Knowledge of electronic components series-triode
Time：2021.09.13
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The structure and type of transistor
Transistor is one of the basic semiconductor components. It has the function of current amplification and is the core component of electronic circuits. The triode is made of two PN junctions very close to each other on a semiconductor substrate. The two PN junctions divide the positive semiconductor into three parts. The middle part is the base area, and the two sides are the emitter area and the collector area. The arrangement is PNP And NPN two types, as shown in the figure, the corresponding electrodes are drawn from the three areas, namely the base b, the emitter e and the collector c.
The PN junction between the launch area and the base area is called the launch junction, and the PN junction between the collector area and the base area is called the collector. The base region is very thin, while the emitter region is thicker, and the impurity concentration is large. The PNP transistor emitter region "emits" holes, and its moving direction is consistent with the current direction, so the emitter arrow points inwards; the NPN transistor emitter region "emits" "The free electrons move in the opposite direction of the current, so the emitter arrow points outward. The emitter arrow points outward. The emitter arrow points to the conduction direction of the PN junction under the forward voltage. Both silicon and germanium crystal transistors have two types: PNP type and NPN type.
Package form and pin identification of triode
Commonly used triodes are packaged in two types: metal package and plastic package. The pin arrangement has certain rules. As shown in the figure, for low-power metal packaged transistors, place them according to the bottom view of the figure, so that the three pins form an isosceles. On the vertices of the triangle, it is ebc from left to right; for small and medium-power plastic transistors, according to the figure, the plane faces itself, and the three pins are placed downward, which is ebc from left to right.
At present, there are many kinds of transistors of various types in China, and the pin arrangement is not the same. When using a transistor whose pin arrangement is uncertain, it must be measured to determine the correct position of each pin, or look up the transistor user manual to make it clear The characteristics of the triode and the corresponding technical parameters and data.
The current amplification effect of the transistor
The transistor has a current amplifying effect, and its essence is that the transistor can control a large change in the collector current with a small change in the base current. This is the most basic and important characteristic of the triode. We call the ratio of ΔIc/ΔIb the current amplification factor of the transistor, which is represented by the symbol "β". The current amplification factor is a fixed value for a certain triode, but it will also change with the change of the base current when the triode is working.
Three working states of transistor
Cut-off state: When the voltage applied to the emitter junction of the transistor is less than the on-voltage of the PN junction, the base current is zero, the collector current and the emitter current are both zero, and the transistor loses the current amplification function at this time, the collector and the emitter Between is equivalent to the off state of the switch, we call the triode in the off state.
Amplified state: When the voltage applied to the emitter junction of the transistor is greater than the turn-on voltage of the PN junction and is at an appropriate value, the emitter junction of the transistor is forward biased, and the collector junction is reverse biased. At this time, the base current Play a control role on the collector current, so that the transistor has a current amplification effect, its current amplification factor β = ΔIc/ΔIb, at this time the transistor is in an amplified state.
Saturation conduction state: When the voltage applied to the emitter junction of the transistor is greater than the conduction voltage of the PN junction, and when the base current increases to a certain extent, the collector current no longer increases with the increase of the base current. But it does not change much near a certain value. At this time, the triode loses its current amplification effect, the voltage between the collector and the emitter is very small, and the collector and emitter are equivalent to the on-state of the switch. This state of the triode is called the saturated conduction state.
According to the voltage level of each electrode when the triode is working, the working state of the triode can be judged. Therefore, electronic maintenance personnel often use a multimeter to measure the voltage of each pin of the triode during the maintenance process to determine the working condition and working state of the triode.
