We had a lot basic components videos on this channel. But I’ve realized that we never
talked only about the BJT transistors. I have a full video about diodes, which I recommend
you to watch, because it will help you understand this video. And why is that? Well, as you
can learn in that video a diode is basically a PN junction and if you put a PN junction
together with another PN junction you get a PNP or a NPN transistor which are called
BJT standing for bipolar junction transistors. In another previous vide we’ve talked about
the differences between transistors such as the MOSFET, IGBT and the BJT but today we
will go in depth only about BJTs, so let’s get started.
What’s up my Friends, welcome back. These here are two of the most common used by makers
BJT transistors, the BD 139 and the BD140. One is a NPN transistor and the other one
is a PNP type and together they are both BJT transistors. Any BJT has 3 pins and these
are labeled as E B and C. That stands for Emitter, Base and the Collector. But you can
find these transistors in all sort of shapes. This one is the TO220 but we also have the
TO92, the SOT23 for SMD transistors and we can also get to bigger sizes or metal cover
for some more powerful components. But they all have 3 pins. Now let’s see how they
work from inside. ANIMATION. From my past diodes episode, we know these are made out
of silicon. Silicon by itself has no free electrons so is not conductive, meaning no
current could flow trough it. That’s because the molecular structure of silicone has 4
electrons, and each atom will share these 4 electrons with their neighbors creating
a stable crystal formation where no electron could flow because everything is already fixed
in place. That’s why foundries will dope the silicone with another material, such as
for example phosphor or boron which have 5 or 3 electrons. This doping process will chance
the material properties and will create a N type doped silicone or an P type doped silicone.
N type will have more free electrons that could flow around, and the P type material
will have the so-called holes, meaning it could move electrons around and fill these
holes. For more details about this process, please see my diodes episode where I go in
to more details about the internal structure, the depletion zone and more. When we merge
together these two types of silicon, they form a PN junction and in the middle is the
so-called depletion zone. Depending on the order, we can arrange these to be a NPN or
PNP transistor. Now this depletion zone is important. The extra electrons from the N
type will jump and fill the holes of the P type and that will create a stable zone once
again, and in this moment, the transistor is not conductive since we have these barriers.
What we want is to make a current flow from the collector to the emitter so let’s imagine
this example. We want to turn ON this light. Right now, current is not flowing trough the
transistor since we have those barriers. But when I apply a small voltage to the base,
between the emitter and the base, we have a diode in direct polarization and a small
current will start flowing like this. But now this middle zone is not stable anymore
so the other electrons will take this chance and start flowing as well. But have in mind
that this process will only happen because the P type semiconductor in the middle is
very narrow. Placing two diodes in series won’t create this effect. So guys, that’s
how we basically create a switch. Using a small current at the base, we can enable a
bigger current flow between the collector and emitter. That’s why we can also use
the transistor as amplifiers. We apply a small current and we can get a bigger current at
the output with the same shape but bigger amplitude. To understand why we have two types
of BJT transistors you first have to know there are 4 different states that a BJT could
work in. First state is cut off where obviously, the base voltage is smaller than the emitter
and collector voltage so the transistor is OFF and no current is flowing. The seconds
state is forward active. In this mode we start increasing the current at the base and according
to that value a bigger current will flow between the collector and emitter. This current value
is equal to the base current multiplied by a gain factor, and we can know this gain factor
by checking the datasheet of the manufacturer. Then we reach the third state called saturation.
In this state the base current has its maximum value so the transistor is fully open so we
can’t use it as an amplifier anymore since it will act just as a copper cable, allowing
all the current to flow. Finally, the fourth state is reverse active
which is basically the same as the second state but the current flow is reversed. This
process is possible since a transistor is symmetric. If you flip an NPN is still an
NPN. This is not entirely true. Yes, we have the same N type on both sides, but during
manufacturing, these are doped with a different amount of free electrons that’s why the
NPN is better of conducting zeros and the PNP type is better for conducting ones. Let
me explain. Usually, we use an PNP transistor connected to the positive supply voltage,
which in digital is known as a 1, and the load is connected after the transistor. Applying
a voltage at the base the current will flow like this. With NPN we do it backwards, We
connect the load to the positive supply and the transistor is connected between the load
and ground, which in digital is known as a 0. Applying a voltage at the base the current
will flow like this. We use these icons for NPN or PNP transistors, and the small arrow
on the emitter will point the direction of the conventional current. In this way we know
how to connect them on our schematics. Finally, let’s make a quick list of why transistors
are so good. They work as a switch but they have no moving parts. That means they will
last a lot longer and we can switch them millions of times. They are very, very, small, so we
can pack a lot of them in a very small area. That’s how we create the microprocessors
and other digital components. Being so small will also result into more resistance to vibrations.
We can also use these as a switch for some very fast signals so they are perfect for
high frequency audio signals for example. Finally, the materials used to make transistors
are very cheap. The machine we use to manufacture these components are very expensive and precise,
but the silicon is actually not that expensive. Now you should know how a BJT transistor works
internally, why we have a NPN and a PNP type and how to connect each in your circuit. I
hope you have learned something new and if so, consider giving me a like or comment below.
Thanks again and see you later guys.