P-type.
slide12

The conduction band edge Ecp in the P-type

  • material is higher than the conduction band
  • edge Ecn in the N-type material.
  • Similarly, the valence band edge Evp in the
  • P-type material is higher than the valence
  • band edge Evn in the N-type material.
  • E1 & E2 indicate the shifts in the Fermi level from the intrinsic conditions in the P & N materials respectively.
slide13

Therefore, the total shift in the energy level

  • E0 is given by

E0 = E1 + E2 = Ecp – Ecn = Evp - Evn

  • The energy E0(in eV) is the potential
  • energy of the electrons at the PN-junction,
  • which is equal to qV0.

Where,

V0= contact potential (OR) barrier potential

( exists across an open circuited PN- junction)

slide15

Forward bias

When positive terminal of the battery is connected

to the P-type & negative terminal is to the N-type

of the PN-junction diode, known the diode is kept

in forward bias.

slide16

Open circuit PN -junction

P

N

Space charge region

P

N

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

Forward bias

VF

slide17

When the diode is in forward bias the following

points are noted.

  • The applied +ve & -ve potential repels the
  • holes & electrons in P-type & N-type materials.
  • Hence, they can move towards the junction.
  • When the applied potential is more than the
  • internal barrier potential the depletion region
  • & internal potential barrier disappear.
  • Hence, high current flows through the junction.
  • In forward bias the current is due to majority
  • charge carriers (mA).
slide18

Reverse bias

When negative terminal of the battery is connected

to the P-type & positive terminal is to the N-type

of the PN-junction diode, known the diode is kept

in reverse bias.

slide19

Open circuit PN -junction

P

N

Space charge region

P

N

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Reverse bias

VR

slide20

When the diode is in reverse bias the following

points are noted.

  • The holes move towards the –ve terminal of the battery & the electrons towards +ve terminal of the battery.
  • Hence, the potential barrier & width is increased
  • which prevents the flow of charges.
  • Therefore, no current flow across the junction.
  • But in practice a very small current flows in order
  • of microamperes, due to minority carriers.
slide21

Volt–Ampere characteristics

(V-I curves)

The graph is plotting in between the voltage is taking on X-axis & current is on Y-axis, is known

as V-I characteristics of a PN- junction.

Note:

These curves are drawn on the basis of diode

is connected in forward & reverse bias.

slide22

Forward bias

I

(i = mA)

Knee voltage

V

0

slide23

Reverse bias

-V

Breakdown

voltage

(i = μA)

-I

slide24

I

Forward bias

i = mA

Knee voltage

V

-V

Breakdown

voltage

i = μA

Reverse bias

-I

slide25

From the graph the following points are noted.

  • The region between knee voltage & breakdown
  • voltage is known as non-ohmic region.
  • Above the knee & breakdown voltage the current
  • increases.
  • Breakdown voltage is due to thermally broken
  • covalent bonds.
  • Diode is conducting in forward bias &
  • non-conducting in reverse bias.
slide26

Applications of a diode

  • It works as an electronic switch.
  • It works as a rectifier.
  • It works as a demodulator(detector).
  • It works as a d.c. restorer in TV receiver
  • and voltage multipliers……
slide27

Diode as a rectifier:

A rectifier is an electronic circuit which converts alternating current to direct current

(OR) unidirectional current.

Rectifiers are mainly three types

1.Half wave rectifiers

2.Full wave rectifiers

3.Bridge rectifiers

slide28

1.Half-wave rectifier:

An electronic circuit which converts

alternating voltage (OR) current for

half the period of input cycle hence

it is named as half-wave rectifier.

slide30

During the +ve half cycle of input,end A

  • becomes +ve w.r.t. B. Hence,the diode is
  • in forward biased.Therefore it offers
  • verysmall resistance conducts the current.
  • During the -ve half cycle of input, end A
  • becomes -ve w.r.t. B. Hence, the diode is
  • in reverse biased. Therefore it offers very
  • high resistance does not conducts the
  • current.
slide31

Rectifier efficiency(η)

The ratio of D.C power output to applied A.C

power input is known as rectifier efficiency.

