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What Is A Transistor? Basic Working Principles (Video)

Introduction(related video)

Transistors make our electronics world go round. They're critical as a control source in just about every modern circuit. Sometimes you see them, but more-often-than-not they're hidden deep within the die of an integrated circuit. 


Article CoreTransistors

General ViewApplication and Importance
Transistors Structure and Operation


Development of Transistors1) vacuum triode
2) point-contact transistor
3) bipolar and unipolar transistors
4) silicon transistors
5) integrated circuit (IC)
7) CPU

Transistor AdvantagesFewer Consumption
No need to preheat
Solid and reliable

Tansistors ClassificationAccording to material
According to craft
According to current capacity
According to service frequency
According to packaging Types
According to applications

Specific Types Expressions1) transistors
2) giant transistors
3) phototransistors
4) bipolar transistors
5) bipolar junction transistors—BJT
6) field effect transistors(FET)
7) static induction transistors
8) single electron transistor

Control Power

Test Replacement

Method of Judging ElectrodeA. method for judging resistances of collector and emitter
B. PN junction forward resistance method
C. amplification coefficient method

Transistors Replacement PrincipleOne—same type
Two—similar characteristics
Three—similar appearance

Related News

Introduction(specific details)

Transistor is a kind of solid semiconductor device. It has many functions, such as detection, rectifier, amplifier, switch, voltage stabilizer, signal modulation and so on. As a variable current switch, transistors can control output currents based on input voltages. Unlike conventional mechanical switches (such as relay, switch), transistors use telecommunication signals to control their opening and closing, and the switching speed can be very fast, for example, the switching speed in the labs can be higher than 100GHz. Strictly speaking, transistors refer to all single components based on semiconductor materials, including diodes, transistors, field effect transistors, silicon control and so on. In addition, transistors usually mean crystal triodes.

The transistors are divided into two main categories: bipolar junction transistors (BJT) and field effect transistors (FET).

The transistor has three poles. The three poles of bipolar junction transistor, composed of emitter(made up of N-type and P-type), base and collector respectively. For field effect transistor, they are source, gate and drain respectively.

Because the transistor has three polarities, there are also three ways to use them, namely, emitter grounding (called common emitter amplification, CE configuration), base grounding (called common base amplification, CB configuration) and collector grounding (called common set amplification, CC configuration, emitter coupled logic).

General View


Transistors are semiconductor devices, which are commonly used as amplifiers or electrically controlled switches. Transistors are important components that regulate the operation of computers, mobile phones, and all electronic devices. Due to their high response speed and accuracy, transistors can be used for a wide variety of digital and analog functions design, including amplifiers, switches, and voltage stabilizers, signal modulation and oscillator circuits. Transistors can be packaged independently or in a very small area, which can accommodate 100 million or more transistors integrated into a part of the circuit.

The Structure and Operation of Transistors

Transistors are made by stacking three different layers of semiconductor material together. Some of those layers have extra electrons added to them, which called “doping”, and others have electrons removed (doped with “holes” – the absence of electrons). A semiconductor material with extra electrons is called an N-type (negative) and a material with electrons removed is called a P-type (positive). 

With some hand waving, we can say electrons can easily flow from N-regions to P-regions, if they have a little force (voltage) to push them. But flowing from a P-region to a N-region is really hard (requiring more force—voltage). 

The NPN transistor is designed to pass electrons from the emitter to the collector (the conventional current flows from collector to emitter). The emitter emits electrons into the base, which controls the number of electrons. In fact, most of the electrons emitted are “collected” by the collector, which sends them along to the next part of the circuit.

A PNP has a little special area. The base still controls current flow, but that current flows in the opposite direction, that is, from emitter to collector, instead of electrons, the emitter emits “holes” which are collected by the collector.

The transistor is kind of like an electron valve. The pin of base is likely to a handle you can adjust to allow more or less electrons to flow from emitter to collector. 

active transistor current flow


The invention of transistors can date back to the middle& later 1920s, an engineer Physicist Julius Edgar Lilienfeld filed a patent for a field-effect transistor (FET) in Canada in 1925, which was intended to be a solid-state replacement for the triode. Lilienfeld also filed identical patents in the United States in 1926 and 1928. However, it was limited to the technical level at the time, the material used to make it couldn’t meet the high-quality requirement, making it impossible to actually construct a working device at that time.

