- A diode allows current to flow in only one direction, exhibiting different characteristics and behaviors under forward and reverse bias. When forward biased, current flows easily but requires a minimum voltage of around 0.7V. When reverse biased, very little current flows. - There are three models used to analyze a diode: the ideal model, which assumes it acts as a closed switch under forward bias; the practical model, which adds a barrier potential and small resistance; and the complete model, which also includes a dynamic resistance. - Diodes can be tested using a multimeter to check the forward and reverse bias voltages are around 0.7V and close to the supply voltage respectively. This confirms the diode's functionality and polarity.

1. SEMICONDUCTOR
BASICS
ByBy
ROHIT KUMAR (15EL30)ROHIT KUMAR (15EL30)

2. Introduction to Semiconductor -Introduction to Semiconductor -
Chapter Outline :Chapter Outline :
1.81.8 Voltage Current Characteristic of a DiodeVoltage Current Characteristic of a Diode
1.91.9 Diode ModelsDiode Models
1.101.10 Testing a DiodeTesting a Diode

3. -When a forward bias voltage is
applied – current called forward
current,
-In this case with the voltage
applied is less than the barrier
potential so the diode for all
practical purposes is still in a
non-conducting state. Current is
very small.
-Increase forward bias voltage –
current also increase
FI
FIGURE 1-26FIGURE 1-26 Forward-bias measurementsForward-bias measurements
show general changes in Vshow general changes in VFF and Iand IFF as Vas VBIASBIAS isis
increased.increased.
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

4. -With the applied voltage
exceeding the barrier
potential (0.7V), forward
current begins increasing
rapidly.
-But the voltage across the
diode increase only above
0.7 V.
FIGURE 1-26FIGURE 1-26 Forward-bias measurementsForward-bias measurements
show general changes in Vshow general changes in VFF and Iand IFF as Vas VBIASBIAS isis
increased.increased.
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

5. -Plot the result of
measurement in Figure 1-
26, you get the V-I
characteristic curve for a
forward bias diode
- Increase to the right
- increase upward
FFd IVr ∆∆= /'
dynamic resistancedynamic resistance r’r’dd decreases as you move up thedecreases as you move up the
curvecurve
FV
FI
VVF 7.0<
zero
bias
VVF 7.0≈
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

6. Reverse
Current
Breakdown
voltage
-not a normal
operation of pn
junction devices
- the value can be
vary for typical Si
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( V-I Characteristic for Reverse bias)( V-I Characteristic for Reverse bias)

7. Combine-Forward bias
& Reverse bias  Complete
V-I characteristic curve
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( Complete V-I Characteristic curve)( Complete V-I Characteristic curve)

8. • Forward biased
dioed :
for a given value
of
• For a given
• Barrier potential
decrease as T
increase
• Reverse current
breakdown –
small & can be
neglected
↑↑ FIT ,
FV
↑FF VI ,
1.81.8 Voltage-Current Characteristic of a DiodeVoltage-Current Characteristic of a Diode
( Temperature effect on the diode V-I Characteristic)( Temperature effect on the diode V-I Characteristic)

9. Directional of current
cathodeanod
1.91.9 Diode ModelsDiode Models
( Diode structure and symbol)( Diode structure and symbol)

10. DIODE
MODEL
The Ideal
Diode Model
The Complete
Diode Model
The Practical
Diode Model
1.91.9 Diode ModelsDiode Models

11. •Assume
•Forward
current, by
Ohm’s law
Ideal model of diode-
simple switch:
•Closed (on) switch -> FB
•Open (off) switch -> RB
VVF 0=
LIMIT
BIAS
F
R
V
I =
BIASR
R
VV
AI
=
= 0
(1-2)
1.91.9 Diode ModelsDiode Models
( The ideal Diode model)( The ideal Diode model)

