Integrated Circuit Devices

Professor Ali Javey

Summer 2009

PN Junctions

Reading: Chapter 5

PN Junctions

A PN junction is present in every semiconductor device.

N-type

P-type

Donors

V

I

Reverse bias Forward bias

N P

V

I

diodesymbol

– +

N-region P-region(a)

(b)

(c)

(d)

Depletionlayer

Neutral P-region

NeutralN-region

Ef

Energy Band Diagram and Depletion Layer of a PN Junction

A depletion layer exists at the PN junction. n 0 and p 0 in the depletion layer.

Ec

Ef

Ev

Ec

Ev

Ef

Ec

Ev

Ef

Ev

Ec

Doping Profile of “Idealized Junctions”

p n p n

Qualitative Electrostatics

Band diagram

Built in-potential

From =-dV/dx

When the junction is formed, electrons from the n-side and holes from the p-side will diffuse leaving behind charged dopant atoms. Remember that the dopant atoms cannot move! Electrons will leave behind positively charged donor atoms and holes will leave behind negatively charged acceptor atoms.

The net result is the build up of an electric field from the positively charged atoms to the negatively charged atoms, i.e., from the n-side to p-side. When steady state condition is reached after the formation of junction (how long this takes?) the net electric field (or the built in potential) will prevent further diffusion of electrons and holes. In other words, there will be drift and diffusion currents such that net electron and hole currents will be zero.

Formation of pn junctions

Equilibrium ConditionsUnder equilibrium conditions, the net electron current and hole current will be zero.

NA = 1017 cm3 ND = 1016 cm3

hole diffusion current

hole drift current

net current = 0

E-fieldP-type N-type

2lni

adbi

n

NN

q

kTV

Built-in Potential

d

ckTAqcd N

N

q

kTAeNNn ln

2

2

lni

ackTBqc

a

i

n

NN

q

kTBeN

N

nn

d

c

i

acbi N

N

n

NN

q

kTABV lnln 2

N-region

P-region

(b)

E f

E c

E v

qVbi qB

qA

The Depletion Approximation

We assume that the free carrier concentration inside the depletion region is zero.

We assume that the charge density outside the depletion region is zero and q(Nd-Na) inside the depletion.

On the P-side of the depletion layer, = –qNa

On the N-side, = qNd

(a)

N P

Nd

Na

D eple tion La yer N e utral R egi on

–xn

0 xp

(b)

x x

p

–xn

(c)

qNd

–qNa

x

E

–xn xp

(d)

(f)

Ec

Ef

Ev

bi , built-in potential

P N

0

–xn

xp

x

bi

(e)

N eut ra l Re gion

V

s

aqNdxd

E

E

)()( xxqN

x p

s

a E

)()( n

s

d xxqN

x E

Field in the Depletion Layer

Field in the Depletion Layer

The electric field is continuous at x = 0.

Naxp = Ndxn

A one-sided junction is called a N+P junction or P+N junction

(a)

N P

Nd

Na

D eple tion La yer N e utral R egi on

–xn

0 xp

(b)

x x

p

–xn

(c)

qNd

–qNa

x

E

–xn xp

(d)

(f)

Ec

Ef

Ev

bi , built-in potential

P N

0

–xn

xp

x

bi

(e)

N eut ra l Re gion

V

Depletion Width

EXAMPLE: A P+N junction has Na=1020 cm-3 and Nd

=1017cm-3. What is a) its built in potential, b)Wdep , c)xn ?

Solution:a)

b)

c)

V 1cm10

cm1010lnV026.0ln

620

61720

2

i

adbi

n

NN

q

kT

μm 12.010106.1

11085.812222/1

1719

14

d

bisdep qN

W

μm 12.0 depn Wx

Reverse-Biased PN Junction

Forward Biased PN Junction

Junction Breakdown

A Zener diode is designed to operate in the breakdown mode.

V

I

VB, breakdown

Forward Current

Small leakageCurrent

voltage

Quantum Mechanical Tunneling

Potential energy barrier

d

E

x

Tunneling Breakdown

Dominant breakdown cause when both sides of a junction are very heavily doped.

(a)

(b)

Empty StatesFilled States

V

I

(c)

Breakdown

-

Ev

Ec

Ev

Ec

Ef

Avalanche Breakdown

impact ionization

avalanche breakdown

EcE

fn

Ec

Ev

Efp

originalelectron

electron-holepair generation

The PN Junction as a Temperature Sensor

What causes the IV curves to shift to lower V at higher T ?

)1(0 kTVqeII

an

n

dp

pi NL

D

NL

DAqnI 2

0

Other PN Junction Devices–From Solar Cells to Laser Diodes

Solar Cells

Also known as photovoltaic cells, solar cells can convert sunlight to electricity with 15-30% energy efficiency

Solar Cells

N P

-

+

short circuit

lightIsc

(a)

V0.7 V

–Isc

Maximumpower-output

Solar CellIV

I

Dark IV

0

(b)

Ec

Ev

Eq.(4.9.4)

Eq.(4.12.1)

p-i-n Photodiodes•Only electron-hole pairs generated in depletion region (or near depletion region) contribute to current

•Only light absorbed in depletion region contributes to generation

–Stretch depletion region

–Can also operate near avalanche to amplify signal

Light Emitting Diodes (LEDs)

•LEDs are typically made of compound semiconductors

–Why not Si