the invention of transistors
revolutionized human civilization like
no other technology
at the heart of a smartphone lies a
processor
and this processor holds about 2 billion
transistors
what do these incredibly tiny devices do
how do they work
transistors can act like a switch with
no moving parts
they can amplify a weak signal
in fact amplification is the basic
function of a transistor
first let's understand the basis of
transistors we will come back to the
application part later
transistors are made of semiconductors
such as silicon
each silicon atom is bonded with four
neighboring silicon atoms
silicon has four electrons in its
valence shell
let's replace this silicon atom with a
four-handed smiley
each hand holds one electron
each one of these electrons goes for
sharing with a neighboring silicon atom
this is known as a covalent bond
currently the electrons are in their
valence band
if the pure silicon has to conduct
electricity the electrons have to absorb
some energy and become free electrons
thus the pure silicon will have a low
electrical conductivity
a technique called doping is used to
improve the conductivity of
semiconductors
for example say you inject phosphorus
with five valence electrons
here one electron will be free to move
in the system this is known as n-type
doping
on the other hand if you inject boron
with three valence electrons there will
be a vacant position for an electron
this vacant position is known as a hole
and a neighboring electron can fill this
hole at any time
this electron movement is visualized as
holes moving in opposite direction
we call this p-type doping
if you dope a silicon wafer in the
following manner a transistor is born
but if you really want to understand how
a transistor works we have to get a
clear idea of what happens at the
electron level of a more basic component
a diode
a diode is formed when you dope one part
of silicon as a p-type and the other
part as an n-type
something very interesting happens at
the boundary of the n and p joint
the abundant electrons on the inside
will have a natural tendency to migrate
to the holes that are available on the p
side
this will make the p side boundary
slightly negatively charged and the end
side slightly positively charged
the resulting electric field will oppose
any further natural migration of the
electrons
if you apply an external power source
across the diode as shown the power
source will attract the electrons and
holes
electricity flow is impossible in this
case
however if you reverse the power
connection the situation is quite
different assume that the power source
has enough voltage to overcome the
potential barrier
you can immediately see that the
electrons will be pushed away by the
negative terminal when the electrons
cross the potential barrier they will be
drained of energy and will easily occupy
the holes in the p region
but due to the attraction of the
positive terminal these electrons can
now jump to the holes nearby in the p
region and flow through the external
circuit
this is known as the forward biasing of
a diode
just keep this simple principle of a
diode in mind you will understand the
operation of a transistor very easily
now back to the transistor
note that the p layer is really thin and
lightly doped
you can easily see that the transistor
is essentially two diodes sandwiched
back to back
so whichever way you connect the power
source one diode will always be reverse
biased and block the electricity flow
this means the transistor is in the off
state
now let's connect the second power
supply as shown the power supply should
have enough voltage to overcome the
potential barrier
so this is just a forward biased diode
thus a high number of electrons will be
emitted from the n region
just like in a diode a few electrons
will combine with the holes and jump
across the neighboring holes and flow to
the base
but there are a lot more electrons that
has crossed to the p side what will
these remaining electrons do
think for a moment
the remaining electrons will get
attracted by the positive terminal of
the first power source and will flow
straight as shown
note that the p region is very narrow
which ensures that no remaining
electrons flow to the positive terminal
of the second power source
in short a small base current is
amplified to a high collector current
you can easily correlate the naming of
the transistor terminal with the nature
of electron flow
if you can increase the base current the
collector current will increase
proportionally this is a clear case of
current amplification
the kind of transistor we have discussed
is called a bipolar junction transistor
let's replace this representative
transistor with a realistic one
you can further improve the
amplification by introducing one more
transistor
the base of this transistor is connected
with the emitter of the first transistor
if you introduce a weak fluctuating
signal at the input like what you would
find in a microphone you will get an
amplified signal at the loudspeaker
the other interesting thing you can note
about this basic circuit is that
depending on the value of the applied
voltage the transistor can be either on
or off
here the transistor acts as a switch
this property of the transistor opens
the doors to the world of digital
electronics and digital memory
using two bjts you can build the basic
dynamic memory element of computer a
flip flop
to know more about it please watch the
video on the transistor as a switch
please support us at patreon.com so that
we can continue our free educational
service thank you