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Home > Technology List > The working principle of Transistor and the method of dentification

The working principle of Transistor and the method of dentification

Published time: 2018-06-27

The main content of the article:

The definition of transistor and the simple description of transistor

2.What is the working principle of a transistor

3.The introduction of several working states and amplification principle of triode

4.The description of the transistor's amplification

5.How to identify the quality, polarity, and position of bipolar transistors

The definition of Transistor and the simple description of Transistor.

Transistor, full name should be semiconductor transistor, also known as bipolar transistor.Transistor, is a semiconductor device to control the current.The transistor’s role is to amplify the weak signal into a larger amplitude of the electrical signal,and it can also be used as a contactless switch.

A transistor, which is one of the basic semiconductor components, has a current amplification function.It is a core component of an electronic circuit. The triode is to make two PN junctions on a semiconductor substrate with short distances. The two PN junctions divide the whole semiconductor into three parts. The middle part is the base area and the two sides are the emitter area and the collector area. There are two ways to arrange.the one is PNP.the other one is NPN.

the simple description of Transistor 

What is the working principle of a transistor

Crystal transistors (hereinafter referred to as triodes) are divided into two types according to materials: manifolds and silicon tubes. Each of them has two types of NPN and PNP structure, The most widely used transistors are silicon NPN and germanium PNP transistors(where N is the negative meaning , N-type semiconductors are produced by adding phosphorus in high-purity silicon and replacing part of silicon atoms to generate free electron conduction under voltage stimulation. P-type semiconductors replace the silicon element by adding boron to generate a large number of holes and form a good conductive environment. ). In addition to the different polarity of the power supply, the working principle is the same. The following only introduces the current amplification principle of the NPN silicon tube.

As for an NPN tube, it is composed of two P-type semiconductors sandwiched between two N-type semiconductors.The emitter junction is a PN junction formed between the emitter region and the base region, and the collector junction is a PN junction formed between the collector region and the base region, the three leads are called Emitter, Base, and Collector, respectively. As shown below

NPN TUBE AND PNP TUBE 

When the potential at point b is more than a few volts at the potential of point e, the emitter junction is in a forward-biased state, and the potential at point C is a few volts higher than the potential at point b, the collector junction is in a reverse biased state, and the collector power Ec is higher than the base Power supply Eb.

When the transistor is manufactured, the majority carrier concentration in the emitter region is consciously greater than that in the base region, while the base region is made very thin, and the impurity content must be strictly controlled so that once the power is turned on, the emitter junction is biased. The majority carriers (electrons) in the emitter region and the majority carriers (holes) in the base region easily spread across the emitter junction toward each other, but the former has a greater concentration base than the latter, the current through the emitter junction basically, the electron flow is called the emitter current.

Transistor circuit diagram 

Due to the thin base region and the reverse bias of the collector junction, most of the electrons injected into the base region cross the collector junction into the collector region to form a collector current Ic, leaving only a few (1-10%) of the electrons left. The holes in the base region are recombined, and the base holes that are recombined are replenished by the base power source Eb, thereby forming a base current Ibo. According to the current continuity principle:

Ie=Ib+Ic

That is to say,by adding a small Ib to the base, a large Ic can be obtained at the collector. This is the so-called current amplification. Ic and Ib maintain a proportional relationship, ie:

β1=Ic/Ib

In the formula: β1--called DC magnification,

The ratio of the collector current variation ΔIc to the base current variation ΔIb is:

β= △Ic/△Ib

In the formula, β- is called AC current amplification factor. Since the values of β1 and β are not much different at low frequencies, sometimes to make it convenient, the two are not strictly distinguished, and the β value is about ten to one hundred.

α1=Ic/Ie (Ic and Ie are currents in the DC path)

Where: α1 is also referred to as DC amplification.It is generally used in a common-base configuration amplifier circuit to describe the relationship between emitter current and collector current.

α =△Ic/△Ie

The alpha in the expression is the ac common base current amplification. Similarly, α and α1 have little difference in the input of small signals.

For the two magnifications describing the current relationship there is the following relationship:

two magnifications describing the current relationship there is the following relationship 

The current amplification of the transistor actually uses a small change in the base current to control the huge change in the collector current.The triode is a current amplifier device,in reality, the current amplification effect of the triode is often converted into voltage amplification by a resistor.

The introduction of several working states and amplification principle of triode

Cut-off state overview

When the voltage applied to the triode emitter junction is less than the PN junction's turn-on voltage, the base current is zero, the collector current and the emitter current are zero, and the triode loses its current amplification affection, and the collector and emitter are equivalent. In the off state of the switch, we say that the transistor is in the off state.