Use a multimeter to test the triode
Judgment of the base of the triode: According to the structure diagram of the triode, we know that the base of the triode is the common pole of the two PN junctions in the triode. Therefore, when judging the base of the triode, just find the common pole of the two PN junctions. It is the base of the triode. The specific method is to adjust the multimeter to the R×1k block of the electric barrier, first place the red test lead on one leg of the transistor, and use the black test lead to touch the other two legs of the transistor. If it is fully passed twice, the red test lead The foot you put is the base of the triode. If it is not found once, change the red test lead to the other pin of the transistor and test it twice; if it is not found, change the red test lead and test it twice. If you haven't found it yet, use a black test lead to put it on one foot of the transistor, and use a red test lead to test twice to see if it is all-passed. If it fails once, change it again. In this way, there is no measurement at most 12 times, and the base can always be found.
Discrimination of the type of triode: There are only two types of triode, namely PNP type and NPN type. It is enough to know whether the base is a P-type material or an N-type material when making the judgment. When using the multimeter R×1k block, the black test lead represents the positive pole of the power supply. If the black test lead is connected to the base, it means that the base of the transistor is P-type material, and the transistor is of NPN type. If the red test lead is connected to the base, it means that the base of the transistor is N-type material, and the transistor is of PNP type.
Electronic triode
While Fleming invented the diode to improve the radio detector, the American Ph.D. Forrester was also working on the detector. As his research was deepening, the news came that Fleming from England had successfully invented the vacuum diode, which shocked him greatly. Should it change course or continue? He thought that Fleming's diodes can be used for rectification and detection, but they cannot yet amplify electrical signals. Therefore, after two years of research and development, De Forest finally improved Fleming's diode and made a new invention. Insert a third electrode (shed electrode) with the function of controlling the movement of electrons between the cathode and anode of the diode. The weak signal change of the voltage on the shed can modulate the current flowing from the cathode to the anode, so that a current that is the same as the input signal change but greatly increased can be obtained. This is the "amplification" effect of the triode invented by De Forrester.
In 1912, De Forrester successfully did several triode connection experiments and obtained a much larger amplification capability than a single triode. Soon, De Forrester developed the first electronic amplifier for telephone repeaters to amplify weak telephone signals. He was the first person to use electronic products in telephones. In addition, the triode can also oscillate to produce electromagnetic waves, that is to say, many people in foreign countries regard the invention of the triode as the true birth of the electronics industry.
MOS field effect tube
That is, the metal-oxide-semiconductor field effect transistor, abbreviated as MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), belongs to the insulated gate type. Its main feature is that there is a silicon dioxide insulating layer between the metal gate and the channel, so it has a high input resistance (up to 1015Ω). It is also divided into N-channel tube and P-channel tube, the symbol is shown in Figure 1. Usually the substrate (substrate) and the source S are connected together. According to the different conduction mode, MOSFET is divided into enhancement type and depletion type. The so-called enhanced type refers to: when VGS=0, the tube is in an off state, and after adding the correct VGS, the majority of carriers are attracted to the gate, thereby "enhancing" the carriers in this area and forming a conductive channel . The depletion type means that when VGS=0, a channel is formed, and when the correct VGS is added, the majority of carriers can flow out of the channel, thus "depleting" the carriers and turning the tube off.
Taking the N channel as an example, it is made on a P-type silicon substrate with two source diffusion regions N+ and drain diffusion regions N+ with a high doping concentration, and then the source S and the drain D are led out respectively. The source and the substrate are connected internally, and the two always maintain the same potential. The front direction in the symbol of Figure 1(a) is from the outside to the electricity, which means that the P-type material (substrate) refers to the N-type channel. When the drain is connected to the positive pole of the power supply, the source is connected to the negative pole of the power supply and VGS=0, the channel current (that is, the drain current) ID=0. With the gradual increase of VGS, attracted by the positive voltage of the gate, negatively charged minority carriers are induced between the two diffusion regions, forming an N-type channel from the drain to the source. When VGS is greater than the tube When the turn-on voltage VTN (usually about +2V) is reached, the N-channel tube begins to conduct, forming a drain current ID.
The typical products of domestic N-channel MOSFET are 3DO1, 3DO2, 3DO4 (the above are all single-gate tubes), and 4DO1 (dual-gate tubes). Their pin arrangement (bottom view) is shown in Figure 2.
