lecture 5: pn junction...

ECE 142: Electronic Circuits

Lecture 5:

PN Junction (Diode)

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The PN JunctionSteady State1

P N

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

NA ND

Metallurgical Junction

Space Charge Regionionized

acceptorsionized

donors

E-Field

++_ _

h+ drift h+ diffusion e- diffusion e- drift= =

The PN Junction

At steady state, when no

external source is connected to

the pn junction, diffusion and

drift balance each other out for

both the holes and electrons

Depletion Region: This region includes the net positively and negatively charged regions.

The space charge region does not have any free carriers. The width of the space charge

region is denoted by W in pn junction formulae.

Metallurgical Junction: The interface where the p- and n-type materials meet.

P N

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

NA ND

Metallurgical Junction

Space Charge Regionionized

acceptorsionized

donors

E-Field

++_ _

h+ drift h+ diffusion e- diffusion e- drift= =

The Biased PN Junction

P n

+_

Applied Electric

Field

Metal

Contact

“Ohmic

Contact”

(Rs~0)

+_

Vapplied

I

The pn junction is considered biased when an external voltage is applied.

The Biased PN Junction

Forward Bias: • Depletion region shrinks slightly in width.

• Energy required for charge carriers to cross the depletion region

decreases exponentially.

• As the applied voltage increases, current starts to flow across the

junction.

• The barrier potential of the diode is the voltage at which appreciable

current starts to flow through the diode.

• The barrier potential varies for different materials.

Reverse Bias: Depletion region widens.

A small leakage current, Is (saturation current) flows under reverse bias

conditions.

This saturation current is made up of electron-hole pairs being produced

in the depletion region.

Vapplied > 0

Vapplied < 0

Properties of DiodesFigure 1.10 – The Diode Transconductance Curve2

• VD = Bias Voltage

• ID = Current through Diode. ID is Negative for Reverse Bias and Positive for Forward Bias

• IS = Saturation Current

• VBR = Breakdown Voltage

• Vφ = Barrier Potential

Voltage

VD

ID (mA)

(nA)

VBR

~Vφ

IS

Diode I-V (Shockley) Equation:

ID = IS(eVD/ηηηηVT – 1)

• As described in the last slide, ID is the current through the diode, IS is the saturation current and VD is the applied biasing voltage.

VT = kTq

k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C

• ηηηη is the emission coefficient for the diode. It is determined by the way the diode is constructed. It somewhat varies with diode current. For a silicon diode η is around 2 for low currents and goes down to about 1 at higher currents

Types of Diodes and Their Uses

PN Junction

Diodes:

Are used to allow current to flow in one direction while blocking

current flow in the opposite direction. The pn junction diode is the

typical diode that has been used in the previous circuits.

A K

Schematic Symbol for a PN

Junction Diode

P N

Representative Structure for a PN

Junction Diode

Zener Diodes: Are specifically designed to operate under reverse breakdown

conditions. These diodes have a very accurate and specific reverse

breakdown voltage.

A K

Schematic Symbol for a Zener

Diode

Types of Diodes and Their Uses

Schottky Diodes: These diodes are designed to have a very fast switching time which

makes them a great diode for digital circuit applications. They are

very common in computers because of their ability to be switched

on and off so quickly.

A K

Schematic Symbol for a Schottky

Diode

Shockley Diodes: The Shockley diode is a four-layer diode while other diodes are

normally made with only two layers. These types of diodes are

generally used to control the average power delivered to a load.

A K

Schematic Symbol for a four-layer

Shockley Diode

Types of Diodes and Their Uses

Light-Emitting

Diodes (LED):

• Light-emitting diodes are designed with a very large bandgap so

movement of carriers across their depletion region emits

photons of light energy.

• Lower bandgap LEDs (Light-Emitting Diodes) emit infrared

radiation, while LEDs with higher bandgap energy emit visible

light.

• Many stop lights are now starting to use LEDs because they are

extremely bright and last longer than regular bulbs for a

relatively low cost.

A K

Schematic Symbol for a Light-

Emitting Diode

The arrows in the LED

representation indicate

emitted light.

Types of Diodes and Their Uses

Photodiodes: • While LEDs emit light, Photodiodes are sensitive to received

light. They are constructed so their pn junction can be exposed

to the outside through a clear window or lens.

• In Photoconductive mode the saturation current increases in

proportion to the intensity of the received light. This type of

diode is used in CD players.

• In Photovoltaic mode, when the pn junction is exposed to a

certain wavelength of light, the diode generates voltage and can

be used as an energy source. This type of diode is used in the

production of solar power.

