Handout 4
Biasing the PN Junction
„
„
„
Forward Bias
Biasing means applying a fixed Direct Current (DC)
voltage
„
The primary usefulness of a pn junction is its
ability to allow current in only one direction
„
„
„
There are two types of PN junction biasing
„ Forward Bias
„ Reverse Bias
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Forward Bias …..
„
„
Hole current
Current
limiting
Resistor
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+
„
Electron current
VB
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Forward Bias….
n-region
p-region
Permits current across the pn-junction
(-ve) terminal of the battery is connected to the n
region (Cathode)
(+ve) terminal of the battery is connected to the p
region (Anode)
The battery pushes conduction electrons in nregion towards the junction while holes in the pregion are pushed towards the junction
The external battery overcomes the barrier
potential
The n-region will have enough energy to penetrate
the depletion layer and cross the junction and
combine with p-region holes
Once the depletion region is collapsed, electrons
will flow from –ve terminal of the battery to the
+ve terminal of the battery
-
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Handout 4
Forward Bias…
N-region
P-region
Forward Bias…
Pn-junction
equivalent
Circuit symbol
Barrier
Potential
+
+
P
-
Rp
VB
N
Conventional
Current flow
Rn
+
Cathode
(K)
Anode
(A)
R
-
I
+
V
Electrons
flow
-
Battery
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Reverse Bias
‰
‰
‰
‰
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Reverse Bias…
Prevents current across the pn-junction
(+ve) terminal of the battery is connected to the n
region (Cathode)
(-ve) terminal of the battery is connected to the p
region (Anode)
The holes are attracted to the –ve terminal and
electrons are attracted to the +ve terminal of the
battery
n-region
p-region
Widened Depletion
Layer
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Handout 4
Reverse Bias…
‰
‰
‰
‰
Reverse Bias…
As electrons and holes are moved away from the
pn-junction, the depletion region widens
More +ve irons and –ve irons are produced in the
n-region and p-region respectively
The depletion layer widens until the potential
difference across is equal to the battery voltage
The depletion layer acts like an insulator
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‰
‰
‰
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Reverse Bias…
‰
‰
There is very small reverse current produced by
minority carriers due to small number of thermally
generated electron-hole pairs
The amount of reverse current depends on the
temperature and not on the reverse voltage
The reverse current is known as Saturation
Current (IS) and is in µA and nA range and it
doubles in value for every 5OC rise in temperature
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I-V Characteristic of Junction Diode
Increased Reverse-bias voltage will avalanche
breakdown of the pn-junction
„
Consider forward bias condition
IF
Most diodes are not operated in the reverse
breakdown because they are destroyed
However, there is special type of diode designed to
be operated in the reverse-breakdown, Zener
diodes
IF
(Forward diode current)
R
‰
VF
+
VB ≈ 0.7V (Si)
≈ 0.3V (Ge)
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VB
VF
(Knee voltage)
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Handout 4
I-V characteristic…
„
I-V characteristic…
The forward region I-V characteristic is approximated by exponential
function:
„
The Thermal voltage, VT is given by:
⎞
⎛ VF
I F = I s ⎜⎜ e nVT − 1⎟⎟
⎠
⎝
VT =
IF = Diode forward current
η (eta)
VF = Diode forward voltage
IS = Saturation current
VT = Thermal voltage
η is between 1 and 2 depending on the material and
physical structure of diode
Where:
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Where:
„
q = Electronic charge 1.602 x 10-19 coulomb
13
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e
»1
Therefore diode equation can be approximated by:
VF
I F = I se
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„
„
η VT
ηVT
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I-V characteristic…
The value of η for silicon is usually assumed to be 1 for VF ≥
0.5V and approached 2 as VF approaches 0:
For VF several times VT, makes:
VF
k = Boltzmann’s constant 1.38 x 10-23 J/Kelvin
T = Absolute Temp. in kelvin 273 + OC
I-V characteristic…
„
kT
q
At room temperature (20OC – 25OC), VT is 25mV to
26mV
Break down
voltage
The maximum reverse
IS
VR
voltage that a diode can
withstand without break
down is known as
Peak Inverse Voltage
(PIV)
IR
15
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Handout 4
First Approximations
Diode Approximations
„
„
„
Approximations are used in electronics to simplify analysis of
electronic circuits to avoid difficulty and time consuming
calculations
First Approximation: Ideal Diode
Forward barrier voltage is ignored (0V)
Reverse saturation current is ignored (0A)
No breakdown voltage (infinity)
VR
VB
In the forward bias the diode conducts at 0V
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IF
IF
IR
17
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First Approximation…
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Uin
+5V
Equivalent: Closed Switch
Zero voltage drop
t
0
Uin
Uout
-5V
Uout
+5V
Equivalent: Open Switch
Uin
Infinite Resistance
Uout
t
0
S ON
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First Approximation…
S
Reverse Biased Diode
VF
0
Ideal Diode
I
Forward Biased Diode
VR
VF
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S OFF
S ON
20
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Handout 4
Second Approximation
„
Second Approximation…
The forward voltage drop across the real diode
(Barrier voltage) is taken into consideration
IF
Ideal
‰
VB
‰
Vertical line
VR
VB
‰
VF
VB
(Knee
voltage)
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Second Approximation…
‰
0.7V
Uout
Uin
t
0
‰
-5V
S
0.7V
‰
Uout
+5V
4.3V
Uin
Uout
t
0
S ON
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Third Approximation
Uin
+5V
Si
The diode will conduct in the forward direction if the
external voltage exceeds the Barrier Voltage (VB)
The equivalent circuit is an ideal diode in series with
the a battery whose voltage equals the knee voltage
(0.7V Si, 0.3V Ge)
In the reverse direction, the diode is equivalent to an
open switch
S OFF
‰
Third approximation can be used to account more
accurately for diode forward voltage drop
The diode I-V characteristic is not vertical above the
knee voltage but actually slopes upwards to the right
This means that as more current flows through the
real diode, more voltage is dropped across it
Once the diode conducting, it acts like a resistor
S ON
23
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Handout 4
Third Approximation…
Third Approximation….
