Diodes Physics

8/10/2019 Diodes Physics

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PNJUNCTION DIODES

The V-Icharacteristics of a junction diode

from the physics point of view.


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Physical structure

If donor impurities are introduced into one side and

acceptors into the other side of a single crystal of a

semiconductor, apn-junction is formed.

_+

_+

_+

_+

_+

_+

_+

_+

_+

p - type n - type

Acceptor ionJunction

Donor ion


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Diffusion Current

! A density gradient across the junction established.

! Holes from thep-type will diffuse to the right andelectrons from the n-type to the left.! Diffusion current

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_+

_+

_+

_+

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_+

_+

_+

p - type n - type

Acceptor ionJunction

Donor ion

Hole Electron


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Re-combination

! Holes inp-type combine with the diffused electrons.

! Electrons in the n-type combine with diffused holes.

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_ +

_+

_+

_ +

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_ +

_+

p - type n - type

_

_

_

+

+

+

Depletion region

M5


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Depletion Region

! At the junction, there are no mobile charges.! This region is called the depletion region, the space-

charge region, or the transition region.

_+

_+

_+

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p - type n - type

_

_

_

+

+

+

Depletion region

M5


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Contact Potential

! Holes from the n-type will tend to flow across the junction.

! Electrons from thep-type tends to flow into the n-type.! Drift current

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_ +

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_ +

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_ +

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p - type n - type

_

_

_

+

+

+

Depletion region

M5


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Contact Potential

! The potential established by this electric field is calledthe contact potential VD.

! Under steady state conditions

! Diffusion current = drift current

! Net current = 0


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Reverse Biased (cont.)

! As there are very few holes (electrons) in the n-type (p-

type), zero current results.

! In practice, a small current does flow because a small

amount of hole-electron pair is generated throughout the

crystal as a result of thermal energy.

! This current is called the reverse saturation current,Is.


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Forward Biased

! The potential barrier established by the electric field willbe lowered by the applied voltage.

p n

+


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Forward Biased (cont.)

! The equilibrium initially established at the junction will be

disturbed.

! Current starts to flow

! If the applied voltage is increased to the contact potential VD,the current will become arbitrary large.


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Forward Biased (cont.)

The V-I characteristics of apn-junction with forward-biased is approximated as a straight line.

I

VCutin voltageOffset voltageThreshold voltage


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The V-ICharacteristics

When a reversed-bias voltage is increased to a large value, alarge reverse current will flow.

! Breakdown

I

V

Breakdown


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The V-ICharacteristics of an Ideal

Junction Diode

For most applications in the course, we can use this diode

model to maximize simplicity.

I

V


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Modeling of a Junction Diode

In practice, a real diode can be modeled by an ideal diode in

series with a small battery.

I

V

V0.7


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Schematic Symbol

+ _Dv

Di


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Summary

! The diode is a non-linear 2-terminal device.

! It allows current to flow in one direction (i.e. theforward direction).

! In the forward direction, it acts almost like a shortcircuit.

! It does not allow any current to flow in the oppositedirection (i.e. reverse direction).! In practice, only the reverse saturation current flows in this

direction.


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Diode Applications

1. Rectification

Rectification is the process of turning an

alternating signal (ac) into one that is restricted to

only one direction (dc).

Rectification is classified as

! (i) half-wave

! (ii) full-wave


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(i) Half-wave Rectifier

! With one diode, there are two possible states to

consider.

! ON

! OFF.

+_

i +

_

VLRL= cosim

Vvi

tO

w

r Dv Di


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Half-wave Rectifier (cont.)

The diode characteristics indicates that only positive

current can flow in this circuit. This requires vi> 0.

+_

i

VLRLvi

r Di

Li

iD

Rr

vi

+

=

(i) The diode is ON.


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The happens when the voltage across the diode is

negative. This requires vi< 0.

+_

i

VLRLvi

r Di

0=Di

(ii) The diode is OFF.


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! The average output voltage VL(avg) is

!

""

!

!

!

Lm

LmavgL

V

ttVV

=

= #$

2

2

)d(cos21)(


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(ii) Full-wave Rectifier

A full-wave rectifier transfers input energy to the outputduring both halves of the cycle.

+_vi

+

_

4

1 3

2

RL vL

i L


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Full-wave Rectifier (cont.)

! It provides increased average current per cycle over

that obtained by using the half-wave rectifier.

! Since there are 4 diodes, there are 16 possible states

to consider. However, only 2 are self-consistent.


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When the input voltage is positive, diodes #1 and #4 are ONand diodes #2 and #3 are OFF.

wt

im

i

2

v

V

wt

&

D 1i

i D 4

+_vi

+

_

4

1 3

2

RL vL

i L


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wt

wt

Lv

Li

VLm


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(iii) Filtering

The waveform resulted from the half-wave or full-waverectifier can be converted to a nearly constant level byusing a capacitor as a simple filter.

+_ Civ VLRL


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(iv) Full-wave peak rectifier

! As in the half-wave case, the output voltage will be

almost equal to the peak value of the input signal.

