Download transistor theory

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Transistors
According to Dictionary.com a transistor is:
“a semiconductor device that …switches the flow of current between two termina
ls by varying the current or voltage between one of the terminals and a third…”
For us, a transistor is a switch that allows us to
use a small current to control a larger current.
http://cen.acs.org/articles/92/web/2014/12/Record-Breaking-NanowireTransistors.html
http://learn.mikroe.com/ebooks/componentsofelect
ronicdevices/chapter/introduction-to-transistors/
Semiconductors
The most common materials for transistors are semiconductors.
What is a semiconductor?
Material that can be either a conductor or an
insulator, depending on the conditions.
What is the most common semiconductor?
Silicon (Not Silicone)
In which periodic table column is Silicon?
Column 14
How many valence electrons does it have?
4 Valence Electrons.
http://sciencenotes.org/printable-periodic-table/
Silicon
Silicon has four valence electrons in its monocrystalline
e
state.
e
When surrounded by other Silicon,
each atom has eight valence electrons.
e
In its base state, is Si an
insulator or a conductor?
Insulator
How do we make it conduct?
e
Si
e
e
Si
e
e
We break up the valence shell
How?
We add other atoms through a process known as doping.
e
e
e
Si
Si
e
e
e
e
e
Si
e
e
e
e
Doping
Doping is the intentional addition of impurities (non-SI atoms).
Column 14 elements have four valence electrons.
Which column of elements will have five?
Column 15
Which column will have three?
Column 13
The most common dopants are:
• Boron, B (3 Valence electrons)
• Phosphorus, P (5 Valence electrons)
http://sciencenotes.org/printable-periodic-table/
n-type doping
n-type doping: adding atoms to make structure more negative.
e
Column 15 elements (like P) have 5
valence electrons.
What happens when we
surround P with Si?
An extra electron is added to an
already full valence band.
e
e
e
This extra electron is
available for conduction.
Extra electrons make this Si block
negatively (n-type) doped.
e
Si
Si
e
Si
e
e
e
e
e
P
e
e
e
e
e
e
Si
e
e
e
e
p-type doping
p-type doping: adding atoms to make structure more positive.
e
Column 13 elements (like B) have 3
valence electrons.
What happens when we
surround B with Si?
The structure has one electron
fewer than what is required for a
full valence band.
This vacancy (hole) also
allows conduction to occur
These holes make this Si block
positively (p-type) doped.
e
e
e
Si
h
e
Si
e
Si
e
e
e
e
e
e
B
e
e
e
Si
e
e
e
e
Note on doping
Silicon is more than doped or not doped.
It is doped in concentrations.
Silicon doping is shown by “+” signs.
n, n+, and n++ are all increasing n-type doping.
Each + represents orders of magnitude change
p
Lightly doped
p-type
p+
Heavily doped
p-type
p++
Very heavily doped
p-type
p-n junction
We can dope a single crystal so that one part is p-type and the
other part n-type.
What happens when we place a voltage across it?
Voltage will push the charges towards each other.
At the pn junction, the electrons cross the border to recombine
with holes. This allows current to flow.
p
n
+
-
p
n
+
-
p-n junction
What happens if we apply a voltage the other way?
Voltage will again push the charges. This time away from the junction.
Electrons cannot recombine with holes. This makes an open circuit.
Open circuits allow no current to flow.
What electrical component do we have that works like this?
This is the working model for our diodes.
p
n
-
+
p
n
-
+
Recombination
What happens at the pn junction when electrons and holes meet?
Voltage pushes the electrons across the junction. They recombine with holes.
Without this voltage pushing the electrons, the junction stays at thermal equilibrium
Remember that electrons scatter due to thermal effects.
The average distance an electron travels before it combines with a
hole is the recombination distance.
The recombination distance is on the order of tens of nanometers.
p
n
Transistors
Sandwiching n-type or p-type semiconductors allows us to make
another electronic component.
These are known as transistors. They come in two types and are
named by their dopings:
n
p
n
npn
p
p
n
pnp
npn Transistors
What happens if we attach a voltage source to an npn transistor, as
shown?
Electrons and holes recombine at the left junction. No recombination occurs at the
right junction. No current can flow.
What if we attach the voltage source the other way?
We have the same problem, reversed. Current remains blocked.
n
p
n
+
n
p
n
How can we make current flow?
We start by changing the doping. We dope the emitter with orders
of magnitude more electrons.
The higher concentration of electrons wants to move to an area
with low electron concentration.
Thermal equilibrium and the base region keep this from occurring.
Emitter
n
Base
p
Collector
n
Emitter
n++
Base
p
Collector
n
How can we make current flow?
Next we shorten the width of the Base region.
If we want electrons to cross the Base without recombining with
holes, how short should it be?
Shorter than the recombination distance of an electron-hole pair.
Tens of nanometers
Emitter
n++
Base
p
Collector
n
Emitter
n++
Base Collector
p
n
How can we make current flow?
Finally, we apply a second voltage source between the Base and
the Emitter.
This attracts Emitter electrons to the Base.
As the Base is shorter than an electron-hole recombination
distance, the electrons continue on into the Collector.
Emitter
Base Collector
p
n++
-
n
Emitter
p
n++
-
+
+
Base Collector
n
+
-
+
Transistors as Switches
Current is flowing and the transistor is on.
VCE can push a large current. Without the second voltage source
VBE no current will be allowed to flow
VBE can be used as a switch to turn VCE on and off
Emitter
Base Collector
p
n++
VBE
-
n
+
-
VCE
+
Transistors and the Arduino
Transistors can be used to have a small current toggle a larger one.
Your Arduino can output ~ 40 mA of current.
Using transistors with your Arduino allows you to toggle a much
larger amperage (such as your 1.5A power supplies).
Emitter
Base Collector
p
n++
VBE
-
n
+
-
VCE
+