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Chapter 2
DIODE
MENJANA MINDA KREATIF DAN INOVATIF
Introduction
Biasing pn junction
Load line concept & graphical analysis
Diode Resistance
Diode Model
Review on PN Junction
With the formation of the p
and n materials combination
of electrons and holes at the
junction takes place.
This creates the depletion
region and has a barrier
potential. This potential cannot
be measured with a voltmeter
but it will cause a small voltage
drop.
Forward Bias
Voltage source or bias connections are +
to the p material and – to the n material.
Bias must be greater than 0.3 V for
Germanium or 0.7 V for Silicon diodes.
+
-
Negative side of bias voltage ‘pushes’
The electrons moves to the external circuit
free electrons towards pn junction, and
across it into the p region, & combine with becoming conducting electrons in metal.
holes.
As more electrons move into the depletion
Positive side of voltage bias attracts the region, the number of + ions is reduced.
valence electrons toward the end of p
As more holes move into the depletion region
region, holes providing the path.
on the other side, the number of - ions is also
reduced. The depletion region narrows.
Holes appear to move towards the
junction.
Forward Bias
FIGURE :A forward-biased diode showing the flow of majority carriers and the
voltage due to the barrier potential across the depletion region.
Reverse Bias
Voltage source or bias connections are – to the
p material and + to the n material.
Bias must be less than the breakdown voltage.
Positive side of battery pulls the free
electrons, (majority in n ) away from the
junction. As electrons move away from junction,
more positive are created.
In p region, electrons from negative side of
battery enter as valence electrons.
It moves from hole to hole toward the
depletion region, creating more negative ions.
This can be viewed as holes being pulled
towards the negative side.
The depletion region widens.
The electric field increases in strength
until the potential across depletion region
equals the bias voltage.
Only a very small reverse current Exist.
Current flow is negligible in most cases.
Reverse Bias
FIGURE : The diode during the short transition time immediately after reversebias voltage is applied.
Reverse breakdown voltage
Reverse current is normally small and can be neglected. They result from
movement of minority carriers.
But if applied voltage is bigger than the breakdown voltage, the current will
increase drastically.
The high reverse biased voltage, imparts energy to the free minority
electrons so that they speed through the p region.
They collide with atoms with enough energy to knock valence electrons out
of orbit into the conduction band.
These electrons have high energy, and repeat the process, & they quickly
multiply.
They have high energy to move though pn junction, & not combine with
holes. Known as avalanche – results in high reverse current
Forward bias measurement
Current & Voltage in a
forward biased pn junction
Figure: Forward-bias measurements show general changes in VF and IF as VBIAS is increased.
Structure & Symbol
Diodes packages
LED (Light Emitting
Diode)
Signal Diode
Zener diode
Power Diode
FIGURE :Typical diode.
Diode Operating Conditions
No Bias
No external voltage is applied : VD = 0V
No current is flowing: ID = 0A
Only a modest depletion layer exists
Diode Operating Conditions
Forward Bias
External voltage is applied across the p-n
junction in the same polarity as the p-type and ntype materials.
 The forward voltage causes the depletion layer
to narrow.
The electrons and holes are pushed toward the
p-n junction.
 The electrons and holes have sufficient energy
to cross the p-n junction.
Diode Operating Conditions
Reverse Bias
External voltage is applied across the p-n
junction in the opposite polarity of the p-type
and n-type materials.
The reverse voltage causes the depletion
layer to widen.
 The electrons in the n-type material are
attracted toward the positive terminal.
 The holes in the p-type material are
attracted toward the negative terminal.
Forward bias
a) Circuit connections showing
the diode symbol.
b) V-I characteristic
Reverse Bias
a) Circuit connections showing
the diode symbol.
b) V-I characteristic
Ideal Diode equations
A fit to the I-V characteristics of a diode yields the following
equation, known as the ideal diode equation or the Shockley
equation:
VT = 26 mV when T = 300K, room temperature.
η=1 for Ge; η= 2 for Si
x
The y intercept is equal to IS.
The slope is proportional to 1/η.
When η = 1, ID increased by approximately
one order of magnitude for every 60 mV
increase in VD.
Example
Plot the I-V characteristic for a silicon diode where IS=25nA at room
temperature (25°C)
Answer
η = 2(silicon); IS=20nA; VT =26mV
If VD =0.4V,
= 0.04mA
VD(V)
0.8
0.6
0.4
0.2
0
-0.1
-1.0
-100
VD(A)
120m
2.56m
0.04m
0.0011m
0
-21.25n
-25n
-25n
Cont..
