chapter 4. diodes. copyright 2004 by oxford university press, inc. diode simple non-linear device 2...

Chapter 4. Diodes

Copyright 2004 by Oxford University Press, Inc.

Diode

• Simple non-linear device

• 2 terminal device, uni- or bi-directional current flow

• Semiconductor

• Many types– pn-junction diode– Photo diode– Light emitting diode– Zener diode


Ideal Diode

Cut-off Turned-on


Rectifier


Diode Logic Gates

“OR” gatevY is high if any of vA, vB, and vC is high.

Y = A + B + C

“AND” gatevY is high if all of vA, vB, and vC are high.

Y = A • B • C


1 2Example. By inspection, we don't know if the diodes and are on.

We make assumptions. Then check for the consistency at the end.

Assume they are both on. The

D D

2 2 2

1

circuit becomes the right one.

0 and 0.

10 01 mA is conducting in the right direction. is on. (OK)

100 ( 10)

At node , write KCL (Kirchhoff's Current Law): 1 2 mA5

1 mA. is

B

D

V V

I D D

B I

I D

1conducting in the right direction. is on. (OK) D

Given circuit is on the left. Assume the diodes are on (right.)


Junction Diodes• Semiconductor junction diodes made of silicon.

– Forward bias region: v > 0– Reverse bias region: v < 0– Breakdown region: v < - VZK

- VZK

Forward bias regionReverse bias regionBreakdown region


pn Junction of Diode

Silicon crystals can be deliberately made impure to create an electrically unstable region. This process is called the “doping.” For example, boron makes p-type where more holes are present. Phosphorus makes n-type where more electrons are present.


Open Circuit pn Junction

ID = IS

The depletion region in the middle creates a barrier voltage. Diode must be biased more than the barrier voltage to start conducting.

Diffusion currentDrift current


Reverse Biased pn Junction

Let I < IS to prevent the breakdown. Injection of additional electrons increases the width of depletion region by combining with more holes and electrons drifted to the middle.→ Increase the barrier voltage.


Forward Biased pn Junction

I supplies holes to p-region, creating more drift current IS. Depletion region narrows. → Decrease the barrier voltage.


Forward Bias Region

1

: Saturation current. Constant physical parameter for a given diode at a given temperature.

: Thermal voltage. Around

T

v

nVS

S

T

i I e

I

V

-23

o

25 mV in room temperature.

=

Boltzmann's constant = 1.38 10 joules / kelvin

Absolute temperature in kelvins = 273 temperature in C

T

kTV

q

k

T

-19 Magnitude of electronic charge = 1.60 10 coulomb

: 1 or 2 depending on how the diode is made. Typically equals 1.

q

n


1

1

ln

Consider a certain diode.

From the above equation, we assume that

T

T

v

nVS

TS

V

nVS

i I e

iv nV

I

I I e

2

2 1

2

2 2 22 1

1 1 1

and are satisfied.

Then, . ln 2.3 log

Current is neglible for 0.5.

Model realistic diode to have a 0.7 V drop.

Approximate value for 2.3 = 0.1 V

T

T

V

nVS

V V

nVT T

T

I I e

I I Ie V V nV nV

I I I

v

nV


Reverse Bias Region

The current is constant at the saturation value.Si I


Breakdown Region

Not all diodes exhibit this characteristics.

"Z" stands for zener.

"K" stands for knee.

ZKv V


D

T

V

nV DD DD S

V VI I e

R

Exponential Model

Graphical Method

22 1

1

Let 5 V and 1 .

Initially, assume that diode has 1 mA at 0.7 V.

5 0.74.3 mA

14.3

0.1log 0.7 0.1log 0.763 V1

4.3 mA and 0.763 V

Use these values of and , and repeat

DD

DD DD

D D

D D

V R k

V VI

RI

V VI

I V

I V

the whole process.

4.237 mA and 0.762 V

Continue until and stop changing (significantly).D D

D D

I V

I V

Numerical Method (Iteration)


Piecewise-Linear Model


0

00

0 for

= for

D D D

D DD D

D

i v V

v Vv V

r


Constant Voltage Drop Model

Usually assume VD = 0.7 V.Ideally, we can also assume VD = 0 V.


No voltage drop.


Small Signal ModelFind the DC operating point (i.e., VD and ID) first using any model.Then, for the small signal, model the diode as a resistor ( whose valueequals the inverse of the slop at the Q point.


( ) ( ) ( )

( ) ( )

( ) =

( )If 1 (for small signal ( )): ( ) ( ) (using 1 for small ).

( ) ( )

D

T

D d d dD D

T T T T T

V

nVD S

D D d

V v t v t v tv V

nV nV nV nV nVD S S S D

xd Dd D D d

T T

D D d

I I e

v t V v t

i t I e I e I e e I e

v t Iv t i t I v t e x x

nV nV

i t I i t I

( ) and .

1Or more accurately,

( )

( )D D

D TD d d

T D

d

D

D i I

I nVv t r

nV I

ri t

v t


Voltage Regulator

Three diodes provide a constant voltage of 2.1 V.

Assume 2.

10 2.17.9 mA

12 25

6.3 .7.9

For three diodes, total 3 18.9 .

If the power supply changes by 2 V peak-to-peak:

2

Td

d

o

n

I

nVr

Ir r

rv

r R

18.92 37.1 mV

18.9 1,000


Zener Diodes

VZ = VZ0 + rZ IZ


Zener Diodes as Shunt Regulator

Put a Zener diode in parallel with a load.Voltage on the load is limited by the Zener voltage, VZ.


Rectifier Circuits

transformer


Half Wave Rectifier

Approximation

Diode model

0 0 0

0

0 for and for

If , - .

o S D o S D S DD D

D o S D

R Rv v V v v V v V

R r R r

r R v v V


Full Wave Rectifier


Bridge Rectifier


Peak Rectifier


=C

Assuming the input sine wave moves slowly, charging curve follows the inputs.

Once the sine wave passes the peak, , discharging begins with time constant .

drops to . The

oID C L L

p

o r

vdvi i i i

dt R

V RC

v V

range of between and is called the ripple.

1DC values: and

2

At the end of discharging interval:

For ,

o p r

pL O p r

t T

CR CRo p p r p

pr p

v V V

VI V V V

R

v V e V V V e

VTCR T V V

CR fCR


Precision Half Wave Rectifier

Diode in feedback loop of voltage follower.Diode turns on by the open loop gain. → 0.7 V turn-on threshold not necessary.


Limiting Circuit

For circuits with passive elements only, K ≤ 1.


DC Restorer

Initially, the capacitor is charged to a certain voltage with diode on. At this point vO = 0V. Now the diode goes off when vI swings +10V from -6V to 4V. Since there is no discharging path, the capacitor keeps the same voltage; i.e., vC remains constant. Thus, the output must also swing +10V making vO = 10V.


DC Restorer with Load R


Voltage DoublerClamp

Peak rectifier

During a positive cycle of input, C1 is charged to Vp. When the input passes the maximum, D1 cuts off and D1 turns on. This makes C2 charged to 2Vp


Special Types of Diodes

• Schottky-Barrier Diode: Forward drop of 0.3 – 0.5 V

• Varactor: Acts as a voltage variable capacitor• Photodiode: Conducts under the light in the reverse direction

• Ligh Emitting Diode: Converts the forward currents to lights.

chapter 4. diodes. copyright 2004 by oxford university press, inc. diode simple non-linear device 2...

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