In December 1947, the first practically implemented device was a point-contact transistor invented by American physicists John Bardeen, Walter Brattain, and William Shockley from Bell Labs. Due to the complex manufacturing process of point-contact transistors, many products fail, and it also has disadvantages, such as high noise, difficult to control when power is high, and narrow application range. To overcome these shortcomings, Shockley put forward a idea of replacing metal semiconductor contacts with a "rectifier junction", and they also proposed the working principle of it. The transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things. The transistor is on the list of IEEE milestones in electronics, and Bardeen, Brattain, and Shockley shared the 1956 Nobel Prize in Physics for their achievement.

In 1950, the first P-N junction transistor came out, and its performance was exactly the same as the assumption of William Shockley. Most of today's transistors are still P-N junction transistors. (the so-called P-N junction is a combination of P-type and N-type, and P-type multiplex with holes, N-type multiplex with electrons.)

In the first test, it can amplify the audio signal 100 times, its shape is shorter than the firewood stick, but more thicker. In naming the device, Walter Brattain thought of its resistive conversion properties, that is, it works on a transfer current from "low-resistance input" to "high-resistance output," so it's called trans-resistor, later this abbreviated as transistor.

The innovation of transistors was a major invention in the 20th century and the forerunner of the microelectronics revolution. Because the transistor is the key active component in practically all modern electronics. With it, a small, low-power-consuming electronic device can be used to replace a large, high-power-consuming electronic tube. What's more, the development integrated circuits based on the invention of transistors.

In 2016, a team at Lawrence Berkeley National Laboratory broke the physical limit and cut the most sophisticated transistor process available from 14nm to 1nm, making a breakthrough in computing technology.

three electrodes of transistor

Development of Transistors

1) vacuum triode

In February 1939, there was a great discovery in the Bell Labs, the birth of a silicon PN junction. In 1942, a student, Seymour Benzer, was on a team led by Lark_Horovitz at Purdue University,  found that monocrystalline germanium has excellent rectifying performance which other semiconductors do not. These findings laid the groundwork for the later invention of transistors. 

Triode is a vacuum tube with three electrodes which are cathode, anode and a control grid. The function of additional third electrode is to serve as an electrostatic screen which shield the cathode from the electrostatic field of anode triode are used for amplification of weak AC signals of frequency ranging from 0 to 100 MHz.

2) point-contact transistor

The point-contact transistor is the first type of transistor to be successfully demonstrated. It was developed by research scientists John Bardeen and Walter Brattain at Bell Laboratories in December 1947. Bardeen and Brattain applied two closely-spaced gold contacts held in place by a plastic wedge to the surface of a small slab of high-purity germanium. The voltage on one contact modulated the current flowing through the other, amplifying the input signal up to 100 times. The group had been working together on experiments and theories of electric field effects in solid-state materials, with the aim of replacing vacuum tubes with a smaller device that consumed less power.

3) bipolar and unipolar transistors

On the basis of bipolar transistors, Shockley put forward the concept of unipolar junction transistors in 1952, which is called junction transistors today. Its structure is similar to that of PNP or NPN bipolar junction transistors, but there is a depletion layer at the interface of P_N to form a rectifier contact between the gate and the conductive channels of source and drain. At the same time, both ends of the semiconductor as the gate to adjust the current between the source and drain. 

4) silicon transistors

The first working silicon transistor was developed at Bell Labs on January 26, 1954 by Morris Tanenbaum. The first commercial silicon transistor was produced by Texas Instruments in 1954. Silicon transistors and germanium transistors have the function of current amplification. The difference is that the threshold voltage(Even if the positive voltage is applied, it must reach a certain value before it can start to turn on. This is called threshold voltage, for silicon transistor, it is about 0.7V and for germanium transistor is about 0.3V) of silicon transistor is larger than that of germanium transistor; the reverse current of the silicon transistor is much smaller than that of the germanium transistor; the maximum operating temperature of the silicon transistor is higher than that of the germanium transistor; the stability of the silicon transistor is better than that of the germanium transistor.

