1. More Applications of PN Junction
By D. P. Narsale

2. PN Junction Based Devices
Energy Band Diagram
Zener Diode
Light to current conversion
Solar Cell
Photodiode
Current to light conversion
LED

3. Energy Band Diagram Conduct current
Electrons tend to fill up the low energy bands first.
Note: A totally filled band can not conduct current, just as the water
In a totally filled bottle does not slosh about.

4. Photoconductor
When light is absorbed by a semiconductor sample and electron-hole
pairs are created, the number of electrons and holes increase
in proportion to the light intensity.
By putting two electrodes on the semiconductor and applying a voltage
between the electrodes, we can measure the change in resistance
and detect changes in light intensity.

5. Example: CdS Cell
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

6. Energy Band Diagram of a Semiconductor Under an Applied Voltage
Electrons roll downhill like stones.
Holes float up like bubles!

7. Energy band
Diagram of
A pn junction

8. 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

9. Energy Band Diagram of a Reverse Biased PN Junctionnp pn np
Stronger E field in the
depletion region pn

10. c02f31 IS=Reverse Saturation=leakage current
Small since np and pn are small.

11. c02f32
The diode current is proportional to area.

12. Complete I-V Characteristic
If the current is limited to a reasonable value
by the external circuit so that the heat dissipation
is not excessive, the PN junction can operate
in the breakdown region safely.

13. Tunneling(Zener) Breakdown
P side
N side
Only a small distance separates the large
number electrons on the P-side valence band and
the empty states in the N-side conduction band.
Tunneling of electrons can occur
Appear for reverse voltage from 3-8V.

14. Avalanche Breakdown
(P side)
(N-side)

15. Application: Zener Diode
A Zener diode is a PN junction diode designed to operate in the breakdwon
Region with a breakdown voltage that is tightly controlled by a manufacturer.

16. Forward Biased Diode

17. Graphical Illustration

18. Solar Cell
A solar cell is structurally identical to a PN junction. The only difference is that it is transparent on one side to allow light reaching the PN junction.

19. Solar Cell
When light shines on the PN junction, the minority carriers that are generate by light close to the junction can diffuse to the junction and be swept across the junction by the built-in E field and cause a current to flow out of the P terminal through the external short circuit and back into the N-terminal. This is current is called short circuit current , ISC. Note the direction of ISC.

20. The total diode (solar cell ) current is the sum of the current generated
by the voltage and that generated by light.
Generated by light
PN junction

21. Photodiode
A reverse-biased PN junction is called a photodiode. Photodiode
are used for light-sensing applications.

22. Example of a Photodiode Circuit
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.

23. Light Emitting Diode
When the injected minority carriers recombine with the majority carriers, photons are emitted.

24. Direct and Indirect Semiconductor
k, the wave vector, represents the direction and the wavelength of the electron.
To be an efficient LED, the electrons and the holes need to have
matching k, just so that they can recombine easily.

25. Optoelectronic Devices