Electronics surround us everywhere. But almost no one thinks about how this whole thing works. It's actually quite simple. This is exactly what we will try to show today. Let's start with such an important element as the transistor. We'll tell you what it is, what it does, and how the transistor works.
Transistor– a semiconductor device designed to control electric current.
Where are transistors used? Yes everywhere! Almost no modern electrical circuit can do without transistors. They are widely used in the production of computer equipment, audio and video equipment.
Times when Soviet microcircuits were the largest in the world, have passed, and the size of modern transistors is very small. Thus, the smallest devices are on the order of a nanometer in size!
Console nano- denotes a value of the order of ten to the minus ninth power.
However, there are also giant specimens that are used primarily in the fields of energy and industry.
There are different types of transistors: bipolar and polar, direct and reverse conduction. However, the operation of these devices is based on the same principle. A transistor is a semiconductor device. As is known, in a semiconductor the charge carriers are electrons or holes.
The region with excess electrons is indicated by the letter n(negative), and the region with hole conductivity is p(positive).
To make everything very clear, let's look at the work bipolar transistor (the most popular type).
(hereinafter referred to simply as a transistor) is a semiconductor crystal (most often used silicon or germanium), divided into three zones with different electrical conductivities. The zones are named accordingly collector, base And emitter. The device of the transistor and its schematic representation are shown in the figure below
Separate forward and reverse conduction transistors. P-n-p transistors are called forward conduction transistors, and n-p-n transistors are called reverse conduction transistors.
Now let's talk about the two operating modes of transistors. The operation of the transistor itself is similar to the operation of a water tap or valve. Only instead of water there is electric current. There are two possible states of the transistor - operating (transistor open) and rest state (transistor closed).
What does it mean? When the transistor is turned off, no current flows through it. In the open state, when a small control current is applied to the base, the transistor opens and a large current begins to flow through the emitter-collector.
And now more about why everything happens this way, that is, why the transistor opens and closes. Let's take a bipolar transistor. Let it be n-p-n transistor.
If you connect a power source between the collector and the emitter, the collector's electrons will begin to be attracted to the positive, but there will be no current between the collector and the emitter. This is hampered by the base layer and the emitter layer itself.
If you connect an additional source between the base and emitter, electrons from the n region of the emitter will begin to penetrate into the base region. As a result, the base area will be enriched with free electrons, some of which will recombine with holes, some will flow to the plus of the base, and some (most) will go to the collector.
Thus, the transistor turns out to be open, and the emitter-collector current flows in it. If the base voltage is increased, the collector-emitter current will also increase. Moreover, with a small change in the control voltage, a significant increase in the current through the collector-emitter is observed. It is on this effect that the operation of transistors in amplifiers is based.
That, in a nutshell, is the essence of how transistors work. Need to calculate a power amplifier using bipolar transistors overnight, or do laboratory work to study the operation of a transistor? This is not a problem even for a beginner if you use the help of our student service specialists.
Don't hesitate to seek professional help in important matters like studying! And now that you already have an idea about transistors, we suggest you relax and watch the video by Korn “Twisted transistor”! For example, you decide to contact the Correspondence Student.
For experiment, we will take a simple and beloved transistor KT815B:
Let's put together a diagram that is familiar to you:
Why did I put a resistor in front of the base?
On Bat1 I set the voltage to 2.5 volts. If you supply more than 2.5 Volts, the light bulb will no longer burn brighter. Let's just say this is the limit after which a further increase in the voltage at the base does not play any role on the current strength in the load
On Bat2 I set it to 6 Volts, although my light bulb is 12 Volts. At 12 Volts, my transistor got noticeably hot, and I didn’t want to burn it out. Here we see how much current our light bulb consumes and we can even calculate the power it consumes by multiplying these two values.
Well, as you saw, the light is on and the circuit is working normally:
But what happens if we mix up the collector and emitter? Logically, the current should flow from the emitter to the collector, because we did not touch the base, and the collector and emitter consist of N semiconductor.
But in practice, the light does not want to light up.
The consumption on the Bat2 power supply is about 10 milliamps. This means that current still flows through the light bulb, but very weak.
Why does the current flow normally when the transistor is connected correctly, but not when connected incorrectly? The point is that the transistor is not made symmetrical.
In transistors, the contact area between the collector and the base is much larger than that between the emitter and the base. Therefore, when electrons rush from the emitter to the collector, almost all of them are “caught” by the collector, and when we confuse the terminals, then not all electrons from the collector are “caught” by the emitter.