5) integrated circuit (IC)

After the invention of silicon transistor in 1954, the great application prospect of transistor has become more and more obvious. The next goal of scientists is how to connect transistors, conductors, and other devices efficiently. Due to the invention of transistors, it gives birth to the integrated circuit as the time require. As we all know, an IC is a collection of electronic components—resistors, transistors, capacitors, etc.—all stuffed into a tiny chip, and connected together to achieve a common goal today.

6) field-effect transistors(FET) and metal-oxide-semiconductor field-effect transistor(MOSFET)

The field-effect transistor was first patented by Julius Edgar Lilienfeld in 1926 and by Oskar Heil in 1934, but practical semiconducting devices (the junction field-effect transistors) were developed later after the transistor effect was observed and explained by the team of William Shockley at Bell Labs in 1947. The basic principle of the field-effect transistor was first patented by Julius Edgar Lilienfeld in 1925. In 1959, Dawon Kahng and Martin M. (John) Atalla at Bell Labs invented the metal-oxide-semiconductor field-effect transistor (MOSFET) as an offshoot to the patented FET design. In 1962, Stanley, Heiman and Hofstein in an RCA device integrated study group found that a MOS tube can be constructed by a conductive strip, a high-resistance channel region, an oxide-layer and an insulating layer on a Si substrate through diffusion and thermal oxidation.

7) CPU

A central processing unit (CPU), also called a central processor or main processor, is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions. But fewer people know that modern CPUs contain millions of individual transistors that are microscopic in size. Because transistors are the building blocks of integrated circuit, and more transistors in CPUs means higher processing efficiency.


Compared with the electron tube, the transistor has many advantages:

Fewer consumption

No matter how good an electron tube is, it will gradually deteriorate due to changes in cathode atoms and chronic gas leakage. For technical reasons, the same problem existed at the beginning of transistor fabrication. With advances in materials and improvements in many ways, transistors live typically 100 to 1000 times longer than electron tubes.

Consumption of electric energy is only 1/10 or dozens of times of the electron tube. It does not require heating the filament to produce free electrons like an electron tube. For example, a transistor radio can be listened to for half a year or more longer with a few dry batteries, which is difficult for an electronic tube radio.

No need to preheat

Work as soon as you turn on the machine. For example, a transistor radio, you can hear the sound as soon as it turns on, and pictures comes up quickly when turn on a transistor TV. But electron tube equipment cannot do this. Obviously, transistors have great advantages in electric equipment, medical treatment, industrial measurement, etc.

Solid and reliable

More reliable than the tube because of its shock resistance and vibration resistance. In addition, transistors release less heat due to its smaller size, so it can be used in small, complex, reliable circuits. Although the fabrication process of transistors is precise, but the process is simple, it is helpful to increase the installation of it on the devices.


Transistors are the key active components in all modern electrical appliances. The importance of transistors in today's society is mainly due to their ability to use highly automated processes for mass production, which greatly reducing unit production costs.

While millions of monolithic transistors are still in use, but most transistors are assembled on microchips (chips) with diodes, resistors, and capacitors to make complete circuits. Analog or digital design or both are integrated on the same chip. The cost of designing and developing a complex chip is quite high, but the price per chip is minimal when the cost apportioned to millions of units.


According to material

The semiconductor material used as a transistor can be divided into silicon material transistors and germanium material transistors. Further more, the polarity of the transistor can be divided into four types: germanium NPN transistors and PNP transistor, silicon NPN transistors and PNP transistors.

According to craft

Transistors can be divided into diffusion transistors, alloy type transistors and planar transistors according to their structure and fabrication process.

According to current capacity

Transistors can be divided into low-power transistors, medium-power transistors and high-power transistors by current capacity.

According to service frequency

Transistors can be divided into low-frequency transistors, high-frequency transistors and ultra-high-frequency transistors.

According to packaging Types

The transistors can be divided into metal, plastic, glass, and ceramic packaging transistors.

