Lecture 4

pn Junctions (Diodes)

pn junctions 1-1 Wednesday 27/9/2017

Agenda Continue pn junctions

Equilibrium (zero bias) • Depletion rejoins • Built-in potential

Reverse and forward bias

I-V characteristics • Bias voltage • Forward and reverse currents • Reverse breakdown

Special Purpose Diodes

pn junctions 1-2

Test Yourself

Choose the correct answer for the following two statements: i. The resistance of …………. increase with rise in temperature degrees.

1. Insulators 2. Conductors 3. Semiconductors 4. Dielectrics

ii. Charge carrier motion which is caused by an electric field due to an externally applied voltage is known as:

1. Carrier Drift 2. Thermal Motion 3. Carrier Diffusion 4. Brownian motion

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Last Lecture

Thermal equilibrium: balance between drift and diffusion

We can divide

semiconductor into

three regions:

• Two quasi-neutral n and p regions (QNR’s)

• One space charge region (SCR)

pn junctions 1-4

depletion region

Equilibrium (zero bias)

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The Depletion Approximation Assume the QNR’s are perfectly charge neutral

Assume the SCR is depleted of carriers (depletion region)

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Space charge density

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Electric Field

The electric field is proportional to the integral of the space charge density and will be zero in the (charge) neutral regions outside of the depletion region

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Carrier Profiles: in thermal equilibrium (zero bias)

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In equilibrium: dynamic balance between drift and diffusion for electrons and holes inside SCR

Built-in Potential (zero bias)

Because of the electric field across the junction, there exists a built-in potential. Its derivation is shown above

pn junctions 1-10

,

,

2

1

p

n

p

p

p

x

x

p

pp

p

dpDdV

dx

dpqDpEq

n

p

p

p

pp

p

pDxVxV

dx

dpD

dx

dVp

ln)()( 12

200

ln , ln

i

DA

n

p

n

NN

q

kTV

p

p

q

kTV

ndiffndrift

pdiffpdrift

II

II

,,

,,

pn junction under bias

Upon application of voltage

Electrostatics upset: depletion region

widens or shrinks

Current flows With rectifying

behavior

Carrier charge storage

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pn junction in reverse bias

pn junctions 1-12

When the n-type region of a diode is connected to a higher potential than the p-type region, the diode is under reverse bias, which results in wider depletion region and larger built-in electric field across the junction

Carrier Profiles: under reverse bias

At reverse bias voltage depletion region

widens

Net drift current

in SCR ⇒ minority

carrier extraction

from QNRs

Carrier flow is small

because there are few

minority carriers

extracted from QNRs

from the minority side

pn junctions 1-13

- ve

terminal

+ ve

terminal Holes

(majority)

attraction

Electrons (majority)

attraction

holes

electrons

p n

pn junction in forward bias

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• When the n-type region of a diode is at a lower potential than the p-type region, the diode is in forward bias

• The depletion width is shortened and the built-in electric field decreased

Carrier Profiles: under forward bias

At forward bias voltage depletion region shrinks

Net diffusion current in

SCR ⇒ minority carrier

injection into QNRs

Carrier flow can be high

because lots of minority

carriers are injected

into QNRs from the

majority side

pn junctions 1-15

+ ve

terminal

- ve

terminal

Holes

(majority)

pushing

Electrons (majority)

pushing

holes

electrons

p n

IV characteristics of pn juntions

Non-linear device

pn junction (Diode) current equation:

pn junctions 1-16

)1( e T

D

V

V

SD II

p n

VD

ID

Physics of forward bias:

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• Potential drop across SCR reduced by V • minority carrier injection in QNRs

• Minority carrier diffusion through QNRs • Minority carrier recombination at contacts to the QNRs

(and surfaces) • Large supply of carriers injected into the QNRs

e T

D

V

V

DI

Physics of reverse bias

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• Potential drop across SCR increased by V • minority carrier extraction from QNRs

• Minority carrier diffusion through QNRs • Minority carrier generation at surfaces & contacts of

QNRs • Very small supply of carriers available for extraction I saturates to small value

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pn junctions 1-20

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Reverse Breakdown

When a large reverse bias voltage is applied, breakdown occurs and an enormous current flows through the diode

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Special Purpose Diodes

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Zener Light-emitting Photo

Schottky

PIN Tunnel

Laser

Current-regulator

Varactor

Special Purpose Diodes

Light Emitting Diode (LED) In a forward-biased p-n junction,

recombination of the holes and electrons requires energy possessed by the unbound free electrons

In Si and Ge, most of the energy is dissipated in the form of heat and photons

But in other material such as GaAs, the energy generate light but it is invisible for the eye to see (infrared)

Other materials that emit light during forward-bias operation

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Color Construction Forward Voltage

Green

Orange

Red

GaP

GaAsP

GaAsP

2.2

2.0

1.8

Special Purpose Diodes

Photodiode a special light sensitive diode with a clear

window to the pn junction

operated with reverse bias

Reverse current increases with greater incident light

pn junctions 1-25 Irradiance, H

Rev

erse

cu

rren

t, (

)I l

0

Dark current

Special Purpose Diodes

Schottky diode a metal-to-semiconductor contact diode that is used

primarily in high frequency and fast-switching applications

has a low forward voltage drop and high efficiency but rather low reverse voltage rating.

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Cathode Anoden

n region Metal region

Metal-silicon junction

Special Purpose Diodes

PIN diode a three layer diode consisting of p and n layers

separated by a narrow intrinsic layer

In microwave applications, the pin diode acts as a voltage-controlled resistor

Certain types are used as photodetectors in fiber optic systems

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Anode Cathode

n region p regionintrinsicregion

p i n

A K

Special Purpose Diodes

Tunnel diode has a characteristic curve that shows a negative

resistance reading between B and C with a small forward voltage

The negative resistance region is unstable. Taking advantage of this characteristic, the tunnel diode can be used in an oscillator circuit at microwave frequencies

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Tunnelingcurrent

Negative-resistanceregion

IF

VFA

B

C

0

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Lecture Summary Covered material Continue pn junctions

Depletion rejoins Built-in potential (zero bias) Forward and reverse bias I-V characteristics

• Bias voltage • Forward and reverse currents • Reverse breakdown

Special purpose diodes (LED)

Material to be covered next lecture Diode equivalent circuits

DC and AC analysis