By the way, it was a miracle that the P-N junction of the emitter-base did not break through, since the voltage was supplied in reverse polarity. Parameter in the datasheet U EB max. For this transistor, the critical voltage is considered to be 5 Volts, but for us it was even a little higher:
So, we learned that the collector and emitter unequal. If we mix up these terminals in the circuit, then a breakdown of the emitter junction may occur and the transistor will fail. So, do not confuse the leads of the bipolar transistor under any circumstances!
I think it's the simplest. Download the datasheet for this transistor. Every normal datasheet has a picture with detailed inscriptions about where the output is. To do this, enter into Google or Yandex the large numbers and letters that are written on the transistor, and add the word “datasheet” next to it. So far there has never been a situation where I didn’t look for a datasheet for some radio element.
I think there should be no problems with finding the base output, given that the transistor consists of two diodes connected in series either as cathodes or anodes:
Everything is simple here, put the multimeter on the continuity icon “ )))” and start trying all the variations until we find these two diodes. The conclusion is where these diodes are connected either by anodes or cathodes - this is the base. To find the collector and emitter, we compare the voltage drop across these two diodes. Between collector and base ohm it must be less than between emitter and base. Let's check if this is true?
First, let's look at the KT315B transistor:
E – emitter
K – collector
B – base
We set the multimeter to test and find the base without any problems. Now we measure the voltage drop across both junctions. Base-emitter voltage drop 794 millivolts
The voltage drop across the collector-base is 785 millivolts. We have verified that the voltage drop between the collector and the base is less than that between the emitter and the base. Therefore, the middle blue pin is the collector, and the red one on the left is the emitter.
Let's also check the KT805AM transistor. Here is its pinout (location of pins):
This is a transistor with an NPN structure. Let's assume that the base has been found (red pin). Let's find out where the collector is and where the emitter is.
Let's take the first measurement.
Let's take the second measurement:
Therefore, the middle blue pin is the collector, and the yellow one on the left is the emitter.
Let's check one more transistor - KT814B. He is our PNP structure. Its base is the blue output. We measure the voltage between the blue and red terminals:
and then between blue and yellow:
Wow! Both here and there are 720 millivolts.
This method did not help this transistor. Well, don't worry, there is a third way for this...
Almost every modern one has 6 small holes, and next to them there are some letters, something like NPN, PNP, E, C, B. These six tiny holes are precisely intended for measuring. I will call these holes holes. They don't look much like holes))).
We put the multimeter knob on the “h FE” icon.
We determine what conductivity it is, that is, NPN or PNP, and push it into such a section. Conductivity is determined by the location of the diodes in the transistor, if you haven’t forgotten. We take our transistor, which showed the same voltage drop in both directions at both P-N junctions, and put the base into the hole where the letter “B” is.
We don’t touch the base, but simply swap the two pins. Wow, the cartoon showed a lot more than the first time. Therefore, in hole E there is currently an emitter, and in hole C there is a collector. Everything is elementary and simple ;-).
I think this is the easiest and most accurate way to check the pinout of a transistor. To do this, it is enough to purchase Universal R/L/C/Transistor-meter and insert the transistor leads into the terminals of the device:
It will immediately show you whether your transistor is alive. And if he is alive, he will give out his pinout.
Good afternoon friends!
Recently, you and I began to become more closely acquainted with how computer hardware works. And we met with one of his “building blocks” - a semiconductor diode. is a complex system consisting of individual parts. By understanding how these individual parts (big and small) work, we gain knowledge.
By acquiring knowledge, we get a chance to help our iron computer friend if he suddenly goes haywire.. We are responsible for those we have tamed, aren’t we?
Today we will continue this interesting business and try to figure out how perhaps the most important “building block” of electronics works - the transistor. Of all the types of transistors (there are many of them), we will now limit ourselves to considering the operation of field-effect transistors.
The word “transistor” is derived from two English words translate and resistor, that is, in other words, it is a resistance converter.
Among the variety of transistors, there are also field-effect ones, i.e. those that are controlled by an electric field.
An electric field is created by voltage. Thus, a field-effect transistor is a voltage-controlled semiconductor device.
In English literature the term MOSFET (MOS Field Effect Transistor) is used. There are other types of semiconductor transistors, in particular bipolar transistors, which are controlled by current. In this case, some power is also spent on control, since some voltage must be applied to the input electrodes.
The field effect transistor channel can only be opened by voltage, no current flowing through the input electrodes (except for very small leakage current). Those. no power is spent on control. In practice, however, field-effect transistors are mostly used not in static mode, but are switched at a certain frequency.
