EE105 Fall 2011Lecture 5, Slide 1Prof. Salahuddin, UC Berkeley Lecture 5 OUTLINE PN Junction Diodes – I/V – Capacitance – Reverse Breakdown – Large and

EE105 Fall 2011 Lecture 5, Slide 1 Prof. Salahuddin, UC Berkeley

Lecture 5

OUTLINE• PN Junction Diodes

– I/V– Capacitance– Reverse Breakdown– Large and Small signal modelsReading: Chapter 2.2-2.3,3.2-3.4


Recap: Law of the Junction

0 a-b

Law of the junction:

TD VV �i enapan /2)()(


Recap: Minority Carrier Concentrations at the Edges of the Depletion Region

n

TDLx

A

VVi

p

ppp

eN

enxn

xnnxn

//2

0

1)(

)()(

x'

Notation:Ln electron diffusion length (cm)

nTD LxVV

nA

inpndiffn ee

LN

nqD

xd

dnqDJ //

2

, 1

0x=a

-b ;x‘=0


Hole Diffusion

X’’0

pTDLxVV

pD

ipdiffp ee

LN

nqDJ //

2

,

''

1

x’’


Distribution of Diffusion Current

x0 a-b

nTD LxVV

nA

indiffn ee

LN

nqDJ //

2

, 1 pTDLxVV

pD

ipdiffp ee

LN

nqDJ //

2

, 1

•Assume: No Recombination in the depletion region•Known: Total Current is the same everywhere


Diode Current under Forward Bias• The current flowing across the junction is comprised

of hole diffusion and electron diffusion components:

0,0,0,0,

xdiffnxdiffpxdriftnxdriftptot JJJJJ

x0 a-b

nTD LxVV

nA

indiffn ee

LN

nqDJ //

2

, 1 pTDLxVV

pD

ipdiffp ee

LN

nqDJ //

2

, 1

J_total


I-V Characteristic of a PN Junction• Current increases exponentially with applied forward

bias voltage, and “saturates” at a relatively small negative current level for reverse bias voltages.

pD

p

nA

niSS

VVSD

LN

D

LN

DAqnAJI

eII TD

2

/ 1

“Ideal diode” equation:


Practical PN Junctions• Typically, pn junctions in IC devices are formed by

counter-doping. The equations provided in class (and in the textbook) can be readily applied to such diodes if– NA net acceptor doping on p-side (NA-ND)p-side

– ND net donor doping on n-side (ND-NA)n-side

)1( kTVqSD

DeII

Dp

p

An

niS NL

D

NL

DAqnI 2

VD (V)

I D (A

)


How to make sure that current flow in a forward-biased p-n junction diode is mainly due to electrons?


Diode Saturation Current IS

• IS can vary by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations.– typical range of values for Si PN diodes: 10-14 to 10-17 A/m2

• In an asymmetrically doped PN junction, the term associated with the more heavily doped side is negligible:

– If the P side is much more heavily doped,

– If the N side is much more heavily doped,

Dp

p

An

niS NL

D

NL

DAqnI 2

Dp

piS NL

DAqnI 2

An

niS NL

DAqnI 2


Depletion Width at Equilibrium

on the P side: xaqN

Esi

A

(see slide 3 in lecture 4)(x)

x-qNA

qND

a

-b

V(x)

xa-b

V0

0

on the N side: bxqN

Esi

D

--(1)

--(2)

DA bNaN --(3)

Let us set the reference point at x=aThen V(a)=0

V(-b)=V0; Built in potential


Depletion Width at Equilibrium xa

qNE

si

A V(x)

xa-b

V0

0

bxqN

Esi

D

--(1)

--(2)

DA bNaN --(3)

V(a)=0 V(-b)=V0; Built in potential


Depletion Width at Equilibrium V(x)

xa-b

V0

0


Depletion Width at biased conditionsV(x)

xa-b

V0

0


PN Junction Depletion Capacitance• A reverse-biased PN junction can be viewed as a

capacitor, for incremental changes in applied voltage.

dep

sij W

C

si 10-12 F/cm is the permittivity of silicon


Voltage-Dependent Capacitance• The depletion width (Wdep) and hence the junction

capacitance (Cj) varies with VR.

VD


Reverse-Biased Diode Application• A very important application of a reverse-biased PN

junction is in a voltage controlled oscillator (VCO), which uses an LC tank. By changing VR, we can change C, which changes the oscillation frequency.

LCfres

121


Forward Bias Diffusion capacitance

x’’

At small forward bias, putting a small ac signal changes the concentration of diffused minority carriers and therefore gives a capacitance. This is called a diffusion capacitance

Documents

EE105 Fall 2011Lecture 5, Slide 1Prof. Salahuddin, UC Berkeley Lecture 5 OUTLINE PN Junction Diodes – I/V – Capacitance – Reverse Breakdown – Large and