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Conductors – many electrons free to move
Insulators – few free electrons
Semi-conductors – insulators when pure but can be made
conductive by adding impurities (doping), stimulating with
heat, light, or a voltage
Intrinsic (pure ) semi-conductor has 4 electrons in the
outer shell all form bonds unless there is a disruption to
the crystal pattern.
Doping
p – type semiconductor
n – type semiconductor
+
Charge carrier - electron
-
+
Charge carrier - hole
-
+
-
Forward bias +
p
n
-
-
+ Reversed bias -
p
n
+
Experiment
PN Junction Diode Characteristics
Light emitting diodes
Forward biased pn junction
e
+
p
n
-
When electron hole pairs
recombine they give out
energy E = hf
If the energy is large
enough the frequency of
the light will be visible
Photodiode – reacts very quickly to light
Photovoltaic mode (forward biased)
When light shines on the junction electrons are given
enough energy to escape the atom forming electron
hole pairs. This gives a voltage across the junction.
There will be no other power source in the circuit.
Used in solar cell for calculators etc.
Photoconductive mode (reverse biased)
When light shines on it the resistance decreases as
electrons are given enough energy to escape the
atom so the number of charge carriers increases.
The current in the circuit will increase.
Current is proportional to irradiance
Transistors
NPN
b
PNP
c connected
to +Vs
b
I
+0.7 V
- 0.7V
e at 0V
c connected to
- Vs
I
e at 0V
MOSFET
drain
Metal Oxide
Semiconductor Field
Effect Transistor
gate
source
MOSFET – when the potential difference between the
source (0V) and the gate is greater than a threshold value
(usually about 2V) it allows current to flow.
source
gate
n
p-type substrate
drain
n
metal
Oxide
layer
semiconductor