Dr. Nasim ZafarElectronics 1

EEE 231 – BS Electrical EngineeringFall Semester – 2012

COMSATS Institute of Information TechnologyVirtual campus

Islamabad

Semiconductor device lab.KwangwoonUniversity Semiconductor Devices.

I-V Characteristics of PN Junctions

Lecture No: 7

PN Junction

Ideal I-V Characteristics: Assumptions

1) The space-charge region boundaries represent an a step junction.

2) The abrupt depletion layer approximation applies. - abrupt boundaries & neutral outside of the depletion region

3) No carriers exist in the space-charge region.

4) In the bulk of the diode outside the depletion region, the semiconductor is neutral.

5) Diode operation is considered at a temperature at which all impurity atoms are ionized.

6) Perfect ohmic contacts are made to the ends of the p and n regions.

7) The Maxwell-Boltzmann approximation applies to carrier statistics.

8) The Concept of low injection applies.

Qualitative Description of Current Flow

Equilibrium Reverse bias Forward bias

Semiconductor device lab.KwangwoonUniversity Semiconductor Devices.

Current-Voltage Relationship

Quantitative Approach

Current-Voltage Characteristics

THE IDEAL DIODE

Positive voltage yields finite current

Negative voltage yields zero current

REAL DIODE

Voltage-Current Characteristics of a P-N Junction

Built-in-Potential

):(ln barrierpotentialinbuiltVnNNVV bii

datbi

Boundary Conditions:

If forward bias is applied to the PN junction

)exp(

)exp(

kTeVPP

kTeVnn

anon

apop

The Steady state :

• Under the idealized assumptions, no current is generated within the depletion region; all currents come from the neutral regions.

•In the neutral n region, there is no electric field , thus in the steady-state the solution of the continuity equation, with the boundary conditions gives:

)exp(]1)[exp()(n

n

t

anon L

xxVVpxp

)exp(]1)[exp()(n

papop L

xxkTeVnxn

Semiconductor Devices

Minority Carrier Distribution

)exp(]1)[exp()(n

papop L

xxkTeVnxn

)exp(]1)[exp()(n

n

t

anon L

xxVVpxp

0,0',0))((

EgxP

t

n

<p-region>

<n-region>Steady state condition : Steady

state condition :

Semiconductor Devices

Ideal PN Junction Current

]1)[exp()(

)()(

,

]1)[exp()(

)()(

t

a

n

ponpn

xx

pnpn

t

a

p

nopnp

xx

npnp

VV

LpeD

xJ

dxxdn

eDxJ

Similarly

VV

LpeD

xJ

dxxdpeDxJ

p

n

)()()( 1eJxJxJJ tVaVsnppn

)(n

pon

p

nops L

neDL

peDJ

Effect of Temperature on diode Curves:

• Doping Levels

• Junction Area

• The Junction Temperature.

All other factors may be regarded as being constant. However, temperature dependence is very strong.

Semiconductor Devices Semiconductor Devices

Total PN Junction Current

)(],exp[]1)[exp()(

)(],)(exp[]1)[exp()(

pn

p

n

ponn

np

n

p

nopp

xxL

xxkTeVa

LneD

xJ

xxL

xxkTeVa

LPeD

xJ

Semiconductor Devices

Temperature Effect

)(

)1exp(

n

pon

p

nops

s

LneD

LpeD

J

kTeVaJJ

a

ipo

d

ino N

nnNnpstatesteady

22

,:

)exp(2

kTE

nJ gis

Js : strong function of temperature

Semiconductor Devices

Reverse Bias-Generation Current

GWenRdxeJ

nR

npnEE

o

igen

o

i

onopo

iit

2

2

때일

일때

)'()'()( 2

ppCnnCnnpNCC

Rpn

itpn

GpCnC

nNCCR

pn

itpn

''

2

Recombination rate of excess carriers (Shockley-Read-Hall model)

In depletion region,

n

pon

p

nops L

neDL

peDJ WenJ

o

igen

2

Total reverse bias current density, JR

gensR JJJ n=p=0

Semiconductor Devices

Forward Bias Recombination Current

)()()( 2

ppnnnnpR

nopo

i

wa

o

irec

ai

kTeVeWneRdxJ

kTeVnR

0

0max

)2

exp(2

)2

exp(2

)'()'()( 2

ppCnnCnnpNCC

Rpn

itpn

Recombination rate of excess carriers (Shockley-Read-Hall model)

