Transcript The p-n junction and applications

Problems
All problems up to p.28 Q 4 can be done.
The p-n Junction and Applications
The p-n junction
n-type
p-type
•Electrons in Conduction band
•Spaces in Valence band
•So electrons are mobile
•Positive holes are mobile
•Overall neutral
•Overall neutral
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.
p – n Junction - Band Energy Diagram
p-type
Conduction
band
n-type
Conduction
band
Valence
band
Valence
band
Depletion
zone
Applications of the p-n Junction
1. 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.
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.
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!
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.
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.
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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