A K

A Kλ

ECE 142: Electronic Circuits

Lecture 6:

Diode Applications

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Diode Applications

• Half Wave Rectifier

• Full Wave Rectifier

• Clipping Circuits

• Clamping Circuits

• Regulator

• Regulated Power Supply

Diode Applications

• Half wave rectifier and equivalent circuit with

piece-wise linear model

vi

v i = VM sin (ωt)

Half Wave Rectifier

• We initially consider the diode to be ideal,

such that Vφ =0

Half Wave Rectifier

• The (ideal) diode conducts for vi >0 , thus

v0 ≈ vi

• For vi < 0, the (ideal) diode is an open circuit

(it doesn’t conduct) and

v0 ≈ 0.

Half Wave Rectifier

• In this simplified (ideal diode) case the

input and output waveforms are as shown

The diode must withstand a peak inverse voltage

of VM

Half Wave Rectifier

• The average d.c. value of this half-wave-

rectified sine wave is

+= ∫

π

θθπ

0

0sin2

1dVV MAV

[ ]π

ππ

MM VV=−−= 0coscos

2

Half Wave Rectifier

• So far this rectifier is not very useful.

• Even though the output does not change polarity it has a lot of ripplei.e. variations in output voltage about a steady value.

• To generate an output voltage that more closely resembles a true d.c. voltage we can use a reservoir or smoothing capacitor in parallel with the output (load) resistance.

Smoothed Half Wave Rectifier

Circuit with reservoir

capacitor

Output voltage

The capacitor charges over the period t1 to t2 when the diode is on

and discharges from t2 to t3 when the diode is off.

Smoothed Half Wave Rectifier

• When the supply voltage exceeds the output voltage the (ideal) diode conducts. During the charging period (t1 < t< t2)

vo = VM sin (ωt)

Smoothed Half Wave Rectifier

• When the supply voltage falls below the output voltage the diode switches off and the capacitor discharges through the load.

• During the discharge period (t2 < t< t3 ) and

vo = VM exp {- t’ /RC}

where t’= t- t2

• At time t3 the supply voltage once again exceeds the load voltage and the cycle repeats

Smoothed Half Wave Rectifier

• The resistance in the discharge phase is the

load resistance R.

• RC can be made large compared to the wave

period.

• The change in output voltage (or ripple) can

then be estimated using a linear

approximation to the exponential discharge.

Smoothed Half Wave Rectifier

• vo = VM exp {- t’ /RC} ≈ VM [ 1- (t’ /RC)]

• The change in voltage ∆V is therefore

approximately given by VM t’ /RC

• For a the half wave rectifier this discharge

occurs for a time (t3 - t2 ) close to the period T

= 1/f, with f= frequency.

• Giving the required result:

RC

TV∆V

M≈

Smoothed Half Wave Rectifier

• We can define a ripple factor as

where Vd.c. = (VM - ∆V/2)

The lower the ripple factor the better

d.cV

∆Vfactor Ripple =

Non-Ideal Half Wave Rectifier

VM

Vφ

Vφ

Full-Wave (Bridge) Rectifier

• We initially consider the diodes to be ideal, such

that VC =0 and Rf =0

• The four-diode bridge can be bought as a package

vi

Full-Wave (Bridge) Rectifier

• During positive half cycles vi is positive.

• Current is conducted through diodes D1, resistor R and diode D2

• Meanwhile diodes D3 and D4 are reverse biased.

vi

Full-Wave (Bridge) Rectifier

• During negative half cycles vi is negative.

• Current is conducted through diodes D3, resistor R and diode D4

• Meanwhile diodes D1 and D2 are reverse biased.

vi

Full-Wave (Bridge) Rectifier

• Current always flows the same way through the load R.

• Show for yourself that the average d.c. value of this full-wave-rectified sine wave is VAV = 2VM/π(i.e. twice the half-wave value)

Full-Wave (Bridge) Rectifier

• Two diodes are in the conduction path.

• Thus in the case of non-ideal diodes vo will be

lower than vi by 2VC.

• As for the half-wave rectifier a reservoir

capacitor can be used. In the full wave case

the discharge time is T/2 and

2RC

TV∆V

M≈

Diode Clipper Circuits

• These circuits clip off portions of signal

voltages above or below certain limits, i.e. the

circuits limit the range of the output signal.

• Such a circuit may be used to protect the

input of a CMOS logic gate against static.

Diode Clipper Circuits

Diode Clipper Circuits

• When the diode is off the output of these

circuits resembles a voltage divider

i

SL

L

o vRR

Rv

=

+

Diode Clipper Circuits

• If RS << RL

• The level at which the signal is clipped can be

adjusted by adding a d.c. bias voltage in series

with the diode.

v0 ≈≈≈≈ vi

For instance

Diode Clipper Circuits

• Let’s look at a few other examples of clipper

circuits.

Clipper circuits using zeners

Figure 3.24 A voltage regulator supplies constant voltage to a load.

Voltage Regulator

Designing a power supply

Diode Clamper Circuits

• The following circuit acts as a d.c. restorer.

Diode Clamper Circuits

• A bias voltage can be added to pin the output

to a level other than zero.