According to applications

Transistors can be divided into low noise amplification transistors, middle and high frequency amplification transistors, low frequency amplification transistors, switching transistors, Darlington transistors, high reversion voltage transistors, damping transistors, photo transistors and magnetic sensitive transistors and so on.

The low cost, flexibility and reliability of transistors make them as the general choice for non-mechanical tasks, such as digital computing. In the control of electric appliances and machinery, transistor circuits are also replacing motor equipment because of its lower cost and high efficiency.

Specific Types Expressions

1) transistors

It is a semiconductor device with two PN junctions inside and usually three eliciting electrodes outside. Transistor is divided into two main categories: bipolar junction transistor (BJT) and field effect transistor (FET), which have slight differences of their application in a circuit. A bipolar junction transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing between the base and the emitter) can control or switch a much larger current between the collector and emitter. For a field-effect transistor, the terminals are labeled gate, source, and drain, and voltage at the gate can control the current between source and drain.

2) giant transistor

Power transistor is a high voltage, high current bipolar transistor (Bipolar Junction Transistor-BJT), so it is sometimes called Power BJT;. Its characteristics are: high voltage, high current, good switching characteristics, but the driving circuit is complex, driving power is large; the principle of GTR and ordinary bipolar junction transistor is the same.

3) phototransistor

Phototransistor is a device that is able to sense light and alter the current flowing between emitter and collector according to the level of light it receives.

Phototransistors and photodiodes can both be used for sensing light, but the phototransistor is more sensitive in view of the gain provided by the transistor. This makes phototransistors more suitable in a number of applications.

Phototransistors adopt the basic transistor concept as the basis of its operation. In general, a phototransistor can be made by exposing the semiconductor of an ordinary transistor to light. Photo transistors were made by not covering the plastic encapsulation of the transistor with black paint in early stage.

4) bipolar transistor

This is a transistor widely used in audio circuits. The bipolar means the flow of current in two kinds of semiconductor materials. Bipolar transistors can be divided into NPN type or PNP type according to the polarity of operating voltage. 

5) bipolar junction transistor—BJT

The bipolar junction transistor (BJT) is a type of transistor that uses both electron and hole charge carriers. On the contrary, unipolar transistors, such as field-effect transistors, only use one kind of charge carrier. For their operation, BJTs use two junctions between two semiconductor types, N-type and P-type.

BJTs have two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, often in large numbers.


BJTs have amplification function, concretely, it can amplify current, mainly depending on its emitter current transmission through the base area to the collector. To ensure this transmission process, on the one hand, it requires to meet the internal conditions, that is, the impurity concentration in the emission region needs much larger than the impurity concentration in the base region, and meanwhile, the thickness of the base region should be very small. On the other hand, the external conditions should be satisfied, that is, the emission junction should be positive bias (adding positive voltage), and the collector junction should be inversely biased. This allows BJTs to be used as amplifiers or switches, giving them wide applicability in electronic equipment, including computers, TVs, mobile phones, audio amplifiers, industrial control, radio transmitters and so on.

There are many kinds of BJT, according to frequency, high frequency, low frequency, according to power, small, medium, high power, according to semiconductor material, silicon and germanium tube. The amplifier circuit consists of common emitter, common base and common collector.

6) field effect transistor(FET)

The meaning of "field effect" is that the principle of the transistor is based on the electric field effect of the semiconductor. The field effect refers to the modulation of the electrical conductivity of a material by the application of an external electric field.

There are two main types of FET: junction FET (JFET) and metal-oxide semiconductor FET (MOS-FET). Unlike BJT, FET is conducted by only one carrier, therefore, it also known as a unipolar transistor. It belongs to voltage-controlled semiconductor devices have the advantages of high input resistance, low noise, low-power consumption, wide dynamic range, easy integration, no secondary breakdown, wide safe working area and so on.

