PN Junction Band Diagram and free charge carrier distributions

PN-JunctionP-Type N-Type

n(x)

e- diffusion

p(x)

h+ diffusion

Idiffusion

PN-Junction

P-Type N-Type

P+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

-B B-

B-B-

B-

B-

Electric field

n(x)

p(x)

e-

Idrift

PN-Junction

dxx

r

0

)(E(x)

P-Type N-Type

(x)

P+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

qNA

qND

2bi lnVi

pn

n

pn

qkT

dxxE )(V(x)

E(x)

V(x)

PN-Junction

P-Type N-TypeP+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

2bi lnqVi

pn

n

pnkT

e-h+

CONDUCTION BAND

VALENCE BAND

Biasing a PN Junction

VBIAS

Forward Bias

P-TypeP+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

2bi lnqVi

pn

n

pnkT

VALENCE BAND

N-Type

holes electrons

q VBIAS

Reverse Bias

VBIAS

P-TypeP+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

2bi lnqVi

pn

n

pnkT

VALENCE BAND

N-Type

electrons holes

q |VBIAS|

B-

B-

B- P+

P+

P+

Reverse Bias

dxx

r

0

)(E(x)

P-Type N-Type

(x)

P+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

qNA

qND

E(x)

B-

B-

B-

P+

P+

P+

Emax

Emax ≥ Emax threshold

AvalancheZener effect

e-

F=q Emax= m a

PN Junction as a device. ModelsTypes

qkT

VwhereeII T

nV

V

SDT

D

1 Nonlinear expression

Id

VdV

1/Rs

Id

VdV

DC- MODELS

V

Rs

Vd> V Vd< V

V

Id

Vd

AC- MODEL

P-Type N-Type

P+

P+

P+

P+P+

P+

P+P+ P+

B-

B-

B-B-

B-

B- B-

B-B-

B-

B-

Small signal model

R

VAC

VDC

RVDC

R

VAC

Non Linear

Id

Vd

Id

VdV

1/Rs

T

D

T

D

nV

V

SnV

V

SD eIeII

1

Q

rd

Diode types

Zener Diode PIN Diode

P I N

Schottky Diode

Metal N

Light Emitting Diode (LED)

RF/µwaves applications:•Switch•Variable Attenuator•Signal limiterPhotodetector/Photovoltaic cell

Voltage reference for:•Voltage regulators•Clampers•Voltage references in ADC/DAC

Applications where are required:•Low forward Voltage drop (protection to discharge)•Fast recovery time (rectifiers and switched-mode power suplies)