This is a transistor It is one of the most important devices ever to be invented. So, we're going to learn
how they work in detail in this video. What is a transistor? Transistors come in many shapes and sizes. There are two main types,
the bipolar and the field effect. We're going to mostly focus
on the bipolar version in this video. Transistors are small electronic
components with two main functions. It can act as a switch to control circuits and they can
also amplify signals. Small low power transistors are enclosed in a racing case to help
protect the internal parts. But higher power transistors will have
a partly metal case, which is used to help remove the heat which is generated as this
will damage the components over time. We usually find these metal body transistors attached to a heat sink,
which helps remove the unwanted heat. For example, inside this DC Bench power supply We have some mosfet transistors
which are attached to very large heat sinks. Without the heat sink the components quickly reach 45 degrees
Celsius or 113 degrees Fahrenheit. With a current of just 1.2A. They will become much hotter
as the current increases. But for electronic circuits with small
currents, we can just use these resin body transistors which do not require a heat sink. On the body of the transistor. We find some text. This will tell us the part number which we can use to find
the manufacturers datasheet. Each transistor is rated to handle a certain voltage and current,
so it is important to check these sheets. Now with the transistor we have
three pins labelled E, B and C. This stands for the emitter,
the base and the collector. Typically with these resin body type TRANSISTORS with a flat edge, the left pane is the emitter, the middle is the base,
and the right side is the collector. However, not all transistors
use this configuration. So do check the manufacturers datasheet. We know that if we connect a light
bulb to a battery, it will illuminate. We can install a switch into the circuit and control the light
by interrupting the power supply. But this requires a human
to manually control the switch. So how can we automate this? For that, we use a transistor. This transistor is blocking
the flow of current. So the light is off. But if we provide a small voltage
to the base pane in the middle, it causes the transistor to start allowing
current to flow in the main circuit. So the light turns on. We can then place a switch
on the controlling pin to operate it remotely or we can place a sensor
on this to automate the control. Typically, we need to apply at least 0.6V to 0.7 volts to the
base pin for the transistor to turn on. For example, this simple transistor circuit has a red LED and a nine volt
power supply across the main circuit. The base pin is connected to the DC Bench power supply The circuit
diagram looks like this. When the supply voltage to the base pin is 0.5V
the transistor is off. So the LED is also off at 0.6V the transistor is on, but not fully. The LED is dim because the transistor is
not yet letting the full current flow through the main circuit. Then at 0.7V the lead is brighter because
the transistor is letting almost the full current through. At 0.8V, the LED is at full brightness. The transistor is fully open. So what's happening is we're using a small voltage and current to control
a larger voltage and current. We saw that a small change to the voltage on the base pin causes a large
change on the main circuit. Therefore, if we input a signal to the base pin, the transistor acts as an amplifier. We could connect a microphone which varies the voltage signal on the base pin,
and this will amplify a speaker in the main circuit to form
a very basic amplifier. Typically, there is a very small current on the base pin, perhaps just
1mA or even less. The collector has a much higher current,
for example, 100mA. The ratio between these two is known as the current gain and uses
the symbol beta We can find the ratio
in the manufacturers datasheet. In this example, the collector current is 100mA and the base
current is 1mA. So the ratio is 100mA divided
by 1mA, which gives us 100. We can also rearranges formula
to find the currents also. NPN and PNP transistors We have two main types of bipolar transistors, the NPN and the PNP type,
the two transistors look nearly identical. So we need to check the part
number to tell which is which. With an NPN transistor. We have the main circuit
