Lecture Electrical Engineering: Lecture 7 - Dr. Nasim Zafar

In this chapter, you will learn about: The space-charge region boundaries represent an a step junction, the abrupt depletion layer approximation applies, no carriers exist in the space-charge region, in the bulk of the diode outside the depletion region, the semiconductor is neutral.

Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering

Fall Semester – 2012

COMSATS Institute of Information Technology

Virtual campus Islamabad

 PN Junction

1) The  space­charge region  boundaries represent an a step junction.

2) The  abrupt depletion layer  approximation applies.

Qualitative Description of Current Flow

Voltage­Current Characteristics of a P­N Junction

) :

i

d

a t

bi

If forward bias is applied to the PN junction

) exp(

) exp(

kT

eV P

P

kT

eV n

n

a no

n

a po

p

] 1 ) [exp(

)

(

n

n t

a no

n

L

x

x V

V p

x p

) exp(

] 1 ) [exp(

)

(

n

p a

po p

L

x

x kT

eV n

x n

Minority Carrier Distribution

) exp(

] 1 ) [exp(

)

(

n

p a

po p

L

x

x kT

eV n

x

n

) exp(

] 1 ) [exp(

)

(

n

n t

a no

n

L

x

x V

V p

x

p

0 ,

0 ' , 0 ))

(

t n

<p­region>

<n­region>

Steady state condition  : Steady 

state  condition   : 

Ideal PN Junction Current

] 1 ) [exp(

)

(

) ( )

(

,

] 1 ) [exp(

)

(

) ( )

(

t

a n

po n p

n

x x

p n p

n

t

a p

no p n

p

x x

n p n

p

V

V L

p eD x

J

dx

x dn eD x

J

Similarly

V

V L

p eD x

J

dx

x dp eD x

J

p n

) (

) ( )

J

s n

p p

no

p s

L

n

eD L

p eD J

Semiconductor Devices  Semiconductor Devices

Total PN Junction Current

) (

], exp[

] 1 ) [exp(

)

(

) (

], ) (

exp[

] 1 ) [exp(

)

(

p n

p n

po

n n

n p

n p

no p p

x

x L

x

x kT

eVa L

n

eD x

J

x

x L

x

x kT

eVa L

P

eD x

J

Semiconductor Devices  Semiconductor Devices

Temperature Effect

) (

) 1 exp(

n

po n p

no

p s

s

L

n

eD L

p

eD J

kT

eVa J

J

a

i po

d

i no

N

n n

N

n p

state

steady : 2 , 2

) exp(

2

kT

E n

Js : strong function of temperature

Reverse Bias­Generation Current

G W

e

n Rdx

e J

n R

n p

n E

E

o

i gen

o i

o no

po

i i

t

2 2

)' (

)

p p C n

n C

n np N

C

C

R

p n

i t

p n

G p

C n C

n N C

C R

p n

i t p n

' '

no p s

L

n

eD L

2

Total reverse bias current density, JR

gen s

J

n=p=0

Forward Bias Recombination Current

) (

) (

)

p p n

n

n np R

no po

i

w

a o

i rec

a i

kT

eV eWn

eRdx J

kT

eV n

R

0

0 max

) 2

exp(

2

) 2

exp(

2

)' (

)' (

)

p p C n

n C

n np N

C

C R

p n

i t

p n

Recombination rate of excess carriers 

(Shockley­Read­Hall model)

) 2

exp(

kT

eV J

ro rec

R = Rmax  at x=o

Total Forward Bias Current

] 1

exp[

kT

eVa J

D rec J J

J

) 2

exp(

kT

eV J

ro rec

Total forward bias current density, J

kT

eVa J

J

kT

eVa J

J

s D

ro rec

ln ln

2

ln ln

In general,   (n : ideality factor)

) 2 1

( ], 1 )

[exp( n

nkT

eVa I

SUMMARY

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

Critical Electric Field & Voltage at Breakdown

B

crit s

B

eN

E V

Zener test current

Maximum Zener current

V Z

I Z

Z Z

Z

V Z

I

D ZM

Z

P I

V

Metal Contacts I-V Characteristics

– IF >= 6 mA

Fig 2.35­37 Light emitting diodes.

LED symbol

Table 2.4 Common LEDs.

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

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

5 V

Fig 2.39 Multicolor LED.