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EMT111
CHAPTER 1
Introduction to Semiconductor
By
Pn. ‘Aini Syuhada Md Zain
Introduction to Semiconductor Chapter Outline :
1.8 Voltage Current Characteristic of a Diode
1.9 Diode Models
1.10 Testing a Diode
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
-When a forward bias voltage is
applied – current called forward
current, I
F
-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
FIGURE 1-26 Forward-bias measurements
show general changes in VF and IF as VBIAS is
increased.
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
-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 1-26 Forward-bias measurements
show general changes in VF and IF as VBIAS is
increased.
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
-Plot the result of
measurement in Figure 126, you get the V-I
characteristic curve for a
forward bias diode
- VF Increase to the right
- I F increase upward
dynamic resistance r’d decreases as you move up the
curve
zero
bias
VF  0.7V
VF  0.7V
r ' d  VF / I F
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for Reverse bias)
Breakdown
voltage
-not a normal
operation of pn
junction devices
- the value can be
vary for typical Si
Reverse
Current
1.8
Voltage-Current Characteristic of a Diode
( Complete V-I Characteristic curve)
Combine-Forward bias
& Reverse bias  Complete
V-I characteristic curve
1.8
Voltage-Current Characteristic of a Diode
( Temperature effect on the diode V-I Characteristic)
•
Forward biased
dioed : T , I F 
for a given value
of VF
•
For a given I F ,VF 
•
Barrier potential
decrease as T
increase
•
Reverse current
breakdown –
small & can be
neglected
1.9
Diode Models
( Diode structure and symbol)
anod
cathode
Directional of current
1.9
Diode Models
The Ideal
Diode Model
The Practical
Diode Model
DIODE
MODEL
The Complete
Diode Model
1.9
Diode Models
( The ideal Diode model)
Ideal model of diodesimple switch:
•Closed (on) switch -> FB
•Open (off) switch -> RB
•Assume V  0V
F
•Forward
current, by
Ohm’s law
IF 
VBIAS
(1-2)
RLIMIT
IR  0A
VR  VBIAS
1.9
Diode Models ( The Practical Diode model)
•Adds the barrier potential
to the ideal switch model
• r ' ‘d is neglected
•From figure (c):VF  0.7V ( Si)
VF  0.3V (Ge)
The forward current [by
applying Kirchhoff’s voltage
low to figure (a)]
VBIAS  VF  VRLIMIT  0
VRLIMIT  I F RLIMIT
•Represent by VF
produced across the pn
junction
Ohm’s Law
VBIAS  VF
IF 
RLIMIT
•Equivalent to close
switch in series with a
small equivalent voltage
source equal to the barrier
potential 0.7V
(1-3)
•Same as ideal diode
model
IR  0A
VR  VBIAS
1.9
Diode Models ( The Complete Diode model)
Complete model of diode
consists:
•Barrier potential
•Dynamic resistance, r ' d
•Internal reverse resistance, r ' R
•The forward voltage:
VF  0.7V  I F rd'
(1-4)
•The forward current:
IF 
VBIAS  0.7V
RLIMIT  rd'
(1-5)
•acts as closed switch in
series with barrier
potential and small r ' d
•acts as open
switch in parallel
with the large r ' R
1.9
Diode Models ( Example)
(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’ = 10 at the determined value of forward
current.
1.0kΩ
1.0kΩ
10V
5V
1.9
a)
Diode Models ( Example)
Ideal Model: VF  0
V
10V
I F  BIAS 
 10mA
R
1000
VRLIMIT  I F  RLIMIT  (10 10 3 A)(1103 )  10V
b) Practical Model: VF  0.7V
IF 
(c) Complete model:
(VBIAS  VF ) 10V  0.7V

 9.3mA
RLIMIT
1000
VRLIMIT  I F  RLIMIT  (9.3 10 3 A)(1103 )  9.3V
IF 
VBIAS  0.7V 10V  0.7V

 9.21mA
'
RLIMIT  rd
1k  10
VF  0.7V  I F rd'  0.7V  (9.21mA)(10)  792mV
VRLIMIT  I F RLIMIT  (9.21mA)(1k)  9.21V
1.9
Diode Models ( Typical Diodes)
Diodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in.
1.10
Testing A Diodes ( By Digital multimeter)
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.
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.
K A
A K
1.10
Testing A Diodes ( By Digital multimeter)
Defective Diode
1.10
Testing A Diodes ( By Analog multimeter – ohm
function )
Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100
ohm)
Reverse-bias:
get high reading (0 or infinity)
Summary
 Diodes, transistors, and integrated circuits are
all made of semiconductor material.
 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.
Summary
 A diode conducts when forward biased and does not
conduct when reverse biased
 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.