Answer
Introduction
Biasing pn junction
Load line concept & graphical analysis
Diode Resistance
Diode Model
Load Line Concept
 Simple diode circuit where ID and VD are not known.
 A simple analysis which used the diode characteristic to obtain the
Q-point (operation point)
Graphical Analysis Technique
Load Line concept
Kirchoff Voltage Law:
VD = VPS – IDR
On x-axis, ID = 0 ; => VD = VPS
On y-axis, VD = 0; => ID = VPS/R
In this case,
When ID = 0, VD = 5V and
When VD = 0, ID = (5/2k) = 2.5 mA
Graphical Analysis Technique
Load Line concept
The x intercept of the load line is the
open circuit voltage and the y intercept is
the short circuit current.
The quiescent point or Q-point is the
intersection of diode I-V characteristic
with the load line.
I-V characteristics of diode must be
known.
Q point
The x intercept of the load line is the open circuit voltage and the y
intercept is the short circuit current.
The Q-point is dependent on the power supply voltage and the resistance
of the rest of the circuit as well as on the diode I-V characteristics.
Q point
with different R & voltage applied
Load Line - Reverse Bias
The Q-point is always ID = 0 and VD = the open circuit voltage
Summary
The load line plots all possible current
(ID) conditions for all voltages applied to
the diode (VD) in a given circuit. E/R is the
maximum ID and E is the maximum VD.
Where the load line and the
characteristic curve intersect is the Qpoint, which specifies a particular ID and
VD for a given circuit.
Example 1
For the series diode configuration below, employing the diode
characteristics of figure below, determine Q point,(VDQ, IDQ)and VR.
Solution1
Step 1: Find the maximum ID.
VD = 0V→ ID = IR= E/R
Step 2: Find the maximum VD.
ID=0A → E = VD + IDR
Step 3: Plot the load line
Step 4: Find the intersection
between the load line and
the characteristic curve.
This is the Q-point
Step 5: Checking !!!!
Example 2
Find the Q point for the
circuit if the IV characteristic
as shown in the graph.
Answer
Q(0.5V,80mA)
Introduction
Biasing pn junction
Load line concept & graphical analysis
Diode Resistance
Diode Model
Effect of AC voltage on load line
As VS varies with time, the load line also changes, which changes the
Q-point (VD and ID) of the diode.
Effect of AC voltage on load line
Diode resistance
For a specific applied DC voltage VD, the diode has a specific current
ID, and a specific resistance RD
Diode resistance
In the forward bias region:
•The resistance depends on the amount of current (ID) in the diode.
•The voltage across the diode is fairly constant (26mV for25°C).
•rB ranges from a typical 0.1W for high power devices to 2W for low power,
general purpose diodes. In some cases rB can be ignored.
In the reverse bias region:
The resistance is essentially infinite. The diode acts like an open circuit.
Diode resistance
AC resistance can be determined
by selecting two points on the
characteristic curve developed for a
particular circuit.
Exist due to the semiconductor
medium itself.
Introduction
Biasing pn junction
Load line concept & graphical analysis
Diode Resistance
Diode Model
V-I Characteristic curve
1. Ideal diode model
The p-n junction only
conducts significant current
in the forward-bias region.
ID is an exponential
function in this region.
Essentially no current
flows in reverse bias.
In this characteristic curve
we do not consider the
voltage drop or the resistive
properties.
V-I Characteristic curve
2. Practical diode model
In most cases we consider only
the forward bias voltage drop of a
diode. Once this voltage is
overcome the current increases
proportionally with voltage.
 This drop is particularly
important to consider in low
voltage applications.
V-I Characteristic curve
3. Complete diode model
The voltage drop is not the only
loss of a diode.
In some cases we must take
into account other factors such
as the resistive effects as well as
reverse breakdown.
V-I Characteristic curve
ideal & practical for silicon and germanium.
ID
Ideal
- 150V
Germanium
Silicon
- 50V
0.3V
VD
0.7V
-1uA
+
Silicon knee voltage = 0.7V,
Germanium knee voltage = 0.3V
VD
ID
-
Diode Approximation
Summary
 Diodes made of semiconductor material.
 P and N type materials are joined together to form a PN junction.
 At the junction a depletion region is formed. This creates barrier that
requires approximately .3 V for a Germanium and .7 V for Silicon for
conduction to take place.