JAES Learning

Jaes Sponsor - Basket

How do transistors work?

TRANSISTORS are semiconductor devices widely used both in analog and digital electronics.
They are at the basis of modern electronics and are essential for the control of many circuits or entire processors.

The CPU (Central Processing Unit) or main processor is the core of the Computers elaboration. Only this element is made up of about 5 million transistors in a single Chip.
The transistor is therefore at the same time a very small and ubiquitous electronic component in the Information Technology field.

JAES in its catalog offers a wide selection of transistors from the major manufacturers.

What is a transistor? And what is it for?

A transistor can operate in digital logic, as a elementary switch, or it can be used in the analog environment to amplify a weak input signal into a stronger output signal.

In this video we will focus on the bipolar junction transistor and on its applications, which as you can see, has 3 terminals, namely: the EMITTER, the COLLECTOR and the BASE.

There are two types of bipolar transistors: “NPN"s and “PNP”s. This difference is not only related to the use of opposite polarities for the three electrodes (collector, base and emitter), but allows to obtain a symmetrical operation, very useful in many circuits.

As in the case of the diode, the transistor can also be made of silicon, or in general of semiconductor.

If we enlarge the view in the crystal lattice we note that each silicon atom is bound with 4 other contiguous silicon atoms.

In its outer atomic shell, silicon has 4 electrons, which are called: valence electrons.
Each of these electrons can be shared with an nearby atom, thus creating the well-known COVALENT BOND.
Atoms make this link to make their electronic configuration as stable as possible and thus obtain an electronic state at a lower possible energy level.

At the moment the atoms are in their valence shell.
When the silicon needs to conduct electricity, the electrons will absorb part of the energy to counteract the covalent bond and thus become free electrons.
In this situation, however, pure silicon will have low electrical conductivity.

For this reason, a technique called doping comes into play.
We had already heard about this technique in our previous video about the working principles of diodes, when we explained that small amounts of impurity atoms were added to its silicon lattice to modify the electronic properties of silicon, this so-called donor atoms, which share their own electrons or holes with silicon lattice and not belonging to any covalent bond.

In this case, we consider for example a phosphorus atom that replaces a silicon atom into the lattice.
Unlike silicon, the phosphorus atom has 5 electrons in its outer shell. And for this reason an electron will not be interested in the covalent bond and will be free to move within the lattice.
This is known as type “N” doping.

On the other hand, if a boron atom is inserted into the silicon lattice that has only 3 valence electrons in its shell, a free space will be created for an electron, this space is known as “hole” and neighboring electrons can fill these spaces freely.
This is known as “P” type doping.

Combining these doping techniques on the silicon we obtain a transistor.

To fully understand the transistor function it is useful to remember what happens electronically within another elementary electronic device: the diode.

In our previous video about the diode, we explained how the silicon inside it is doped so as to obtain two distinct parts with two different levels of charge distribution, namely the PN junction. The majority of electrons in one side and the majority of holes available in the other, cause the natural displacement of electrons in abundance on the N side towards the holes available on the P side.

In this situation the border region of the P side is slightly negatively charged and the border region of the N side is slightly positively charged. The consequent formation of a barrier potential, prevents a further migration of electrons from N side to P side.
By connecting the diode’s cathode to the positive pole and the anode to the negative pole of a battery, an INVERSE BIAS condition is obtained, therefore the electrons and the holes are attracted in order to polarize the PN junction and consequently block the current flow by increasing the depletion region.

By reversing the battery poles connection a FORWARD BIAS condition is created, and the depletion region narrows.

Using a battery with a higher voltage than the barrier potential, the electrons, not meeting more resistance, are able to cross the barrier and occupy the holes available on the P side. Thanks to the attraction of the positive pole of the battery, they continue to occupy the subsequent holes and thus flow out of the diode through the electrical circuit.

This condition is known as direct polarization of the diode. The condition of direct diode polarization allows us to understand in a simpler way the transistor’s operation principle.

We can easily notice the P layer in the transistor is much thinner and slightly doped and is between two larger N layers.

We can say that this transistor is substantially formed by the union of two diodes, connected one to each other in a back to back configuration.

So, in any way we connect the battery, one of the two diodes will always be in an INVERSE BIAS condition, by increasing its depletion region one diode will always block the current flow. By blocking the current flow the transistor is turned off.

If we try to connect a battery as shown, with enough voltage to overcome the barrier potential, we get a diode in a forward bias condition.
Also in this case a significant amount of electrons will migrate from the N side to occupy the holes of the P side.

Just as in a diode the electrons, once the first holes are occupied, will continue to occupy the next holes thanks to the attraction of the positive pole of the battery, thus allowing electric current to flow through the electric circuit.

However, a lot of more electrons will move from N side to P side. The P side will have an excess of electrons. These electrons will be attracted by the positive pole of the first battery and will flow in this direction.

The narrow dimensions of the P side ensure that no excess electron inside it flows to the positive terminal of the second battery.

In short, a low base current is amplified to a high collector current.
If the base current is increased, the collector current will increase proportionally, thus demonstrating a clear example of signal amplification.

The type of transistors we’ve talked about so far is called bipolar junction transistor.
Let’s try to replace this transistor with a real bipolar junction transistor.
It is possible to further improve signal amplification by adding more than one transistor.

The base terminal of this transistor is connected to the emitter terminal of the first transistor.
If you introduce a weak floating signal like the one in a microphone, you will get an amplified signal at the speaker output.

A noteworthy aspect in these simple circuits is the fact that depending on the applied voltage value, the transistor can be turned on or off. In this condition, the transistor behaves as a switch.

This transistor’s feature is generally very much used in electronics and specifically in digital memory.

By using two bipolar junction transistors it’s possible to create a simple dynamic memory for computer or a sequential circuit.

Finnally, if you found this video useful let us know by leaving a comment and subscribing to our Youtube channel to stay tuned on the release of new videos. Thanks for watching!