Unit 12: Semiconductor devices.

Diode.

P-N Junction in equilibrium. Diode.

Diode bias. Forward and reverse bias.

Diode current-voltage characteristics. Models.

Applications.

Electrons Holes

PN junction in equilibrium

NP

Jpdrift

Jndif

Jpdif

Jndrift

300 K0 K

V0

rrrrE

At room temperature, holes onp area go by diffusion towards n

area, and e- from n area crossto p area (majority carrierscurrents).

Xp Xn

Diffusion Currents

of majority carriers

Drift Currents of

minority carriers

On junction area, holes and e-

are recombined, appearing anarrow depletion area (withoute- and holes) with a chargedensity due to the ions of theimpurities, negative on p areaand positive on n area.

So, an electric field appears,flowing drift currents of minoritycarriers (e- from p to n area,and holes from n to p area),canceling diffusion currents.

On not biased p-n junction (or in equilibrium), diffusion currents of majority carriers are cancelled by drift currents of minority carriers.

A P-N junction is a DIODE

PN junction in equilibrium. Not biased junction.

V0p nrrrrE

driftJr

difJr

0JJ difdrift =+rr

xp xn

PN

junct

ion in e

quili

brium

. Featu

res.

ρqND

-qNA

-0

+

pp0 ≈ ≈ ≈ ≈ NA nn0 ≈ ≈ ≈ ≈ ND

np0 pn0

Charge carriers density distribution.

Charge distribution

Xp Xn

V 0

rrrr

E

Xp Xn np

V 0

rrrr

E

Xp Xn

Electric field on pn junction

EXp Xn

Drop of potential known as

Built-in potencial

V0

V

Xp Xn

V0 = 0.7 V for Si diodes

V0 = 0.3 V for Ge diodes

at 20 ºC

PN

junct

ion in e

quili

brium

. Featu

res.

Diode bias. Forward bias

VD creates an electric field opposite to the field on the depletion area, being

lower Etotal and the drop of potential on the junction: V´=V0-VD. So, diffusion

majority current is increased, and drift minority current is decreased.

VDI

r r r r

E

p n

Jdes

J

Jdif

r r r r r r r r

r r r r

driftJr

difJr

V’

Diode bias. Forward bias.

If VD>V0, diffusion and drift currents have same direction and current can be

higher. There isn’t opposition for flowing of current.

p n

driftJr

difJr

VD>V0

rrrrE

Jr

Diode bias. Reverse bias.

VR creates an electric field reinforcing the field in the depletion area, increasing the

drop of potential: V´=V0+VR. Depletion area enlarges. So, the diffusion current of

majoritary carriers decreases (holes from p to n area and e- from n to p), and the

drift current of minoritary carriers should increase (e- from p to n area and holes

from n to p). But there are only few minoritary carriers availables (generated by

thermic generation), and this current I0 is very small, and it’s called REVERSE

SATURATION CURRENT.

VR I0 <<<<

r r r r E

p n

Jdes

J

Jdif r r r r

r r r r

r r r r

V’

driftJr

difJr

Diode current-voltage characteristic.

-0,05

0,05

0,15

-70 -20 30 80

I (m

A)

V (mV)

Io

-0,05

0,05

0,15

-70 -20 30 80

I (m

A)

V (mV)

Io I0 < µA

I0: Reverse saturation current

Vu

Vu: Diode forward voltage drop

Symbol for diode:

p area n area

Anode Cathode

Inverse of the slope (m) on high voltage region is the internal resistance of the diode (rd=1/m)

m

R

I I

ε ε

R

I

R

ε ε

R

Not taken in account neither Vu nor rd. Forward biased, the diode is a short-circuit. Reverse biased, the diode is an open circuit.

1st approaching. Ideal diode:

RI

ε=

I=0

Models of diode

Behaviour of diode can be modeled with three approachings:

V

I

voltage-current characteristic for a diode in 1st approaching

Only taken in account diode forward voltage drop:

Vu= 0.3 V for Ge diode

Vu= 0.7 V for Si diode

5.3mA1k

0.76

R

VVI u0 =

−=

−=

Vu V

I

R=1kΩ

I

V0 = 6VR=1kΩ

V0 = 6V

I Vu=0.7 V

Models of diode

2nd approaching. Simplified model:

voltage-current characteristic for a diode in 2nd approaching

R=1kΩ

ε= 6V Ird = 25Ω

Vu=0.7 V

5.4mA251000

0.76

R

VεI u =

+

−=

−=

Models of diode

3d approaching. Linear diode:

Taken in account both diode forward voltage drop as diode internal resistance.

Vu V

I

1/rd

voltage-current characteristic for a diode in 3d approaching

R=1kΩ

Iε= 6VVu=0,7 V

rd=25 Ω

Three models for junction diode

Ideal diode (1st approaching)

I

V

Simplified model (2nd approaching)

Vu

I

VVu

Linear model (3d approaching)

rd Vu

I

V

rd

Vu

Models of diode

t

I

polarizacióndirecta

polarizacióninversa

t

I

tiempo de recuperación inverso

tri

Reverse recovery time of diode

Forward biased Reverse biased

Reverse recovery time

Should be…

Is..…

t

U

~

output

t

U

t

U

~

~

Routput

Half-wave rectifier:

Full-wave rectifier:

Application: Diode as rectifier

input

input

input

outputoutput

output

Applications: logician circuits

AND and OR logic gates

A

B

R

Vs

10 V

“AND” gate with diodes

Vs

R

“OR” gate with diodes

V=10 V 1 Logic

V= 0 V 0 Logic

VA VB VS

0 (0) 0 (0) 0,7 (0)

0 (0) 10 (1) 0,7 (0)

10 (1) 0 (0) 0,7 (0)

10 (1) 10 (1) 10 (1)

VA VB VS

0 (0) 0 (0) 0 (0)

10 (1) 0 (0) 9,3 (1)

0 (0) 10 (1) 9,3 (1)

10 (1) 10 (1) 9,3 (1)

A

B

Rs

Rs R >>>Rs