In a metal, the electron density that responds to applied fields is so large that an external electric field can penetrate only a very short distance into the material. However, in a semiconductor the lower density of electrons (and possibly holes) that can respond to an applied field is sufficiently small that the field can penetrate quite far into the material. This field penetration alters the conductivity of the semiconductor near its surface, and is called the field effect. The field effect underlies the operation of the Schottky diode and of field-effect transistors, notably the MOSFET, the JFET and the MESFET.

The field effect is to change the direction or magnitude of the applied electric field perpendicular to the surface of the semiconductor to control the density or type of most carriers in the conducting layer (channel) of the semiconductor. The current in the channel is modulated by voltage, and the working current is transported by most carriers in the semiconductor. This type of transistor, which has only one polar carrier to conduct electricity, is also called a unipolar transistor. 

Compared with bipolar transistors, FET is widely used in various amplifiers, digital circuits and microwave circuits because of its high input impedance, low noise, high limit frequency, low power consumption, simple manufacturing process and good temperature characteristics. 

7) static induction transistor

The static induction transistor(SIT), which was born in 1970, is actually a junction field-effect transistor. A high-power SIT device can be made by changing the transverse conductive structure of a small-power SIT device used for information processing into a vertical conductive structure. The operating frequency of SIT is comparable to that of the power MOSFET, or even higher than that of the electric MOSFET. The power capacity is also larger than the power MOSFET, so it is suitable for high-frequency and high-power devices. At present, it has been used in radar communication equipment, ultrasonic power amplification, pulse power amplification and high-frequency induction heating and so on.

However, the SIT is conducted when no signal is added to the gate, and the gate is turned off when the negative bias is applied, which is called the normal on-type device, thus it is inconvenient to use. In addition, due to the large on-state resistance and consumption of SIT, it has not been widely used in most power electronic devices.

8) single electron transistor

A kind of transistor that can record signals with one or a small amount of electrons. With the development of semiconductor etching technology, more and more large-scale integrated circuits can be made. It is considered an important component of nanotechnology, single-electron transistors provide high operating speed and low power consumption.

Single-electron transistors are usually made by keeping two tunnel junctions in series. The transistor consists of a source electrode and a source drain, which is joined with the help of a tunneling island that is also connected to a gate capacitively. The electrons can flow to another electrode only through the insulator. There are two categories of single-electron transistors: metallic and semiconducting. The former makes use of a metallic island, and its electrodes using a shadow mask are mostly evaporated onto an insulator. The latter, on the contrary, depends on severing the two-dimensional electron gas that forms at the interface of the semiconductors for the junction.

Insulated-gate bipolar transistor(IGBT) is also a three-terminal device: gate, collector and emitter. It combines the advantages of giant transistor and power MOSFET. Therefore, it is widely used in many fields due to its sound characteristics.

a. main parameters

The main parameters of the transistor include current magnification factor, dissipation power, frequency characteristic, maximum collector current, maximum reverse voltage, reverse current and so on.

b. amplification coefficient

DC current magnification factor, also called static current magnification factor or DC magnification factor. It refers to the ratio of transistor collector current to base current, which is usually expressed by hFE or β when the static signal input is not changed.

c. ac magnification

AC magnification, also called AC current magnification factor or dynamic current magnification factor. It refers to the ratio of transistor collector current variation to base current variation in AC state. In addition, the two parameters are close at low-frequency state.

d. dissipation power

Dissipation power is also called the maximum allowable dissipation power of collector, which refers to the maximum dissipation power of collector when the transistor parameter does not exceed the prescribed allowable value.

The dissipation power is closely related to the maximum allowable junction and collector current of the transistor. The actual power consumption of transistors is not allowed to exceed the maximum allowable dissipation power value, otherwise the transistor will be damaged by overload.

The transistor whose dissipation power is less than 1W is usually called the low-power transistor, that value is equal to or greater than 1W, and less than 5W, such transistor is called the medium-power transistor; whose value is equal to or greater than 5W is called the high-power transistor.

When the operating frequency of the transistor exceeds the cutoff frequency fβ or fα, the current amplification factor β will decrease with the increase of characteristic frequency fT(fT refers to the operating frequency of the transistor when the β value is reduced to 1).