! The ripple frequency, however, will be twice that of

the input.


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(v) The Peak detector as an AM demodulator

Transduction Amplification TransmissionMusic

Speech

Remote

listeners

Consider a simple radio broadcast system.


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The Peak detector as an AM demodulator

(cont.)

Two problems arise:

(i) The transmitting antenna required to convert

the signals to electromagnetic radiation wouldhave to be very long.

(ii) If all radio stations were to transmit the same

frequency band (audio), the listener would find

it difficult to distinguish those signals.


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(cont.)

A solution:

!To shift the frequency band to be transmitted from

the audio range to a location at a much higher

frequency.

! This reduce the required antenna length.


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(cont.)

The information contained in the AM signal can

then be extracted or detected using a diode peakrectifier.


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Diode Applications (cont.)

2. Clipping Circuits

! To select part a signal that lies above or

below a reference level.

! Two general categories:

! Series

!

parallel


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(i) Simple series clippers

Example 3. Determine the output waveform for

the following clipping circuit.

+

_

vi

vo

+

_

V

RtT

2

__To

imV

iv


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Simple series clippers (cont.)

vi V= ( diodes change state )

1. The diode changes state

(vd=0 and id=0) when vi= V .

2. When vi> V, the diode is ON.

By KVL, we have

Vvvio

!=3. When vi< V, the diode is OFF

and vo=0.

+

_

vi

vo

+

_

V

R

id=0

vd=0

iv

tT

2

__T

imV

V

t

ov

imV V_


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Example 4. Determine the output waveform for the

following clipping circuit.

(ii) Simple parallel clippers

t

iv

16

16_

V

V

+

_

vi

vo

+

_

R

V 4= V


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Simple parallel clippers (cont.)

t

iv

16

16_

T

2

__T

V 4= V

V

V

t

16

T

2

__T

4

V

V

V

+

_

vi

+

_

R

V 4=

id = 0

vd = 0

vo

V 4==

V

1. The diode changes state (vd=0 and id=0)

when vi= 4 V . vo= 4 V.

2. When vi> V, the diode is OFF. vo= vi.

3. When vi< V, the diode is ON. vo=4 V


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Diode Applications (cont.)

3. Clamping Circuits

! Clamping circuits clamp a signal to a different

dc level.

! Consist of:

! a capacitor

! a diode

! a resistor

! DC supply (for an additional DC shift)


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Clamping Circuits (cont.)

In solving this type of circuits, we need to

(i) determine the applied voltage that will forward-bias the diode.

(ii) determine the voltage level during the diodeON state by assuming that the capacitor willcharge up instantaneously.

(iii) determine the voltage level during the diodeOFF state by assuming that the capacitor

will hold on its established voltage level.


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Clamping Circuits (cont.)

+

_

vo

+ _

R

C

VV

+

_

+

_

vo

+ _

R

C

VV

+

_

iv

t

T2__T

V

V_

t

V2_

ov

1. The diode will be forward-

biased when vi= V. vo= V+VC.

1. The diode is OFF when vi= -V. vo=

0.


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OTHER DIODE TYPES (cont.)

! A Shockley diode is formed by bonding a metal,

such as platinum, to a n-type silicon.

! This type of diodes has no depletion layer and can

switch faster than ordinary diodes.

+ _

n+

1. Shockley diodes


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Shockley diodes (cont.)

! The most important application of Shockley

diodes is in digital computers.

! Since a Shockley diode has a cut-in voltage of 0.25V, they are frequently used in low-voltage

rectifiers.


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2. Light-emitting diodes (LEDs)

! Special materials (e.g. gallium, arsenic and

phosphorous) are used to convert a portion ofthis energy into light.

! Emit light when forward-biased.


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3. Zener diodes

! Optimize to operate in the breakdown region.

! Use mainly in voltage regulators! circuits that hold load voltage almost constant

despite large changes in input voltage and/or

load resistance.


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Zener diodes (cont.)

The V-I characteristics

I

V

IZ max

IZ min

V_ Z

Breakdown


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Zener diodes (cont.)Schematic symbol

Zener resistance

REIdeal


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Zener regulators

! vS> Vz! Voremains constant even when the input voltage

varies over a relatively wide range.

+

_ L

+

_

RVZ

Ri L

vs

Zi

i


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Summary

! Half-wave and full-wave rectifiers convert an acsignalto an dcsignal.

! A filtering capacitor can be added to rectifier circuits toreduce ripples.

! The filtered rectifier circuits can be used in AM

demodulation process.


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Summary (cont.)

! Diodes can be used in wave-shaping circuits that either

clip portions of a signal or shift the dc voltage level. These

circuits are called clipping and clamping circuits

respectively.

! Shockley diodes, because of their fast switching nature

and low cut-in voltage, are used extensively in digital

computers and low-voltage rectifications.


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Summary (cont.)

! The LED converts an electrical current into light and isused extensively in applications such as the seven-segment display.

! Zener diodes operate in the reverse breakdown regionand behave like a voltage source in that region. Thesedevices are used mainly in regulator circuits.

Diodes Physics

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