Unit 12: Semiconductor devices. Diode. P-N Junction in equilibrium. - - PowerPoint PPT Presentation

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

N P

Jpdrift Jndif Jpdif Jndrift

300 K 0 K

V0

• E

At room temperature, holes on p area go by diffusion towards n area, and e- from n area cross to p area (majority carriers diffusion currents).

Xp Xn Diffusion Currents

• f majority carriers

Drift Currents of minority carriers

On junction area, holes and e- are recombined, appearing a narrow depletion area (without e- and holes) with a charge density due to the ions of the impurities, negative on p area and positive on n area. So, an electric field appears, flowing drift currents of minority carriers (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.

V0 p n

• E

drift

J

• dif

J

• J

J

dif drift

= +

• xp

xn

PN junction in equilibrium. Features.

ρ qND

• qNA
• 0 +

pp0 ≈ ≈ ≈ ≈ NA nn0 ≈ ≈ ≈ ≈ ND np0 pn0 Charge carriers density distribution. Charge distribution

Xp Xn

V 0

• E

Xp Xn

n p

V 0

• E

Xp Xn

Electric field on pn junction

E

Xp 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 junction in equilibrium. Features.

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.

VD I

• E

p n

Jdes J Jdif

• drift

J

• dif

J

• V’

< V0

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

drift

J

• dif

J

• VD>V0
• E

J

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 <<<<

• E

p n

Jdes J Jdif

• V’

drift

J

• dif

J

Diode current-voltage characteristic.

• 0,05

0,05 0,15

• 70
• 20

30 80 I (mA) V (mV)

Io

• 0,05

0,05 0,15

• 70
• 20

30 80 I (mA) 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:

R I 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

5.3mA 1k 0.7 6 R V V I

u

= − = − =

V

u

V I

R=1kΩ I

V0 = 6V

R=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 I

rd = 25Ω Vu=0.7 V

u

ε V 6 0.7 I 5.2mA R 1000 25 − − = = = +

Models of diode

3d approaching. Linear diode:

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

V

u

V I

1/rd

voltage-current characteristic for a diode in 3d approaching

R=1kΩ I ε= 6V

Vu=0,7 V rd=25 Ω

Three models for junction diode Ideal diode (1st approaching) I V Simplified model (2nd approaching) Vu I V Vu Linear model (3d approaching) rd Vu I V rd Vu

Models of diode

t I polarización directa polarización inversa t I tiempo de recuperación inverso tri

Reverse recovery time of diode

Forward biased Reverse biased Reverse recovery time

Should be… Is..…

t U

~

• utput

t U t U

~ ~

Routput Half-wave rectifier: Full-wave rectifier:

Application: Diode as rectifier

input input input

• utput
• utput
• utput

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