Usually, the transistors whose fT is less than or equal to 3MHZ are called low-frequency transistors; transistors whose fT is greater than or equal to 30MHZ are called high-frequency transistors; those whose fT is greater than 3MHZ and less than 30MHZ are called intermediate frequency transistors.

e. maximum frequency fM

The maximum oscillation frequency is the corresponding frequency when the power gain of the transistor is reduced to 1. In general, the maximum oscillation frequency of high frequency transistors is lower than the common base cutoff frequency fα, while fT is higher than the cutoff frequency fα of common base and lower than the cutoff frequency fβ of common collector.

f. maximum current 

Collector maximum current is the maximum current allowed by transistor collector. When the collector current of the transistor exceeds it, the β value of the transistor will change obviously, which will affect the normal operation of the transistor and even damage it.

g. maximum reverse voltage

Maximum reverse voltage is the maximum operating voltage that the transistor is allowed to apply. It includes collector-emitter reverse breakdown voltage, collector-base reverse breakdown voltage and emitter-base reverse breakdown voltage.

(1) Collector-collector reverse breakdown voltage

This voltage refers to the maximum allowable reverse voltage between the collector and emitter when the base of the transistor is open circuit, usually expressed in VCEO or BVCEO.

(2) Base-base reverse breakdown voltage

This voltage refers to the maximum allowable reverse voltage between the collector and the base when the transistor emitter is open circuit, expressed in VCBO or BVCBO.

(3) Emitter-emitter reverse breakdown voltage

This voltage refers to the maximum allowable reverse voltage between the emitter and the base when the collector of the transistor is open circuit, expressed in VEBO or BVEBO.

(4) ICBO: reverse current between collector and base electrodes

ICBO, also called collector junction reverse leakage current. It refers to the reverse current between collector and base when the emitter of transistor is open circuit. ICBO is sensitive to temperature, thus the smaller the value is, the better the temperature characteristic of transistor is.

(5) ICEO: the reverse breakdown current between collector and emitter

refers to the reverse leakage current between the collector and emitter when the base of the transistor is open. The smaller the current, the better the performance of the transistor.

h. switches

It is a most fundamental application of a transistor is using it to control the flow of power to another part of the circuit, that is, using it as an electric switch. Applying it in either cutoff or saturation mode, the transistor can create the binary on/off effect of switches.

Transistor switch is a critical circuit-building block; it is used to make logic gates, which go on to create microcontrollers, microprocessors, and other integrated circuits.

Control Power

Today's power transistors can control hundreds of kilowatts of power, and using power transistors as switches has many advantages, mainly as follows:

(1) Easy to turn off and few auxiliary components needed.

(2) The switching speed is quick and works at a very high frequency.

(3) The voltage resistance range is wide.

Performance improvement of power transistors. Such as

(1) An increase in the effective working area of switching transistors.

(2) Technical processing simplification.

(3) Recombination of transistors.

(4) The progress of base driving technology for high power switch.

Today's base driving circuits not only drive power transistors, but also protect power transistors, which are called "non-centralized protection" (as opposed to centralized protection). The functions of the integrated drive circuit include:

(1) Turning-on and turning-off power switches.

(2) Monitoring auxiliary power supply voltage.

(3) Limiting maximum and minimum pulse width.

(4) Thermal protection.

(5) Monitoring saturation voltage drop of switches.

Test Replacement

The transistors in the circuit mainly include crystal diode, transistor, thyristor, field effect transistor and so on. The most commonly used transistor and diode are the transistor and diode. How to correctly judge the good or bad of the transistors is one of the key to maintenance.

The key function of an ideal diode is to control the direction of current-flow. Current passing through a diode can only go in one direction, called the forward direction. Current trying to flow the reverse direction is blocked. They’re like the one-way valve of electronics.

If the voltage across a diode is negative, no current can flow, and the ideal diode looks like an open circuit. In such a situation, the diode is said to be off or reverse biased.

As long as the voltage across the diode isn’t negative, it’ll “turn on” and conduct current. Ideally a diode would act like a short circuit (0V across it) if it was conducting current. When a diode is conducting current it’s forward biased (electronics jargon for “on”).