MOS field effect tube is more "squeaky". This is because its input resistance is very high, and the capacitance between the gate and the source is very small, it is very easy to be charged by the external electromagnetic field or electrostatic induction, and a small amount of charge can form a very high voltage on the capacitance between the electrodes (U =Q/C), damage the tube. Therefore, the pins are twisted together at the factory, or installed in metal foil, so that the G pole and the S pole are at the same potential to prevent the accumulation of static charge. When the tube is not in use, all leads should be short-circuited. Be extra careful when measuring, and take corresponding anti-static measures. The detection method is described below.
1. Ready to work
Before measuring, short-circuit the human body to ground before touching the pins of the MOSFET. It is best to connect a wire to the wrist to connect with the earth, so that the human body and the earth maintain an equipotential. Separate the pins again, and then remove the wires.
2. Determination electrode
Set the multimeter to the R×100 gear, and first determine the grid. If the resistance of a pin and other pins are both infinite, it proves that this pin is the grid G. Exchange the test leads and measure again, the resistance value between S-D should be several hundred ohms to several thousand ohms. The black test lead is connected to the D pole and the red test lead is connected to the S pole. For the 3SK series products produced in Japan, the S pole is connected to the shell, so it is easy to determine the S pole.
3. Check amplification capability (transconductance)
Hang the G pole in the air, connect the black test lead to the D pole, and the red test lead to the S pole, and then touch the G pole with your finger, the needle should have a larger deflection. The double-gate MOS field effect transistor has two gates G1 and G2. In order to distinguish, you can touch the G1 and G2 poles with your hands, and the G2 pole is the one with the larger deflection of the hand to the left.
At present, some MOSFET tubes have added protective diodes between the G-S poles, and there is no need to short-circuit each pin.
VMOS FET
VMOS field effect tube (VMOSFET) is abbreviated as VMOS tube or power field effect tube, and its full name is V-groove MOS field effect tube. It is a high-efficiency, power switching device newly developed after MOSFET. It not only inherits the high input impedance of MOS field effect tube (≥108W), small drive current (about 0.1μA), but also has high withstand voltage (up to 1200V), large working current (1.5A～100A), and output High power (1～250W), good linearity of transconductance, fast switching speed and other excellent characteristics. It is precisely because it combines the advantages of electronic tubes and power transistors into one, it is being widely used in voltage amplifiers (voltage amplification up to thousands of times), power amplifiers, switching power supplies and inverters.
As we all know, the gate, source, and drain of a traditional MOS field effect transistor are on a chip where the gate, source, and drain are roughly on the same horizontal plane, and its working current basically flows in a horizontal direction. The VMOS tube is different, as can be seen from Figure 1 its two major structural characteristics: first, the metal gate adopts a V-groove structure; second, it has vertical conductivity. Since the drain is drawn from the back of the chip, the ID does not flow horizontally along the chip, but starts from the heavily doped N+ region (source S), flows through the P channel into the lightly doped N- drift region, and finally reaches vertically downwards Drain D. The direction of the current is shown by the arrow in the figure, because the cross-sectional area of the flow is increased, so a large current can be passed. Because there is a silicon dioxide insulating layer between the gate and the chip, it is still an insulated gate MOS field effect transistor.
The main domestic manufacturers of VMOS field effect transistors include 877 Factory, Tianjin Semiconductor Device Fourth Factory, Hangzhou Electron Tube Factory, etc. Typical products include VN401, VN672, VMPT2, etc. Table 1 lists the main parameters of six kinds of VMOS tubes. Among them, the appearance of IRFPC50 is shown as in Fig. 3.
The method of detecting VMOS tube is introduced below.
1. Judgment grid G
Set the multimeter to the R×1k file to measure the resistance between the three pins. If it is found that the resistance of a pin and its two pins are both infinite, and it is still infinite after exchanging the test leads, it is proved that this pin is the G pole, because it is insulated from the other two pins.