and the control circuit. Both are connected to the
positive of the battery. The main circuit is off until we press
the switch on the control circuit. We can see the current is flowing
through both wires to the transistor. We can remove the main circuit
and the control circuit lED will still turn on when the switch is pressed as the current is returning
to the battery through the transistor. In this simplified example, when this switch is pressed, there are 5mA flowing into the base pin. There are 20mA flowing into the collector pin and 25mA flowing out of the emitter. The current therefore combines in this
transistor With a PNP transistor. We again have the main circuit
and the control circuit, but now the emitter is connected
to the positive of the battery. The main circuit is off until we press
the switch on the control circuit. We can see with this type that some of the current flows out of the base pin and returns to the battery. The rest of the current flows through the transistor and through the main
led and then back to the battery. If we remove the main circuit, the control
circuit, LED will still turn on. In this example,
when the switch is pressed, there are 25mA flowing
into the emitter, 20mA flowing out of the collector
and 5mA flowing out of the base. The current, therefore,
divides in this transistor I'll place these side by side so
you can see how they compare. Transistors are shown on electrical drawings with symbols like these,
the arrow is placed on the emitter. The arrow points in the direction of conventional current so that we know
how to connect them into our circuits. How does a transistor work To understand how a transistor works, I want you to first imagine
water flowing through a pipe. It flows freely through the pipe
until we block it with a disc. Now, if we connect a smaller pipe
into the main one and place a swing gate within this small pipe,
we can move the disc using a pulley. The further the swing gate opens, the more water is allowed
to flow in the main pipe. The swing gate is a little heavy, so a small amount of water
won't be enough to open it. A certain amount of water is
required to force the gate to open. The more water we have flowing in this small pipe, the further the valve opens and allows more and more water
to flow in the main pipe. This is essentially how
an NPN transistor works. You might already know that when we design electronic circuits, we use conventional current. So in this NPN transistor circuit, we assume that the current is flowing
from the batteries positive into both the collector and the base pin
and then out of the emitter pin. We always use this direction
to design our circuits. However, that's not what's
actually occurring. In reality, the electrons are flowing from the negative to the
positive of the battery. This was proved by Joseph Thompson,
who carried out some experiments to discover the electron and also prove
they flowed in the opposite direction. So in reality, electrons flow from the negative
into the emitter and then out of the collector and the base pin.
We call this electron flow. I'll place the side by side so you can
see the difference in the two theories. Remember, we always design circuits
using the conventional current method. But scientists and engineers know
that electron flow is how it actually works by the way, we have also covered how a battery works in detail
in our previous video. Do you check that out links can be found in the video
description down below. OK, so we know that electricity is
the flow of electrons through a wire. The copper wire is the conductor
and the rubber is the insulator. Electrons can flow easily through the copper, but they can't flow
through the rubber insulator. If we look at this basic model of an atom of a metal conductor,
we have the nucleus at the centre and this is surrounded by a number of orbital
shells which hold the electrons. Each shell holds a maximum number of electrons,
and an electron needs to have a certain amount of energy to be
accepted into each shell. The electrons located furthest away
from the nucleus hold the most energy. The outermost shell is
known as the valence shell. A conductor has between one and three
electrons in its valence shell. The electrons are held in place by the nucleus, but there is another
shell known as the conduction band. If an electron can reach this,
then it can break free from the atom and move to other atoms.
