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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

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

Boundary Conditions:

If forward bias is applied to the PN junction

kTeVPP

kTeVnn

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

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

anon L

xxVVpxp

0,0',0))((

<p-region>

<n-region>Steady state condition : Steady

state condition :

Ideal PN Junction Current

]1)[exp()(

Similarly

dxxdpeDxJ

)()()( 1eJxJxJJ tVaVsnppn

nops L

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()(

xxkTeVa

Temperature Effect

)1exp(

kTeVaJJ

nnNnpstatesteady

)exp(2

nJ gis

Js : strong function of temperature

Reverse Bias-Generation Current

GWenRdxeJ

때일

일때

)'()'()( 2

ppCnnCnnpNCC

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

In depletion region,

nops L

peDJ WenJ

Total reverse bias current density, JR

gensR JJJ n=p=0

Forward Bias Recombination Current

)()()( 2

ppnnnnpR

kTeVeWneRdxJ

kTeVnR

)'()'()( 2

ppCnnCnnpNCC

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

exp(kT

eVJJ arorec

R = Rmax at x=o

Total Forward Bias Current

]1exp[ kTeVaJJ s

Drec JJJ

exp(kT

eVJJ arorec

Total forward bias current density, J

kTeVaJJ

In general, (n : ideality factor)

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

SUMMARY

Junction Break Down

Breakdown Characteristics

* Zener Breakdown

* Avalanche Breakdown

Zener Breakdown

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

Critical Electric Field & Voltage at Breakdown

critsB eN

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

Fig 2.28-30 Zener characteristics.

Zener kneecurrent

Zener testcurrent

MaximumZener current

Fig 2.31 Determining Zener impedance.

IZT (20mA)

Fig 2.32 Zener equivalent circuits.

Ideal: ZZ = 0

Prac.: ZZ > 0

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

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)

Metal Contacts I-V Characteristics

• Light emitting diode, made from GaAs

– VF=1.6 V

– IF >= 6 mA

Fig 2.35-37 Light emitting diodes.

LED symbol

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

Fig 2.38 A LED needs a current-limiting resistor.

Drivingcircuit

Currentsinkingcircuit

Fig 2.39 Multicolor LED.

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

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