2. Determine source S, drain D
It can be seen from Figure 1 that there is a PN junction between the source and the drain, so the S pole and the D pole can be identified according to the difference in the forward and reverse resistance of the PN junction. Use the exchange meter pen method to measure the resistance twice, and the one with the lower resistance value (generally several thousand ohms to ten thousand ohms) is the forward resistance. At this time, the black test lead is S pole, and the red one is connected to D pole.
3. Measuring drain-source on-state resistance RDS (on)
Short-circuit the G-S pole, select the R×1 gear of the multimeter, connect the black test lead to the S pole, and the red test lead to the D pole, and the resistance should be a few ohms to more than ten ohms.
Due to different test conditions, the measured RDS(on) value is higher than the typical value given in the manual. For example, use a 500-type multimeter to measure an IRFPC50 VMOS tube with R×1 file, RDS(on)=3.2W, greater than 0.58W (typical value).
4. Check transconductance
Place the multimeter in the R×1k (or R×100) position, connect the red test lead to the S pole, and the black test lead to the D pole, and hold the screwdriver to touch the grid. The needle should deflect significantly. The greater the deflection, the greater the transconductance of the tube. high.
Precautions:
(1) VMOS tubes are also divided into N-channel tubes and P-channel tubes, but most of the products are N-channel tubes. For P-channel tubes, the position of the test leads should be exchanged during measurement.
(2) There are a few VMOS tubes with protection diodes between G-S, items 1 and 2 in this detection method are no longer applicable.
(3) There is also a VMOS tube power module on the market, which is specially used for AC motor speed regulators and inverters. For example, the IRFT001 module produced by the American IR company has three N-channel and P-channel tubes inside, forming a three-phase bridge structure.
(4) VNF series (N-channel) products on the market now are ultra-high frequency power FETs produced by Supertex in the United States. Its highest operating frequency is fp=120MHz, IDSM=1A, PDM=30W, and common source small signal low frequency Transconductance gm=2000μS. It is suitable for high-speed switching circuits and broadcasting and communication equipment.
(5) A suitable heat sink must be added when using a VMOS tube. Taking VNF306 as an example, the maximum power can reach 30W after installing a 140×140×4 (mm) radiator.
Field effect transistor
Field-effect transistor (FET) is abbreviated as field-effect transistor. It is a voltage-controlled semiconductor device. It has the advantages of high input resistance (108～109Ω), low noise, low power consumption, no secondary breakdown phenomenon, and wide safe working area. Has become a strong competitor of bipolar transistors and power transistors.
Field effect transistors are divided into two categories: junction type and insulated gate type. The junction field effect transistor (JFET) is named because it has two PN junctions, and the insulated gate field effect transistor (JGFET) is named because the gate is completely insulated from other electrodes. At present, among the insulated gate field effect transistors, the most widely used is the MOS field effect transistor, referred to as MOS tube (ie, metal-oxide-semiconductor field effect transistor MOSFET); in addition, there are PMOS, NMOS and VMOS power field effect transistors. And recently came out πMOS field effect tube, VMOS power module, etc.
According to the difference of channel semiconductor materials, junction type and insulated gate type are divided into two types: channel and P channel. If divided according to the conduction mode, the field effect tube can be divided into depletion type and enhanced type. The junction field effect transistors are all depletion type, and the insulated gate field effect transistors are both depletion type and enhanced type.
Field effect transistors can be divided into junction field effect transistors and MOS field effect transistors. The MOS field effect transistors are divided into four categories: N-channel depletion type and enhancement type; P-channel depletion type and enhancement type. See attached picture 1.
Precautions for the use of MOS field effect transistors.
MOS field effect transistors should be classified when they are used and cannot be interchanged at will. MOS field effect transistors are easily broken down by static electricity due to their high input impedance (including MOS integrated circuits). Pay attention to the following rules when using them:
1. MOS devices are usually packed in black conductive foam plastic bags when they leave the factory, please do not take a plastic bag casually. You can also use thin copper wires to connect the pins together, or wrap them in tin foil
2. The MOS device taken out cannot slide on the plastic board, and a metal plate should be used to hold the device to be used.