With a metal atom such as copper. The valence shell and the conduction band overlap, so it's very easy for the
electrons to move with an insulator the outermost shell is packed. There's very little to no
room for an electron to join. The nucleus has a tight grip on the electrons and the
conduction band is far away. So the electrons can't
reach this to escape. Therefore, electricity cannot
flow through this material. However, there's another material
known as a semiconductor. Silicon is an example of a semiconductor. With this material, there's one too many electrons in the
valence shell for it to be a conductor. So it acts as an insulator. But as the conduction band is quite close, if we provide some external energy,
some electrons will gain enough energy to make the jump into the conduction
band and become free. Therefore, this material can act as
both an insulator and a conductor. Pure silicon has almost no free electrons. So what engineers do is dope the silicon with a small amount of another material
which changes its electrical properties. We call this P type and N type doping. We combine these materials
to form the PN junction. We can sandwich these together
to form an NPN or PNP transistor. Inside the transistor we have the collector pin and the emitter pin between these in an NPN transistor, we have two layers of N type
material and one layer of P type. The base wire is connected
to the P type layer in a PNP transistor this is just configured the opposite way. The entire thing is enclosed in a resin
to protect the internal materials. Let's imagine the silicon hasn't been doped yet, so it's just
pure silicon inside. Each silicon atom is surrounded
by four other silicon atoms. Each atom wants eight electrons in its valence shell but the silicon atoms only have four electrons in their valence shell, so they sneakily share an electron with their neighbouring atom
to get the 8 desire. This is known as covalent bonding. When we add the N type material such as phosphorus, it will take the position
of some of the silicon atoms. The phosphorus atoms have five
electrons in their valence shell. So as the silicon atoms are sharing
electrons to get their desired eight, they don't need this extra one,
which means there's now extra electrons in the material and these are free
to move around with P type doping we add in a material such
as aluminium. This atom has only three
electrons in this valence shell. It therefore can't provide its
four neighbours with an electron to share. So one of them will have to go without. This means a hole has been created where
an electron can sit and occupy. We now have two doped pieces of silicon, one with too many electrons
and one we not enough electrons. The two materials join to form a PN junction. At this junction we get what's known
as a depletion region in this region some of the excess electrons from the N side will move over
to occupy the holes in the P side. This migration will form a barrier with a build up of electrons
and holes on opposite sides. The electrons are negatively charged
and the holes are therefore considered positively charged, so this Build-Up causes a slightly negatively charged region and a slightly
positively charged region. This creates an electric field and prevents more electrons
from moving across. The potential difference across this region is typically around 0.7V when we connect a voltage source across the two ends with the positive
connected to the P type material. This will create a forward bias
and the electrons will begin to flow. The voltage source has to be greater
than the 0.7V barrier. Otherwise, electrons cannot make the jump when we reverse the power supply so that the positive is connected
to the N type material. The electrons held in the barrier will be
pulled back towards the positive terminal and the holes will be pulled back
towards the negative terminal. This has caused a reverse bias in a NPN transistor. We have two layers of N type material,
so we have two junctions and therefore two barriers, so no current
can flow through it ordinarily. The emitter N type material is heavily doped, so there are a lot
of excess electrons here. The base P type is lightly doped,
so there are a few holes here. The collector N type is moderately doped, so there are a few
excess electrons here. If we connect a battery across the base
and the emitter with the positive connected to the P type layer, this will create a forward bias. The forward bias causes the barrier to collapse as long as the voltage
is at least 0.7V. So the barrier diminishes and the electrons rush across to fill
the space within the P type material. Some of these electrons will occupy a hole and they will be pulled towards
the positive terminal of the battery. The P type layer is thin and was lightly doped on purpose so that there is a low
chance of electrons falling into a hole. The rest will remain free
to move around the material. Therefore, only a small current will flow out of the base pin,
leaving an excess of electrons in the pitot material if we then connect another battery between the emitter and the collector with the positive connected to the collector, the negatively charged
electrons within the collector will be drawn to the positive terminal,
which causes a reverse bias. If you remember, with the reverse bias,
the electrons and holes of the barrier are pulled back across,
so the electrons on the P type side of the barrier are pulled across
to the N type side and the holes on the N type side
are pulled back to the P type side. They are already an excess number
of electrons in the P type material. So they will move to occupy these holes
and some of them will be pulled across because the voltage of this
battery is greater. So the attraction is much higher. As these electrons are pulled across,
they flow into the battery. So a current develops across
the reverse bias junction. A higher voltage on the base pin fully
opens the transistor, which means more current and more
electrons moving into the P type layer. Therefore, more electrons are
pulled across the reverse bias. We also see more electrons flowing in the emitter side of the transistor
compared to the collector side. OK, that's it for this video, but to continue learning about electronics
engineering, click on one of the videos on screen now and I'll catch
you there for the next lesson. Don't forget to follow us on Facebook, Twitter, LinkedIn, Instagram and of
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