The design of the field-effect transistor determines the presence of an internal transition capacitance, through which, when switching, a certain current flows, depending on the frequency (the higher the frequency, the greater the current). So, strictly speaking, some power is still spent on control.
The current level of technology makes it possible to make the open channel resistance of a powerful field-effect transistor (FET) quite small - a few hundredths or thousandths of an Ohm!
And this is a great advantage, since when a current of even tens of amperes flows, the power dissipated by the PT will not exceed tenths or hundredths of a watt.
Thus, you can eliminate bulky radiators or greatly reduce their size.
PTs are widely used in computer and low-voltage switching stabilizers on computers.
Of the variety of types of FETs, FETs with an induced channel are used for these purposes.
An induced-channel FET contains three electrodes—source, drain, and gate.
The principle of operation of the PT is half clear from the graphic designation and the name of the electrodes.
The PT channel is a “water pipe” into which “water” (a stream of charged particles that forms an electric current) flows through a “source” (source).
"Water" flows out of the other end of the "pipe" through the "drain" (drain). A valve is a “tap” that opens or shuts off a flow. In order for “water” to flow through the “pipe”, it is necessary to create “pressure” in it, i.e. apply voltage between drain and source.
If no voltage is applied (“no pressure in the system”), there will be no current in the channel.
If voltage is applied, then you can “open the tap” by applying voltage to the gate relative to the source.
The higher the voltage is applied, the more the “faucet” is open, the greater the current in the drain-source channel and the lower the channel resistance.
In power supplies, PT is used in switching mode, i.e. the channel is either completely open or completely closed.
Honestly, the operating principles of PT are much more complex, it can work not only in key mode. His work is described by many abstruse formulas, but we will not describe all of this here, but will limit ourselves to these simple analogies.
Let's just say that PTs can be with an n-channel (in this case, the current in the channel is created by negatively charged particles) and a p-channel (the current is created by positively charged particles). In the graphical representation, the arrow for a PT with an n-channel is directed inward, while for a PT with a p-channel the arrow is directed outward.
Actually, the “pipe” is a piece of semiconductor (most often silicon) with impurities of various types of chemical elements, which determines the presence of positive or negative charges in the channel.
Now let's move on to practice and talk about
Normally, the resistance between any PT terminals is infinitely high.
And, if the tester shows some slight resistance, then the PT is most likely broken and must be replaced.
Many FETs have a built-in diode between the drain and source to protect the channel from reverse voltage (reverse polarity voltage).
Thus, if you put the “+” of the tester (red probe connected to the “red” input of the tester) to the source, and “-” (black probe connected to the black input of the tester) to the drain, then the channel will “ring” like a regular diode in the forward direction.
This is true for n-channel FETs. For a PT with a p-channel, the polarity of the probes will be reverse.
How to check a diode using a digital tester is described in the corresponding section. Those. in the drain-source section the voltage will drop 500-600 mV.
If you change the polarity of the probes, reverse voltage will be applied to the diode, it will be closed and the tester will record this.
However, the serviceability of the protective diode does not indicate the serviceability of the transistor as a whole. Moreover, if you “ring” the PT without desoldering it from the circuit, then due to the parallel-connected circuits, it is not always possible to draw an unambiguous conclusion even about the serviceability of the protective diode.
In such cases, you can remove the transistor, and using a small circuit for testing, answer the question unambiguously– whether the PT is working or not.
In the initial state, button S1 is open, the voltage at the gate relative to the drain is zero. The PT is closed and the HL1 LED is not lit.
When the button is closed, a voltage drop (about 4 V) appears across resistor R3 applied between the source and gate. The PT opens and the HL1 LED lights up.
This circuit can be assembled as a module with a PT connector. Transistors in the D2 pack package (which is designed for mounting on a printed circuit board) cannot be inserted into the connector, but you can connect conductors to its electrodes and insert them into the connector. To test a PT with a p-channel, the polarity of the power supply and the LED must be reversed.
Sometimes semiconductor devices fail violently, with pyrotechnic, smoke and light effects.
In this case, holes form on the body, it cracks or falls into pieces. And you can make an unambiguous conclusion about their malfunction without resorting to instruments.
In conclusion, the letters MOS in the abbreviation MOSFET stand for Metal - Oxide - Semiconductor (metal - oxide - semiconductor). This is the structure of the PT - a metal gate (“faucet”) is separated from the semiconductor channel by a layer of dielectric (silicon oxide).
I hope you have figured out the “pipes”, “taps” and other “plumbing” today.
However, theory, as we know, is dead without practice! You definitely need to experiment with the field workers, poke around, tinker with checking them, touch them, so to speak.
By the way, buy field effect transistors are possible.
