Diodes Rectifiers - Notes

 Page 1


1
Page 2


1 2
Schottky Barrier Diode
One semiconductor region of the 
pn junction diode can be 
replaced by a non-ohmic
rectifying metal contact.A 
Schottky contact is easily formed 
on n-type silicon.  The metal 
region becomes the anode. An n
+
region is added to ensure that the 
cathode contact is ohmic.
Schottky diodes turn on at a 
lower voltage than pn junction 
diodes and have significantly 
reduced internal charge storage 
under forward bias.
Page 3


1 2
Schottky Barrier Diode
One semiconductor region of the 
pn junction diode can be 
replaced by a non-ohmic
rectifying metal contact.A 
Schottky contact is easily formed 
on n-type silicon.  The metal 
region becomes the anode. An n
+
region is added to ensure that the 
cathode contact is ohmic.
Schottky diodes turn on at a 
lower voltage than pn junction 
diodes and have significantly 
reduced internal charge storage 
under forward bias.
3
Page 4


1 2
Schottky Barrier Diode
One semiconductor region of the 
pn junction diode can be 
replaced by a non-ohmic
rectifying metal contact.A 
Schottky contact is easily formed 
on n-type silicon.  The metal 
region becomes the anode. An n
+
region is added to ensure that the 
cathode contact is ohmic.
Schottky diodes turn on at a 
lower voltage than pn junction 
diodes and have significantly 
reduced internal charge storage 
under forward bias.
3 4
Capacitance diode
Diodes can be designed with hyper-abrupt doping profiles 
that optimize the reverse-biased diode as a voltage 
controlled capacitor. 
Circuit symbol for the variable 
capacitance diode (Varactor)
Chap 3 -14
Page 5


1 2
Schottky Barrier Diode
One semiconductor region of the 
pn junction diode can be 
replaced by a non-ohmic
rectifying metal contact.A 
Schottky contact is easily formed 
on n-type silicon.  The metal 
region becomes the anode. An n
+
region is added to ensure that the 
cathode contact is ohmic.
Schottky diodes turn on at a 
lower voltage than pn junction 
diodes and have significantly 
reduced internal charge storage 
under forward bias.
3 4
Capacitance diode
Diodes can be designed with hyper-abrupt doping profiles 
that optimize the reverse-biased diode as a voltage 
controlled capacitor. 
Circuit symbol for the variable 
capacitance diode (Varactor)
Chap 3 -14
5
Reverse Bias
External reverse bias adds to the built-in potential of the pn
junction.  The shaded regions below illustrate the increase in 
the characteristics of the space charge region due to an 
externally applied reverse bias, v
D
.
Reverse Bias (cont.)
External reverse bias also increases the width of the 
depletion region since the larger electric field must be 
supported by additional charge.
w
d
= (x
n
+x
p
) =
2e
s
q
1
N
A
+
1
N
D
? 
? 
? 
? 
? 
? f
j
+v
R ()
where w
d 0
= (x
n
+x
p
) =
2e
s
q
1
N
A
+
1
N
D
? 
? 
? 
? 
? 
? 
f
j
w
d
=w
d 0
1+
v
R
f
j
Reverse Bias Saturation Current
We earlier assumed that the reverse saturation current was 
constant.  Since it results from thermal generation of 
electron-hole pairs in the depletion region, it is dependent on 
the volume of the space charge region.  It can be shown that 
the reverse saturation gradually increases with increased 
reverse bias.
I
S
= I
S 0
1 +
v
R
f
j
I
S
is approximately constant at I
S0
under forward bias.