3. The soldering iron must be well grounded.
4. Before soldering, the power cord of the circuit board should be short-circuited with the ground wire, and then the MOS device should be separated after the soldering is completed.
5. The welding sequence of each pin of MOS device is drain, source, and gate. When disassembling the machine, the sequence is reversed.
6. Before installing the circuit board, use a grounded wire clamp to touch the terminals of the machine, and then connect the circuit board.
7. If the gate of MOS field effect transistor is allowed, it is best to connect the protection diode. When overhauling the circuit, pay attention to check whether the original protection diode is damaged.
Field effect tube test.
The following takes the commonly used 3DJ type N-channel junction field effect transistor as an example to explain its test method:
The 3DJ type junction field effect transistor can be regarded as an NPN type transistor, the grid G corresponds to the base b, the drain D corresponds to the collector c, and the source S corresponds to the emitter e. So just measure the forward and reverse resistance of the PN junction like a transistor. Set the multimeter to the R*100 block and connect the black test lead to one electrode of the FET, and the red test lead to the other two electrodes respectively. When there are two low resistances, the black test lead is connected to the grid of the FET. The red test lead is connected to the drain or source. For junction field effect transistors, the drain and source can be interchanged. For a junction FET with 4 pins, the other pole is a shielding pole (grounded in use).
The pin sequence of commonly used junction field effect transistors and MOS type insulated gate field effect transistors is shown in Figure 2.
Discrimination of silicon tube and germanium tube
Silicon tubes and germanium tubes are very different in characteristics, so they should be distinguished when they are used. We know that the forward resistance of the PN junction of a silicon tube and a germanium tube is different, that is, the forward resistance of a silicon tube is large, and that of a germanium tube is small. Using this feature, a multimeter can be used to determine whether a transistor is a silicon tube or a germanium tube.
The discrimination method is as follows:
Set the multimeter to R*100 or R*1K. When measuring a diode, the positive terminal of the multimeter is connected to the cathode of the diode, and the negative terminal is connected to the anode of the diode; when measuring an NPN transistor, the negative terminal of the multimeter is connected to the base and the positive terminal is connected to the collector or emitter; when measuring a PNP transistor , The positive terminal of the multimeter is connected to the base, and the negative terminal is connected to the collector or emitter.
After connecting according to the above method, if the pointer of the multimeter is on the right end of the dial or close to the full scale position (that is, the resistance value is small), then the tube to be measured is a germanium tube; if the needle of the multimeter is in the middle or offset of the dial At the point on the right (that is, the resistance is larger), then the tube under test is a silicon tube.
The formula for judging the triode
The determination of the tube type and pin of the transistor is a basic skill for beginners in electronic technology. In order to help readers quickly grasp the method of determination, the author summarizes four formulas: "Three inversions, find the base; PN junction, fix the tube type; Follow the arrow, the deflection is large; the measurement is inaccurate, and the mouth is moved." Let's explain it sentence by sentence.
One or three reversed, find the base
As you all know, a triode is a semiconductor device containing two PN junctions. According to the different connection modes of the two PN junctions, they can be divided into NPN and PNP transistors with two different conductivity types. Figure 1 shows their circuit symbols and equivalent circuits.
To test the transistor, use the ohm block of the multimeter, and select the R×100 or R×1k gear. Figure 2 depicts the equivalent circuit of the ohmic block of a multimeter. It can be seen from the figure that the red test lead is connected to the negative electrode of the battery in the watch, and the black test lead is connected to the positive electrode of the battery in the watch.
Assume that we don’t know whether the tested transistor is NPN or PNP, nor can we tell which electrode each pin is. The first step of the test is to determine which pin is the base. At this time, we can take two electrodes at will (for example, the two electrodes are 1, 2), use the two test pens of a multimeter to measure its forward and reverse resistance, and observe the deflection angle of the needle; then, take 1 Measure the forward and reverse resistance of the two electrodes, 3 and 2 and 3 respectively, and observe the deflection angle of the hands. In these three inversion measurements, there must be two measurement results that are similar: that is, in the inversion measurement, the deflection of the needle is large once, and the deflection is small in the other; the remaining one must be that the deflection angle of the pointer before and after the inversion measurement is very small, this time the unmeasured one The pin is the base we are looking for (see Figure 1 and Figure 2 to understand its reason).