D.C. power output

η =

A.C. power input

slide32

D.C. power output = ( Im /π ) 2 x RL

&

A.C. power input = (Im /2 ) 2 x (rf + RL )

Therefore,

( Im /π ) 2 x RL

(Im /2 ) 2 x (rf + RL )

η =

slide33

4 /π2 x RL

=

=

Conclusion:

0.406 x 100%

40.6%

rf + RL

since,

rf « RL

η =

The maximum efficiency of a half-wave rectifier

is 40.6% of A.C power is converted into D.C power.

slide34

2.Full-wave rectifier:

An electronic circuit which converts

alternating voltage (OR) current into

pulsating voltage (OR) current during

both half cycle of input is known as

full-wave rectifier.

slide36

During the +ve half cycle of input,end A

  • becomes +ve & B becomes negative. This
  • makes the diode D1 in forward biased &
  • D2 in reverse biased.Therefore, D1
  • conducts while D2 does not conduct.
  • During the -ve half cycle of input,end A
  • becomes -ve & B becomes positive. This
  • makes the diode D2 in forward biased &
  • D1 in reverse biased.Therefore, D2
  • conducts while D1 does not conduct.
slide37

Rectifier efficiency(η)

The ratio of D.C power output to applied A.C

power input is known as rectifier efficiency.

D.C. power output

η =

A.C. power input

slide38

&

D.C. power output = ( 2Im /π ) 2 x RL

A.C. power input = (Im / √2 ) 2 x (rf + RL )

Therefore,

(2 Im /π ) 2 x RL

(Im / √2 ) 2 x (rf + RL )

η =

slide39

0.812 x RL

=

=

Conclusion:

0.812 x 100%

81.2%

rf + RL

But,

rf « RL

The maximum efficiency of a full-wave rectifier

is 81.2% of A.C power is converted into D.C power.

Therefore, the full-wave rectifier efficiency is twice

of a half-wave rectifier.

η =

slide40

Diode equation

3/2

3/2

( )

( )

(EF – EC ) / KBT

(EF – EC ) / KBT

n =

nc = Nc

2πme*KBT

2πme*KBT

e

e

2

2

(Diode current equation)

h2

h2

The concentration of electrons in the conduction band of a semiconductor is given by

(OR)

---- (1)

Where,

= Nc

slide41

3/2

( )

(EV – EF ) / KBT

nv = Nv

2πmh*KBT

e

2

Similarly the concentration of holes in the valence band of a semiconductor is given by

h2

---- (2)

Where,

Eq(1) & (2) are applicable for intrinsic semiconductors .

= Nv

slide42

For N-type semiconductor electron density is given by

(EFn – Ecn ) / KBT

(EFp – Ecp ) / KBT

nn = Nc

np = Nc

e

e

Where,

Where,

In eq(1), nc = nn , EF = EFn , Ec = Ecn

In eq(1), nc = np , EF = EFp , Ec = Ecp

Similarly, for P-type semiconductor electron density is

given by

---- (3)

---- (4)

slide43

For N-type semiconductor hole density is given by

(Evn – EFn ) / KBT

(Evp – EFp ) / KBT

pn = Nv

pp = Nv

e

e

Where,

Where,

In eq(2), nv = pn , EF = EFn , Ev = Evn

In eq(2), nv = pp , EF = EFp , Ev = Evp

Similarly, for P-type semiconductor hole density is

given by

---- (5)

---- (6)

slide44

Dividing the eq(3) with (4), we get,

(EFn – Ecn )