First of all, we should know whether the diode belongs to silicon tube or germanium tube. The forward voltage drop of germanium tube is generally between 0.1~0.3V, while that of silicon tube is generally between 0.6~0.7V. The method of measurement is as follows: two multimeters are used. one multimeter is used to measure the forward resistance and another multimeter is measuring the voltage drop of its tube. Therefore, whether germanium tube or silicon tube can be judged according to the voltage drop values. In addition, the greater the difference between the positive and negative resistance of the measured diode, the better. For example, the forward resistance is several hundreds or thousands of ohms, and the reverse resistance is more than tens of kilos, it can concluded that the diode is good. And meanwhile, the positive and negative electrodes of the diodes can be determined: when the measured resistance values are hundreds of ohm or thousands of ohm, it indicates that is the positive resistance. In addition, if the forward and backward resistance is infinite, it indicates the internal breakage; if the forward and backward resistance is same, there is also a problem with such a diode; and the forward and backward resistance are zero to indicate the short circuit.

Crystal Triode: It mainly plays an amplification role, so how to determine the amplification capacity of it? The method is as follows: the multimeter is adjusted to the level R×100 or R×1K. When the NPN tube is measured, the positive meter pen is connected with the emitter and the negative meter pen is connected with the collector, the finally measured value should be thousands of ohm. Then a 100kΩ resistor is connected in series between the base and collector, and the resistance measured by the multimeter should be significantly reduced. The greater the change, the stronger the amplification ability of the transistor. If the change is small or no change at all, that means the transistor does not have amplification ability or this ability is very weak.

Method of Judging Electrode

Using R×100 level of multimeter germanium transistors to measure and for silicon transistors is R×1K. The red meter pen is in contact with any electrode, and the other two electrodes are measured by black meter pen. If you can’t find two small resistors, you can move the red meter pen to the other electrodes to measure continuously. Neither works, you can move the black meter pen.

When two small resistors are found, the measuring electrode of the fixed meter pen is base. If the fixed meter pen is a black pen, the transistor is the NPN type, and if the fixed one is a red pen, the transistor is a PNP type.

A. method for judging resistances of collector and emitter

A multimeter is used to measure the resistance at the extreme poles of the base removal, and the exchange meter pen is measured twice. In the case of a germanium tube, the smaller resistance is measured as the first time. In the case of the PNP type, the black meter pen is connected to the emitter, and the red meter pen is connected with a collector electrode, as if it is a NPN type. The black meter pen is connected to the collector, the red meter pen is connected to the emitter; If it is a silicon tube, the first time the measured resistance is larger, if it is PNP type, the black meter pen is connected with emitter, the red meter pen is connected with the collector, if the type is NPN, the black meter pen is connected with the collector, and the red meter pen is connected with the emitter.

B. PN junction forward resistance method

Measuring the forward resistance of two PN junctions, the value of emitter is larger and the value of collector is smaller.

C. amplification coefficient method

Using the two meter pens of the multimeter to contact the two electrodes except the base, if it is PNP, using the finger to touch the base and the electrode that red meter pen connected to see the swing of pointer. Change the meter pens to test again, selecting the large swing. At this time, the electrode of red meter pen connected is the collector. If it is NPN, using the finger to touch the base and the electrode that red meter pen connected to see the swing of pointer. Change the meter pens to test again, selecting the large swing, at this time, the electrode of black meter pen connected is the collector.

Note: The between analog multimeter and digital multimeter is different. For analog multimeter, the red meter pen is connected to the negative pole of the power supply, whereas the digital meter is the opposite.

Transistors Replacement Principle

The replacement principle of transistors can be summarized as three: same type, similar characteristics and similar appearance.

One—same type

1.The material is the same, that is, germanium tube replaces germanium tube, silicon tube replaces silicon tube.

2.The polarity is the same, that is, npn-type tube replaces npn-type tube and pnp-type tube replaces pnp-type tube. 

Two—similar characteristics

The characteristics of the transistors used for replacement should be similar to those of the original transistors, and their main parameter values and characteristic curves should not differ much. 