Magnification overview

When the voltage applied to the emitter junction of the transistor is greater than the turn-on voltage of the PN junction and at an appropriate value, the emitter junction of the transistor is forward-biased and the collector junction is reverse biased. In this case, the base current is opposite to the collector. The current plays a controlling role, so that the triode has a current amplification function, and the current amplification factor β=ΔIc/ΔIb, at this time, the triode is in an enlarged state.

What is the Saturation conduction

When the voltage applied to the triode emitter junction is greater than the PN junction's turn-on voltage, and when the base current is large enough, the collector current no longer changes with the increase of the base current, but it will be at a certain level. The value does not change very much. At this time, the triode loses the current amplification effect, the voltage between the collector and the emitter is very small, and the collector and the emitter are equivalent to the conduction state of the switch. This state of the transistor is called saturation conduction.

Jotrin Electronics think that, as for digital circuit,, the triode is equivalent to a switch circuit, the off state is the switch off, the saturation state is the switch closed, and the amplified state is just an over-state between the switch off. Next Jotrin Electronics will explain the amplification of the triode in detail.

The description of the triode’s amplification

1. Emission area to the base area

The power supply Ub is applied to the emitter junction via the resistor Rb, and the emitter junction is forward biased. The majority carriers (free electrons) in the emitter region continuously cross the emitter junction into the base region to form the emitter current Ie. At the same time, the majority carrier in the base region also diffuses to the emitter region. However,  the majority carrier concentration is much lower than the carrier concentration in the emitter region, this current may not be taken into account. Therefore, it is considered that the emitter junction is mainly an electron current.

2. Diffusion and compounding of electrons in the base area

After the electrons enter the base region, they are densely concentrated near the emitter junction and gradually form an electron concentration difference. Under the effect of the concentration difference, the electron flow is encouraged to diffuse toward the collector junction in the base region and is pulled into the collector by the collector junction field. The collector current Ic is formed in the region. There is also a small fraction of electrons (since the base region is thin) that recombine with holes in the base region. The ratio of the diffused electron current to the composite electron current determines the amplification capability of the triode.

3. Collecting area collects electrons

Since the collector junction has a large reverse voltage, the electric force generated by this reverse voltage,it will prevent the electrons in the collector region from diffusing to the base region, and at the same time, the electrons that have spread to the collector junction will be drawn into the collector region to form the collector master.-Current Icn. In addition, the minority carriers (holes) in the collector region will also generate drift motions, and the reverse saturation current will flow in the base region, which is represented by Icbo. The value is small, but it is extremely sensitive to temperature.

How to identify the quality, polarity, and position of bipolar transistors

1. Distinguish between good and bad judgment, base and polarity

1) The three electrodes of the triode are numbered, and any two numbered electrodes have two connection sequences (ie, two measurements) with the red and black pens of the analog multimeter. A total of six measurements are performed, and six measurements are performed. Only if the two measuring hands are deflected (the resistance reading is between several KΩ and several tens of KΩ), the tube is intact, otherwise the transistor is bad.

2) Determination of Base (B) and Transistor Polarity:

Prerequisite: The triode is in good condition. There are two situations:

①In the two measurements of pointer deflection, the electrode connected to the black pen at all times is B-pole, and the polarity of the tube is NPN type;

②In the two measurements of pointer deflection, the electrode that is always connected to the red pen is the B pole, and the polarity of this tube is PNP type.

2. Determination of two electrodes C and E

The red and black pens of the analog multimeter are respectively connected to the remaining two electrodes (there are two test sequences), and there are two cases:

①If the triode judged in step 1 is an intact NPN type, in both tests, the left index finger is always overlapped between the electrode connected to the black pen and the B pole, and the resistance readings of the multimeter are recorded separately in the next two tests. For R1 and R2, if R1<R2, in this measurement of reading R1, the electrode connected to the black pen is the C pole and the other electrode is the E pole.

②If the transistor determined in step 1 is an intact PNP type, in both tests, the left index finger is always overlapped between the electrode connected to the red pen and the B electrode. The resistance readings of the multimeter are recorded for the next two tests. R1 and R2, if R1<R2, in this measurement of reading R1, the electrode connected to the red pen is the C pole and the other electrode is the E pole.The above determination method is not applicable to a damper tube or a transistor containing a protection circuit therein.

The point of view from Jotrin Electronics:

From the above description, we learned that the main function of the transistor is switching and amplification. It is particularly important to understand the characteristics of the transistor. The dividing line from the ascending state to the amplifying state is the turn-on voltage of the triode. After entering the amplifying state, the most important factor is the current amplification of the triode. The voltage continues to increase, and the triode enters a saturated state. To prevent the triode from breakdown, we need to understand Transistor maximum reverse withstand voltage. Jotrin Electronics limited has advanced instruments that can fully meet all the measurement work of the transistor.You can also buy related electronic parts at Jotrin Electronics

 

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