1. PN Junction
Section

2. References Section 2.2-2.3. (PN junction without energy band-diagram.)
Supplemental Reference: “Modern Semiconductor Devices for Integrated Circuits”, Chenming Calvin Hu (PN junction without energy band-diagram.)

3. Applications Diodes CdS cell Zener diode
PN junction temperature sensor

4. Review

5. c02f16
What do we get by introducing n-type and p-type dopants into two adjacent sections of a piece of silicon?

6. How Do You Make a PN Junction Diode

7. Ion Implantation Accelerate the ions to high energy
Shoot the ions onto the semiconductor surface
The implanted ions displace semiconductors atoms.
A follow-up anneal (heating) of the wafer is necessary
for removing damage and for placing dopants correctly in the lattice.

8. c02f17

9. Electric Field/Voltage
Definition of Voltage: The work done in moving a unit
positive charge in an electric field.
Alternative definition: + - Vo

10. c02f19 Each e- that departs from the n side leaves
behind a positive ion.
Electrons enter the
P side and create neg.
ion.
P side is suddenly
joined with the n side
The immediate
vincinity of the junction
is depleted of free
carriers.

11. c02f20
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.

12. c02f21 Net charge =0 Net charge =0 E depends on the net charge
(P is neutral, even
though it carries 5 electrons,
one of them being a free electron.)
(B is neutral, even
though it carries 3 electrons. )
Net charge =0
Net charge =0
E depends on the net charge
included in the imaginary surface.
Extra Credit:
Derive Built in Voltage

13. 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)

14. PN Junction under Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger 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. E

15. PN Junction as a capacitor
Smaller capacitance.
(More charge separation)
Large capacitance.
(Less charge separation)
As the reverse bias increases, the width of the depletion region increases.

16. c02f25 Bias dependent capacitance. Useful in cell phone applications.

17. Photodiode Light is applied to the pn junction
Electrons are dislodged from covalent
bonds.
Electron-hole pair is created.
Electron is attracted to the positive terminal
of the battery.
Current flows through the diode is proportional
to light intensity.
Application: Digital camera.

18. 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

19. c02f28 Equilibrium Forward Biased Diode
Majority carriers cross the junction via diffusion.
Minority carriers increased on both sides of the junction.

20. c02f29 (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.

21. c02f30 ID must be constant at all points along x
In the vincinity of depletion region, the current consists
mostly of minority carriers.
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.

22. c02f31
IS=Reverse Saturation=leakage current

23. Measure Forward Biased Diode Current
Listed R1=330 Ohms, Measured R1=327.8 Ohms, % error=-0.66 %

24. Measured Value (Forward Bias)VF
(V)
IF (Computed)
0.455
30.50 uA
0.509
0.10 mA
0.551
0.26 mA
0.603
0.77 mA
0.650
2.10 mA
0.70
5.74 mA
0.748
13.8 mA

25. Measured Diode Voltage

26. On Semilog Plot

27. Application: PN Junction Temperature Sensor

28. Reverse Biased Diode
IS=Reverse Saturation=leakage current

29. Dynamic Resistance VF (V) IF (Computed) 0.70 5.74 mA 0.748 13.8 mA
( )/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms
Dynamic Resistance from the measurement:
( )/(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 V in Figure 4.

30. c02f33
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

31. Reverse Bias
Measured R2 is MOhms. % Error is about -0.3 %

32. Reverse Bias VS
(Measured) IR
(Computed) 5
3 nA 10 15

33. PN Junction Based Devices
PN Junction as a Temperature Sensor
Solar Cell (117)
Zenor Diode
LED