12. •Adds the barrier potential to
the ideal switch model
• ‘ is neglected
•From figure (c):
The forward current [by
applying Kirchhoff’s voltage
low to figure (a)]
Ohm’s Law
dr'
•Equivalent to close
switch in series with a
small equivalent voltage
source equal to the barrier
potential 0.7V
•Represent by
produced across the pn
junction
FV
•Same as ideal diode
model
)(3.0
)(7.0
GeVV
SiVV
F
F
=
=
0=−− LIMITRFBIAS VVV
LIMITFR RIV LIMIT
=
LIMIT
FBIAS
F
R
VV
I
−
=
BIASR
R
VV
AI
=
= 0
(1-3)
1.91.9 Diode ModelsDiode Models ( The Practical Diode model)( The Practical Diode model)

13. Complete model of diode
consists:
•Barrier potential
•Dynamic resistance,
•Internal reverse resistance,
•The forward voltage:
•The forward current:
dr'
Rr'
•acts as closed switch in
series with barrier
potential and small dr'
Rr'
•acts as open
switch in parallel
with the large
'
7.0 dFF rIVV +=
'
7.0
dLIMIT
BIAS
F
rR
VV
I
+
−
=
(1-4)
(1-5)
1.91.9 Diode ModelsDiode Models ( The Complete Diode model)( The Complete Diode model)

14. 10V10V
1.0kΩ1.0kΩ
5V5V
1.0kΩ1.0kΩ
(1) Determine the forward voltage and forward current
[forward bias] for each of the diode model also find the
voltage across the limiting resistor in each cases.
Assumed rd’ = 10Ω at the determined value of forward
current.
1.91.9 Diode ModelsDiode Models ( Example)( Example)

15. a)a) Ideal ModelIdeal Model::
b)b) Practical ModelPractical Model::
(c)(c) Complete model:Complete model:
VARIV
mA
V
R
V
I
V
LIMITFR
BIAS
F
F
LIMIT
10)101)(1010(
10
1000
10
0
33
=Ω××=×=
=
Ω
==
=
−
VARIV
mA
VV
R
VV
I
VV
LIMITFR
LIMIT
FBIAS
F
F
LIMIT
3.9)101)(103.9(
3.9
1000
7.010)(
7.0
33
=Ω××=×=
=
Ω
−
=
−
=
=
−
VkmARIV
mVmAVrIVV
mA
k
VV
rR
VV
I
LIMITFR
dFF
dLIMIT
BIAS
F
LIMIT
21.9)1)(21.9(
792)10)(21.9(7.07.0
21.9
101
7.0107.0
'
'
=Ω==
=Ω+=+=
=
Ω+Ω
−
=
+
−
=
1.91.9 Diode ModelsDiode Models ( Example)( Example)

16. Diodes come in a variety of sizes and shapes. The design and structure isDiodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in.determined by what type of circuit they will be used in.
1.91.9 Diode ModelsDiode Models ( Typical Diodes)( Typical Diodes)

17. Testing a diode is quite simple, particularly if the multimeter
used has a diode check function. With the diode check function
a specific known voltage is applied from the meter across the
diode.
K A A K
With the diode check
function a good diode will
show approximately .7 V or .
3 V when forward biased.
When checking in reverse
bias the full applied testing
voltage will be seen on the
display.
1.101.10 Testing A DiodesTesting A Diodes ( By Digital multimeter)( By Digital multimeter)

18. Defective Diode
1.101.10 Testing A DiodesTesting A Diodes ( By Digital multimeter)( By Digital multimeter)

19. Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100
ohm)
Reverse-bias:
get high reading (0 or infinity)
1.101.10 Testing A DiodesTesting A Diodes ( By Analog multimeter – ohm( By Analog multimeter – ohm
function )function )

20.  P-materials are doped with trivalent impurities
 N-materials are doped with pentavalent impurities
 P and N type materials are joined together to form a
PN junction.
 A diode is nothing more than a PN junction.
 At the junction a depletion region is formed. This
creates barrier which requires approximately .3 V for a
Germanium and .7 V for Silicon for conduction to take
place.
 Diodes, transistors, and integrated circuits are
all made of semiconductor material.
SummarySummary

21.  When reversed biased a diode can only withstand
so much applied voltage. The voltage at which
avalanche current occurs is called reverse breakdown
voltage.
 There are three ways of analyzing a diode. These
are ideal, practical, and complex. Typically we use a
practical diode model.
 A diode conducts when forward biased and does not
conduct when reverse biased
SummarySummary