2. PN junction, fixed tube type
After finding the base of the triode, we can determine the conductivity type of the tube according to the direction of the PN junction between the base and the other two electrodes (Figure 1). Touch the black test lead of the multimeter to the base and the red test lead to any one of the other two electrodes. If the pointer deflection angle of the meter head is large, it means that the tested transistor is an NPN tube; if the deflection angle of the meter head pointer is small, The tested tube is PNP type.
Three, follow the arrow, the deflection is large
I found the base b. Which of the other two electrodes is the collector c and which is the emitter e? At this time, we can determine the collector c and emitter e by measuring the penetration current IEO.
(1) For the NPN transistor, the measurement circuit of the penetrating current is shown in Figure 3. According to this principle, use the black and red test pens of the multimeter to reversely measure the positive and reverse resistances Rce and Rec between the two poles. Although the deflection angle of the multimeter pointer is very small in the two measurements, there will always be a deflection when observed carefully. The angle is slightly larger, the current flow must be: black test lead → c pole → b pole → e pole → red test lead. The current flow is exactly the same as the direction of the arrow in the transistor symbol ("arrow"), so the black test lead at this time The collector c must be connected, and the emitter e must be connected to the red test lead.
(2) For the PNP type transistor, the principle is similar to the NPN type. The current flow must be: black test lead → e pole → b pole → c pole → red test lead. The current flow direction is also consistent with the direction of the arrow in the transistor symbol. Therefore, the black test lead must be connected to the emitter e, and the red test lead must be connected to the collector c.
4. Undetectable, move your mouth
If in the measurement process of "forward arrow, large deflection", if the deflection of the two measurement pointers before and after the reversal is too small to distinguish, you must "move your mouth". The specific method is: in the two measurements of "arrow, large deflection", use two hands to pinch the junction between the two test leads and the pin, and hold (or press the tongue against) the base electrode b with your mouth. Use the "arrow, deflection large" method of discrimination to distinguish the collector c from the emitter e. The human body acts as a DC bias resistor, the purpose is to make the effect more obvious.
Discrimination of high frequency tube and low frequency tube
High-frequency tubes and low-frequency tubes generally cannot substitute for each other due to their different characteristics and uses.
Here is how to use a multimeter to quickly distinguish its high-frequency tube and low-frequency tube. The discrimination method is:
First, use a multimeter to measure the reverse resistance of the emitter of the triode. If it is measuring a PNP tube, the negative end of the multimeter is connected to the base and the positive end of the multimeter is connected to the emitter; if it is measuring an NPN tube, the positive end of the multimeter is connected to the base and the negative end. Connect the emitter. Then use the R*1KΩ block of the multimeter to measure. At this time, the resistance indicated by the needle of the multimeter should be very large, generally not exceeding 1/10 of the full scale value. Then switch the multimeter to the R*10KΩ block. If the resistance value indicated by the needle changes greatly, exceeding 1/3 of the full scale value, the tube is a high-frequency tube; on the contrary, if the multimeter is switched to the R*10KΩ block, the needle The indicated resistance value does not change much, and it does not exceed 1/3 of the full-scale value, then the measured tube is a low-frequency tube.
Judgment of the quality of field effect transistors.
First use the MF10 multimeter R*100KΩ (built-in 15V battery), connect the negative test lead (black) to the grid (G), and the positive test lead (red) to the source (S). Charge between the gate and the source, the pointer of the multimeter is slightly deflected at this time. Then use the multimeter to block R*1Ω, connect the negative test lead to the drain (D), and the positive test lead to the source (S). If the multimeter indicates a few ohms, the FET is good.