Change in voltage is directly proportional to
the change in current
IF
r =
ΛI
ΛV
VF
VB
∆V
∆I
The equivalent circuit is an ideal diode in series with
a battery of value VB and a resistor of value rB
Ideal
This is resistance of the semiconductor
material (p and n regions)
This resistance is called forward or bulk
resistance of the diode (rB)
(Knee
voltage)
rB =
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‰
∆V
∆I
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∆V
rB =
∆I
10
VF (V)
VB
0.80
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Estimating Bulk Resistance…
If the I-V characteristic is available,
select two points well above the knee of
the curve
(mA)
rB
It takes at least 0.3V or 0.7V to turn on the diode.
The diode then acts like a resistor of value rB which
drops additional voltage depending on the amount of
current drawn
Estimating Bulk Resistance
IF
VB
Read the forward current at 1V normally
specified in Manufacturer’s data sheet
IF
Use the knee voltage and current to
workout rB
45mA
0.8V − 0.75V 0.05V
=
=
= 1.2Ω
50mA −10mA 40mA
VB 1.0V
VF
rB =
1V − V B
45 mA − 0
For Si, VB = 0.7V, Ge, VB = 0.3V
0.75
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Handout 4
Third Approximation…
Third Approximation…
Determine the peak Uout
Use third approximation to sketch Uout if the diode is made of
germanium and it has IF of 28mA at 1V, peak Uin = 2V
RL
50 Ω
Uin
S
Uout p =
25 Ω
Uout
Uin
rB =
0.3V
1V − 0 .3V
= 25 Ω
28 mA
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=
Uout
RL
50 Ω
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Third Approximation…
+2V
‰
t
‰
-2V
Uout
‰
+1.133V
t
S ON
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( 2V − 0 .3V )50 Ω
= 1 .133V
50 Ω + 25 Ω
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Using the Diode Approximations
Uin
0
RL + rB
The diode will conduct after the Uin has reached
0.3V
Diode equivalent circuit
using 3rd Approximation
0
(Uin p − VB ) RL
Start with the ideal-diode approach to get basic idea
of how the circuit operates
If the answers obtained by the ideal-diode approach
indicate that 0.3V or 0.7V is significant, use the
second approximation
If the external resistance in series with the diode is
not large compared with the bulk resistance of the
diode, use the third approximation
S ON
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Handout 4
Diode Approximations…
‰
‰
‰
Diode Resistance Levels
The approximations we have studied are called
Large Signal Diode Approximation
It is called Large Signal approximation because the
circuits are driven by large-signal sources whose
voltages are much higher than the diode knee
voltage
Therefore the approximations are suitable whenever
the signal driving the diode is larger than the knee
voltage
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Operating point
IF
VDD
(mA)
R
⎞
⎛ VF
I F = I s ⎜⎜ e nVT − 1 ⎟⎟
⎠
⎝
Operating
point
IFQ
DC load line equation
VDD = VF + IFR
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Operating point…
IF
VF
The type of applied voltage or signal will determine
the resistance level (dc or ac voltage)
The application of dc voltage to a diode circuit will
result in an operating point that will not change with
time
The diode will give a Static or DC Resistance
DC resistance can be found by finding the
correspond levels of VF and IF
VFQ
‰
VF (V)
Diode I-V characteristic
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The values of VF and IF at operating point (VFQ, IFQ)
must satisfy the two equations i.e.
‰ Diode Equation
‰ DC load line equation
VDD = VF + IFR
From the dc load line equation:
VDD and R are constants while VF and IF can
change
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Handout 4
Operating point…
‰
Operating point…
Recall equation of a straight line:
y = ax + b where a and b are real values
y
‰
IF
(mA)
If VF = 0, IF = VDD/R
You need only 2 point
to draw a straight line
0,b
Lets look at the dc load equation:
DC load
line
If IF = 0, VF = VDD
IFQ
-b/a, 0
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Operating
Point
(VF, IF) = (0, VDD/R)
(VF, IF) = (VDD, 0)
x
VDD = VF + IFR
VFQ
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VF (V)
38
DC or Static Resistance
‰
Since the applied voltage (dc voltage) does not
change, the ratio of VFQ and IFQ will give a constant
diode resistance
RD =
‰
VFQ
I FQ
DC resistance in the reverse bias region will be quite
large
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