Unit 12: Semiconductor devices. Diode. P-N Junction in ... 11/Slides unit 11. Diode.pdf · Diode....
Transcript of Unit 12: Semiconductor devices. Diode. P-N Junction in ... 11/Slides unit 11. Diode.pdf · Diode....
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