(EFp – Ecp ) / KBT

e

e

(Ecp – Ecn ) / KBT

e

nn

nn

=

=

np

np

Since the Fermi levels are equal , hence EFn = EFp

Therefore, the above equation becomes as

---- (7)

slide45

As, Ecp – Ecn = eVB

eVB / KBT

- eVB / KBT

Where,

e

e

nn

VB = the barrier potential

=

=

np

nn

np

Therefore, the eq(7) becomes as

(OR)

---- (8)

slide46

Similarly, dividing the eq(5) with (6) & simplifying, we get,

- eVB / KBT

- eVB / KBT

e

e

pn

=

=

pn

pp

pp

(OR)

When the junction is in forward biased with voltage V, then the electron density in the P-region becomes as

- e(VB -V) / KBT

---- (9)

np + ∆np = nn e

slide47

eV / KBT

eV / KBT

e

np + ∆np = np

e

- eVB / KBT

e

=

np

nn

Since,

Therefore, increase in electron density in the p-region

is given by

eV / KBT

e

∆np = np

- 1

---- (10)

- eVB / KBT

(From eq(8)

np + ∆np = nn e

slide48

When the diode is in forward bias more electrons are

move from N-region to P-region hence, increase the

Electron density in P-region by ∆np.

Therefore, diffusion current of electrons is given by

Where,

C = constant (depends on the semiconductor)

eV / KBT

e

ie= C1 ∆np = C1 np

- 1

---- (11)

Similarly diffusion current of holes is given by

slide49

Therefore, total current is given by

I = ie + ih

eV / KBT

eV / KBT

e

e

ih = C2 ∆pn = C2 pn

I = ie + ih = (C1 np+C2 pn)

- 1

- 1

When the junction is in reverse bias V = - V ,

then the reverse current is given by

---- (12)

-- (13)

slide50

At room temperature (T)

- eV / KBT

e

« 1

Therefore,

I = -(C1 np+ C2 pn) = I0--- (14)

- eV / KBT

e

I = ie + ih = (C1 np+C2 pn)

- 1

-- (13)

since, I0 is the steady reverse current.

Substituting eq(14) in eq(13), we get,

slide51

Eq(15) is known as the diode equation.

But, practically the diode equation is written as.

NOTE:

- eV / KBT

- eV / β KBT

e

e

I = I0

I = I0

- 1

- 1

-- (15)

-- (16)

βis a constant depends on the material

of the diode. For Ge = 1 & for Si = 2

slide52

Light emitting diode (LED)

Definition:

LED is a semiconductor PN-junction diode

which converts electrical energy to light

under forward biasing. It emits light in

both visible & IR region.

NOTE:

LEDs are typically made of compound

semiconductors (OR) direct band gap

semiconductors like gallium arsenide.

slide53

Principle:

Injection Luminescence

When LED is forward biased, the majority charge carrier

moves from P to N & similarly from N to P region and

becomes excess minority charge carriers. Then these excess

minority charge carriers diffuse through the junction and

recombines with the majority charge carriers

in N & P region respectively to produce light.

LED is a highly doped diode

slide54

Construction:

Photons

Sio2

P

VF

Ohmic

Contacts(Al)

N

substrate

slide55

The PN-junction is made by doping

  • silicon by GaAs crystal. Here silicon
  • can act both as donor & acceptor.
  • Therefore, a shallow PN-junction is
  • formed on GaAs substrate such that
  • P-layer is formed by diffusion on
  • N-layer as shown in figure.
slide56

In order to increase the probability

  • of radiative recombinations, the
  • thickness of the N-layer is taken
  • higher than that of the thickness
  • of the P-layer.
  • The top layer of the P material is left
  • uncovered for the emission of light.
slide57

Working:

  • If the diode is properly biased the charge
  • carriers move across the junction. If the
  • biasing voltage is further increased, these
  • excess minority carriers diffuse away from
  • the junction and directly recombine with
  • the majority carriers.
  • Therefore, electron-hole recombination
  • occurs more & more and thereby light is
  • emitted through the top layer of the
  • P-material which is left uncovered as
  • shown in figure.
slide58

Symbol:

+

-

Anode

Cathode

slide60

Advantages:

  • They are smaller in size.
  • Its cost is very low.
  • It has long life time.
  • They are available in different colours
  • at low cost.
  • They can operate with low voltage,
  • faster response ≈ 10-9 seconds .
  • its intensity controlled easily & operate
  • wide range temperature………
slide61

Disadvantages:

  • Its output power is low.
  • Its intensity is less than laser.
  • Its light can not travel through
  • longer distance.
  • Its light will not have directionality,
  • incoherent & not in phase……
slide62

Photo diode

Definition:

A PN-junction diode which converts the

photonic energy into its equivalent

Electrical energy under in reversed bias

is called photo diode.

Its operation is quite reverse from LED

& used in optical communication.

slide63

Symbol:

Photo diode are two types.

  • p-i-n photo diode (PIN Diode)
  • Avalanche photo diode (APD)

+

-

Anode

Cathode

slide64

1. p-i-n photo diode(PIN Diode)

Principle:

Under reverse bias when light is made to fall

on the neutral (or) intrinsic region ‘i’ electron

hole pairs are generated. These electrons and

holes are accelerated by the external electric

field, which results in photo-current.

Thus light is converted into electrical signal.

slide65

Electron – hole

pair

P

i

VR

Photons

Figure shows the

reverse bias of

p-i-n diode.

N

slide66

Construction:

  • It consists of three regions called positive (p),
  • intrinsic(i) & negative(n). Hence it is called
  • PIN-diode.
  • The P,N regions are made up of silicon,
  • germanium & their alloys, also heavily doped.
  • The P,N region is separated by an intrinsic
  • region & made as large as possible in order to
  • have more absorption of the incident photons.
slide67

Working:

  • The PIN diode is activated in high reverse bias.
  • Since, the intrinsic region has very less mobile
  • charges hence, the width of the depletion region
  • gets increased.
  • When a photon of energy greater than the
  • band gap energy of the photo diode incidents
  • on the depletion region, the electron-hole
  • pair is created due to the absorption of photon.
slide68

The mobile charges are accelerated by

  • the applied voltage, which gives rise to
  • photo current in the external current.
  • In PIN-diode the photo current is directly
  • proportional to the optical power incident
  • on it. Hence, it is called as a linear device.
slide69

2. Avalanche photo diode(APD)

Principle:

Under reverse bias when light is made to fall

on the neutral (or) intrinsic region ‘i’ electron

hole pairs are generated. By avalanche effect

more number of electron-hole pairs are

Created, which results in large photo current

than of the PIN diode.

Thus light is converted into electrical signal.

slide70

Electron – hole

pair

N+

N+ - heavily doped N-region

P+ - heavily doped P-region

P - lightly doped P-region

Layer 1

P

Layer 2

VR

Photons

i

Layer 3

Figure shows

the reverse bias

of avalanche photo diode.

Layer 4

P+

slide71

Construction:

  • It consists of four layers called P+,i ,P & N+.
  • Layer 1 & 4 are heavily doped, layer 2 & 3
  • are lightly doped.
  • Totally we can imagine the diode as
  • PN-junction diode which the P-region
  • is composed of three layer as P, i, P+.
slide72

Working:

  • Due to reverse bias the depletion region gets
  • widen. Here both i & P are lightly doped.
  • When the light is fall on the intrinsic region,
  • the incident light creates an electron-hole
  • pairs in the intrinsic region.
  • When the biasing voltage is increased the
  • photo electrons are drift through the
  • intrinsic region to P (layer-2) & N (layer-1)
  • junction.
slide73

Here, they collide with free electrons in the

  • valence band & releases more number of
  • free (OR) conduction electrons. Thus
  • avalanche effect is produced.
  • Therefore, a single photon generated 1000’s
  • of electrons by avalanche effect hence,
  • increases the output current enormously.
  • These diodes are termed as highly sensitive
  • detectors.