1. Maximum DC dissipation power (pcm) of collector board

Pcm of the replaced transistor is generally required to be equal to or larger than the original transistor. However, in practical test, if the actual DC dissipation power of the original transistor in the whole circuit is much smaller than its pcm, it can be replaced by a transistor with a smaller pcm.

2. Maximum allowable DC current (icm) of collector

Icm of replacing transistor is generally required to be equal to or larger than the original transistor.

3. Breakdown voltage

Transistors for replacement must be able to withstand the maximum operating voltage throughout the machine.

4. Frequency characteristics

The frequency characteristic parameters of transistors are as follows:

(1) characteristic frequency ft: it refers to the frequency when the test frequency is high enough of the common emitter current magnification factor.

(2) cutoff frequency fb:

When replacing transistors, the main consideration is ft and fb. Transistors usually required for replacement should not be less than the corresponding ft and fb of the original one.

5. Other parameters

In addition to the above main parameters, for some special transistors, the following parameters should be taken into consideration when replacing:

(1) For low noise transistors, transistors with small or equal noise coefficients should be used in replacement.

(2) For transistors with automatic gain control performance, transistors with the same automatic gain control characteristics should be used during replacement.

(3) For the switch tube, the related switching parameters should be considered when replacing the switch tube. 

Three—similar appearance

The small power transistors are similar in shape, so long as the lead line of each electrode is marked clearly, and the order of the lead line is the same as that of the tube to be changed, it can be replaced. The appearance of high-power transistors is quite different. In order to install well and maintain normal heat dissipation conditions, the transistors with similar appearance and same size should be selected for replacement.

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Smaller and faster has been the trend for electronic devices since the inception of the computer chip, but flat transistors have gotten about as small as physically possible. For researchers pushing for even faster speeds and higher performance, the only way to go is up.

University of Illinois researchers have developed a way to etch very tall, narrow finFETs, a type of transistor that forms a tall semiconductor "fin" for the current to travel over. The etching technique addresses many problems in trying to create 3-D devices, typically done now by stacking layers or carving out structures from a thicker semiconductor wafer.

"We are exploring the electronic device roadmap beyond silicon," said Xiuling Li, a U. of I. professor of electrical and computer engineering and the leader of the study. "With this technology, we are pushing the limit of the vertical space, so we can put more transistors on a chip and get faster speeds. We are making the structures very tall and smooth, with aspect ratios that are impossible for other existing methods to reach, and using a material with better performance than silicon."

Typically, finFETs are made by bombarding a semiconductor wafer with beams of high-energy ions. This technique has a number of challenges, Li said. For one, the sides of the fins are sloped instead of straight up and down, making them look more like tiny mountain ranges than fins. This shape means that only the tops of the fins can perform reliably. But an even bigger problem for high-performance applications is how the ion beam damages the surface of the semiconductor, which can lead to current leakage.

The Illinois technique, called metal-assisted chemical etching or MacEtch, is a liquid-based method, which is simpler and lower-cost than using ion beams, Li said. A metal template is applied to the surface, then a chemical bath etches away the areas around the template, leaving the sides of the fins vertical and smooth.

"We use a MacEtch technique that gives a much higher aspect ratio, and the sidewalls are nearly 90 degrees, so we can use the whole volume as the conducting channel," said graduate student Yi Song, the first author of the paper. "One very tall fin channel can achieve the same conduction as several short fin channels, so we save a lot of area by improving the aspect ratio."

The smoothness of the sides is important, since the semiconductor fins must be overlaid with insulators and metals that touch the tiny wires that interconnect the transistors on a chip. To have consistently high performance, the interface between the semiconductor and the insulator needs to be smooth and even, Song said.

Right now, the researchers use the compound semiconductor indium phosphide with gold as the metal template. However, they are working to develop a MacEtch method that does not use gold, which is incompatible with silicon.

"Compound semiconductors are the future beyond silicon, but silicon is still the industry standard. So it is important to make it compatible with silicon and existing manufacturing processes," Li said.

The researchers said the MacEtch technique could apply to many types of devices or applications that use 3-D semiconductor structures, such as computing memory, batteries, solar cells and LEDs

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