)2

exp(kT

eVJJ arorec

R = Rmax at x=o

Semiconductor Devices

Total Forward Bias Current

]1exp[ kTeVaJJ s

Drec JJJ

)2

exp(kT

eVJJ arorec

Total forward bias current density, J

kTeVaJJ

kTeVaJJ

sD

rorec

lnln

2lnln

In general, (n : ideality factor)

)21(],1)[exp( nnkTeVaII S

SUMMARY

Semiconductor device lab.KwangwoonUniversity Semiconductor Devices.

Junction Break Down

Breakdown Characteristics

* Zener Breakdown

* Avalanche Breakdown

Semiconductor Devices

Zener Breakdown

xe-h+

P nEc

Ec

Ev

Ev

Ef

EfZener effect

Doping level > 1018/Cm3

Highly doped junction ( narrow W)

Mechanism is termed tunneling or Zener breakdown

Zener Effect

• Zener Break Down: VD <= VZ:VD = VZ, ID is determined by the circuit.

• In case of standard diode the typical values of the break down voltage VZ of the Zener effect -20 ... -100 V

• Zener Diode– Utilization of the Zener effect– Typical break down values of VZ :-4.5 ... -15 V

Avalanche BreakdownImpact Ionization Mechanism

Mechanism Total current during avalanche multiplication

In(w) = M * Ino

Semiconductor Devices

Critical Electric Field & Voltage at Breakdown

B

critsB eN

EV2

2

Critical electric field at breakdown in a one-sided junction

Total current during avalanche multiplication

The breakdown voltage will decrease for a linearly graded junction

26

Fig 2.28-30 Zener characteristics.

IZK=

IZT=

IZM=

Zener kneecurrent

Zener testcurrent

MaximumZener current

IR

VRVZ

IF

VF

RVSIR

27

Fig 2.31 Determining Zener impedance.

IZT (20mA)

IR

VR

IF

VF

19mA

21mA

VZ

IZ

ZZ

Z

VZI

28

Fig 2.32 Zener equivalent circuits.

IR VZ

ZZ

Ideal: ZZ = 0

Prac.: ZZ > 0

29

Example 2.13 Zener diode.A 1N754A Zener diode has a dc power dissipation rating of 500 mW and a nominal Zener voltage of 6.8 V. What is the value of IZM for the device?

(max) 500mW 73.5mA6.8V

DZM

Z

PI

V

Semiconductor Devices

Metal Contacts

<Ohmic contact>• No rectifying action.• The current can flow in both direction

<Schottky contact> The difference of carrier concentrations of the two materials at the contact. A barrier potential exists. rectifying action occurs. Mostly used in switching circuits. (turn on/off switches)

Semiconductor Devices

Metal Contacts I-V Characteristics

LED

• Light emitting diode, made from GaAs

– VF=1.6 V

– IF >= 6 mA

33

Fig 2.35-37 Light emitting diodes.

LED symbol

34

Table 2.4 Common LEDs.

Elements Forward voltage (VF) Color EmittedGaAs 1.5 V @ IF = 20 mA Infrared (invisible)

AlGaAs 1.8 V @ IF = 20 mA RedGaP 2.4 V @ IF = 20 mA Green

AlGaInP 2.0 V @ IF = 20 mA Amber (yellow)AlGaInN 3.6 V @ IF = 20 mA Blue

35

Fig 2.38 A LED needs a current-limiting resistor.

RS

LED

I

Drivingcircuit

RS

LED

I

Currentsinkingcircuit

5 V

36

Fig 2.39 Multicolor LED.

37

Fig 2.43 Common diodes.Rectifier Zener LED

Schematic symbol

Bias for normal operation

Switched back and forth between forward and reverse.

Reverse Forward

Normal VF Si: VF = 0.7 VGe: VF = 0.3 V

VF = 0.7 V (not normally operated)

Normal VR Equal to applied voltage.

Equal to VZ. Equal to applied voltage.

Primary factors to consider for device substitution

I0 and VRRM ratings. PD(max) and VZ ratings.

VF(min), IF(max), and VBR

1.2V 4.3VFV