1. Problems
All problems up to p.28 Q 4 can be done.

2. The p-n Junction and Applications
n-type
p-type
Electrons in Conduction band
So electrons are mobile
Overall neutral
Spaces in Valence band
Positive holes are mobile
Overall neutral

3. n p + depletion zone (layer)
Some free electrons meet up with holes at the junction – thus the region at the junction is empty (depleted) of mobile charge
Electrons entering the p – type have create a negative barrier at the p – type edge which stops an influx of more electrons
Similarly, electrons leaving the n – type have created a positive barrier at the n – type edge which stops holes.

4. p – n Junction - Band Energy Diagram
p-type
n-type
Conduction band
Conduction band
Valence band
Valence band
Depletion zone

5. Applications of the p-n Junction
p – n Junction in Forward Bias
Bias means a voltage is applied and forward means that the n side is connected to the negative terminal of the battery.

6. n p + If the pd across the junction is sufficient to force
electrons to cross the depletion zone, then electrons in
the n-type conduction band can pass into the conduction
band of the p-type material.
The electron then loses energy (or “deexcites”) so it
moves down to recombine with a hole in the valence band
of the p-type material.
When an electron recombines with a hole it loses
energy and a photon is emitted (also called
“quanta” of radiation).
This is the basis of an L.E.D.

7. 2. PHOTOVOLTAIC MODE (no bias)
n p + V
Incoming photons mobilise electrons in the depletion zone so a potential difference is set up across the p-n junction.
If a circuit is connected between its sides, a current will flow and can power a load.
This is how a solar cell works!

8. 3. PHOTOCONDUCTIVE MODE (reverse bias)
Reverse bias means that the p-type is connected to the negative side of the battery +
n p
Here the mobile charge carriers are separated further, so the depletion layer widens.
This makes a BIG resistance to current.

9. BUT …….
If we shine light on the depletion zone to mobilise electrons, then the resistance decreases.
A photodiode in photoconductive mode is the basis of a light sensor .
Problems All problems up to p30 Q.10.

10. Mode: Photoconductive Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Light sensor
Bias: Reverse Bias
Mode: Photoconductive Mode

11. Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
L.E.D.
Bias: Forward Bias
Mode: no special name

12. Mode: Photovoltaic Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Solar Cell
Bias: no bias
Mode: Photovoltaic Mode

13. Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
L.E.D.
Bias: Forward Bias
Mode: no special name

14. Mode: Photovoltaic Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Solar Cell
Bias: no bias
Mode: Photovoltaic Mode

15. Mode: Photoconductive Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Light sensor
Bias: Reverse Bias
Mode: Photoconductive Mode

16. Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
L.E.D.
Bias: Forward Bias
Mode: no special name

17. Mode: Photoconductive Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Light sensor
Bias: Reverse Bias
Mode: Photoconductive Mode

18. Mode: Photovoltaic Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Solar Cell
Bias: no bias
Mode: Photovoltaic Mode

19. Mode: Photoconductive Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Light sensor
Bias: Reverse Bias
Mode: Photoconductive Mode

20. Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
L.E.D.
Bias: Forward Bias
Mode: no special name

21. Mode: Photovoltaic Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Solar Cell
Bias: no bias
Mode: Photovoltaic Mode

22. Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
L.E.D.
Bias: Forward Bias
Mode: no special name

23. Mode: Photoconductive Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Light sensor
Bias: Reverse Bias
Mode: Photoconductive Mode

24. Mode: Photovoltaic Mode
Let’s Play!
What’s the bias voltage and “mode” – if relevant - for each of the following:
Solar Cell
Bias: no bias
Mode: Photovoltaic Mode