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PN Junction
Section 2.2-2.3
Tentative Schedule
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1/14
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Tuesday
Tuesday
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Thursday
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1/21
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Tuesday
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1/28
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Tuesday
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Thursday
Topic
Diagnostic Test
Lab protocol, cleaning procedure,
Linus/Cadence intro
Fundamental concepts from
Electric Circuits
Basic device physics
I-V characteristics of a diode
(Simulation)
Drift/Diffusion current
Physics of PN junction diode
Diode logic circuit
Applications of diodes: diode
logic/Review
Test #1 
Diode Logic
Class Canceled! 
Section
2.1
2.2-2.3
Important Dates
• 2/4: Test #1
• 2/6: Class Canceled!
Review
• Intrinsic Semiconductor
• Extrinsic Semiconductor
• Currents
• Drift Current
• Diffusion Current
Not A Whole Lot of Free Electrons at
Room Temperature
At T=0K
Electrons gain thermal
energy and break away
from the bonds. They
begin to act as “free
charge carriers”—free
electron.
Add Phosphorous to Silicon to Create an
silicon
Phosphorus has 5 valence electrons. The 5th electron is “unattached”.
This electron is free to move and serves as a charge carrier.
Add Boron to Silicon to Create a p-type
Silicon
if we dope silicon with an atom that provides an insufficient
number of electrons, then we may obtain many incomplete covalent bonds.
A boron has only 3 valence electrons and can form only 3 covalent bonds.
Therefore, it contains a hole and is ready to absorb a free electron.
Two Ways to Produce
Currents
Mechanism: Electric Field
Mechanism: Concentration
Gradient
Drift Current
Drift current is composed of the drift current due to holes and the
drift current due to electrons.
Drift current is caused by the presence of an electric field.
if charge carriers are “dropped” (injected) into a
semiconductor so as to create a nonuniform density.
Even in the absence of an electric field, the carriers
move toward regions of low concentration, thereby
carrying an electric current so long as the
nonuniformity is sustained.
Diffusion current due to Holes
Where does the – sign come from?
Diffusion Current Due to
Electron
Cathode
Anode
(n-type)
(p-type)
What do we get by introducing n-type and ptype dopants into two adjacent sections of a
piece of silicon?
IS=Reverse
Saturation=leakage current
Creation of Depletion Region
P side is suddenly
joined with the n side
Each e- that departs
from the n side leaves
behind a positive ion.
Electrons enter the
P side and create neg.
ion.
The immediate
vincinity of the junction
is depleted of free
carriers.
PN Junction Without Bias Voltage
Electric field within the depletion region points from the left to the right.
The direction of the electric field make it difficult for more free electrons
to move from the n side to the p side.
Equilibrium does not mean that there is no movement of carriers,
but instead
We have the gradient to push holes to the left.
E is there to push the drift current to the right.
Electric Field/Voltage
Definition of Voltage: The work done in moving a unit
positive charge in an electric field.
Alternative definition:
+
Caution:
You can’t use
Vo as a battery!!!
Vo
-
(P is neutral, even
though it carries 5 electrons,
one of them being a free electron.)
Net charge =0
E depends on the net charge
included in the imaginary surface.
Extra Credit:
Derive Built in Voltage
(B is neutral, even
though it carries 3 electrons. )
Net charge =0
Different ways of Crossing PN
Junction
Diffusion
Diffusion
np=ni2
Drift
Drift
Majority carriers cross the pn junction via diffusion (because you have the gradient)
Minority carriers cross the pn junction via drift( because you have the E, not the gradient)
PN Junction under Reverse
Bias Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
E
Minority carrier to cross
the PN junction easily
through drift.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative.
PN Junction as a capacitor
Smaller capacitance.
Large capacitance.
(More charge separation)
(Less charge separation)
As the reverse bias increases, the width of the depletion region increases.
Bias dependent capacitance.
Useful in cell phone applications.
c02f25
Forward Bias Diode
Depletion region shrinks due to charges from
the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side
Equilibrium
Forward Biased Diode
Majority carriers cross the junction via diffusion.
Minority carriers increased on both sides of the junction.
(gradient of minority carriers)
nn,f enters the p side as minority carriers (np,f). np,f will recombine
with the pp,f, which are abundant.
ID must be constant at
all points along x
In the vincinity of depletion region, the current consists
mostly of minority carriers because you have the gradient!
Away from the depletion region, the current consists mostly
Of majority carriers.
At each point along the x-axis, the two components add up
To Itot. (This is the bottom line)
IS=Reverse
Saturation=leakage current
Measure Forward Biased
Diode Current
Listed R1=330 Ohms, Measured R1=327.8 Ohms, % error=-0.66 %
Measured Value (Forward
Bias)
VF
(V)
0.455
0.509
0.551
0.603
0.650
0.70
0.748
IF
(Computed)
30.50 uA
0.10 mA
0.26 mA
0.77 mA
2.10 mA
5.74 mA
13.8 mA
Measured Diode Voltage
15
Measured Data
Barrier Potential is ~ 665 mV
Diode Current (mA)
12
9
6
3
0
400
440
480
520
560
600
640
Diode Voltage
680
720
760
800
Reverse Biased Diode
IS=Reverse
Saturation=leakage
current
Dynamic Resistance
VF
(V)
0.70
0.748
IF
(Computed)
5.74 mA
13.8 mA
Dynamic Resistance from the measurement:
(0.748-0.70)/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms
From the manufacture’s specification=8.33 Ohms, using data from 0.7V
and 0.725 V in Figure 4.
If VD is less than VD, On, the diode behaves like an open circuit.
The diode will behave like an open circuit for VD=VD,on
Reverse Bias
Measured R2 is 0.997 MOhms. % Error is about -0.3 %
Reverse Bias
VS
IR
(Measured) (Computed)
5
3 nA
10
15
3 nA
3 nA