A transistor is a semiconductor device that is commonly used in amplifiers or electronically controlled switches. It’s the basic building block that regulates the operation of computers, cell phones, and all other modern electronic circuits. Due to the fast response time and high accuracy, transistors can be used for a variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators.

I Classification Method of Transistors

Strictly speaking, a transistor refers to all single components based on semiconductor materials, including diodes (two terminals), transistors, field-effect transistors, thyristors (the latter three are of three terminals). 

Three-terminal transistors are mainly divided into two categories: bipolar transistors (BJT) and field-effect transistors(FET). The three terminals of a bipolar transistor are an emitter, a base, and a collector composed of N-type and P-type semiconductors; The three terminals of the field-effect transistor are the source, the gate, and the drain.

Transistors can be classified based on:

 ● Material

Transistors can be divided into silicon transistors and germanium transistors based on semiconductor materials. And according to the polarity, the two types of transistors can be subdivided into germanium NPN type transistor, germanium PNP transistor, silicon NPN type transistor, and silicon PNP type transistor.

 ● Manufacturing Process

There are diffuse-type transistors, alloy type transistors, and planar type transistors according to the manufacturing process of the transistors.

 ● Current Capacity

Transistors can be divided into three groups: small power transistors, medium power transistors, and high power transistors based on their current capacity.

 ● Working Frequency

According to the operating frequency, there are low-frequency transistors, high-frequency transistors, and ultra-high-frequency transistors.

 ● Package Structure

In the light of the package structure, transistors can be classified into metallic packaging transistors, plastic packaging transistors, glass packaging transistors, surface-mounted transistors, and ceramic packaging transistors.

 ● Function and Use

Transistors can be divided into low-noise amplifier transistors, medium and high-frequency amplifier transistors, low-frequency amplifier transistors, switching transistors, Darlington transistors, high-voltage transistors, band-stop transistors, damping transistors, microwave transistors, phototransistors, and magnetic transistors.

II Representative Types of Transistors

A semiconductor transistor is a semiconductor device that usually contains two PN junctions inside and three extraction electrodes outside. Strictly speaking, a transistor refers to all single components based on semiconductor materials, including diodes (two terminals), transistors, field-effect transistors, thyristors (the latter three are of three terminals). 

Three-terminal transistors are mainly divided into two categories: bipolar junction transistora>s (BJT) and field-effect transistors(FET). The three terminals of a bipolar transistor are an emitter, a base, and a collector composed of N-type and P-type semiconductors; The three terminals of the field-effect transistor are the source, the gate, and the drain. The following mainly discuss the bipolar transistors, field-effect transistors, and some other typical types of transistors.

1. Bipolar Junction Transistors (BJT) 

Bipolar Junction Transistor (BJT) is a device that combines two PN junctions through a certain process. Here, "Bipolar" means that both the electron and holes participate in the movement at the same time when they're working. There are two combined structures, PNP and NPN. Three poles are led out externally, which are the collector, emitter, and base. The collector is led from the collector region, the emitter is led from the emitter region, and the base is led from the base region (in the middle). 

PNP schematic symbol (a), layout (b), NPN schematic symbol (c), layout  (d) 

The amplification effect of BJT mainly relies on the transmission of the emitter current from the base region to the collector region. In order to ensure this transmission process, two conditions should be satisfied:

 ● Internal Conditions

The impurity concentration in the emitter region should be much greater than that in the base region, and the thickness of the base region must be small. 

 ● External Conditions

The emitter junction must be forward-biased, and the collector junction must be reverse-biased.

2. Field-Effect Transistors

Field-effect transistors are the transistors that work with the field-effect principle of semiconductors. There are two main types of field-effect transistors: Junction FET (JFET) and Metal-Oxide Semiconductor FET (MOSFET). 

Junction FET circuit symbol

The field effect is used to change the direction or magnitude of the applied electric field perpendicular to the semiconductor surface to control the density or type of the majority carriers in the conducting layer(channel) of the semiconductor. The current in the channel is modulated by the voltage, and the operating currents are from the majority carriers in the semiconductor. 

Unlike BJT, only one kind of carriers (majority carriers) of the FET participate in the conduction process, so it is also called the unipolar transistor. 

The advantages of field-effect transistors are:

  ○ high input impedance 

  ○ low noise

  ○ high limit frequency

  ○ low power consumption

  ○ simple manufacturing process 

  ○ good temperature characteristics 

These features make them widely used in various amplifier circuits, digital circuits, and microwave circuits, etc. Silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs) and GaAs-based metal-semiconductor field-effect transistors (MESFETs) are the two most important field-effect transistors, which are respectively the basic devices of MOS large-scale integrated circuits and MES ultra-high-speed integrated circuits.

3. Other Typical Transistor Types

 ● Giant Transistors(GTR)

The giant transistor is a kind of bipolar transistor that can withstand high voltage and high current, so it can also be called as power BJT. 

Its characteristics are: 

  ○ high voltage resistance 

  ○ large current

  ○ good switching characteristics

  ○ complex driving circuit and large driving power 

The working principle of GTR is the same as that of ordinary bipolar junction transistors.

 ● Phototransistors

The phototransistor is a type of photoelectric device consisting of a three-terminal device such as a bipolar transistor or a field-effect transistor. Light is absorbed in the active region of the device, producing photo-generated carriers, which are amplified by the internal mechanism and generates a photocurrent gain. As the phototransistor operates with three terminals, it is easy to achieve electric control or synchronization.

Schematic and Part Drawing of the Phototransistor

There are mainly two kinds of phototransistors: bipolar phototransistors and field-effect phototransistors. Bipolar phototransistors usually have high gains, but the speed is not fast. For GaAs-GaAlAs bipolar phototransistors, its amplification factor can be greater than 1000, and the response time is larger than nanoseconds. This kind of phototransistor is often used for optical detectors or optical amplification. The field-effect phototransistor has a fast response speed (about 50 picoseconds), but its photosensitive area and gain are small, which is often used as an extremely high-speed photodetector.

The response time of planar optoelectronic devices is tens of picoseconds, making them suitable for optoelectronic integration.

 ● Static Induction Transistors

The static induction transistor(SIT) is actually a junction field-effect transistor. For a low-power SIT used for information processing, if we change its horizontal conductive structure to a vertical conductive structure, it can be transformed into a high-power SIT device. 

The operating frequency of SIT is equivalent to or even higher than that of power MOSFETs, and its power capacity is greater than that of power MOSFETs. Therefore, it is suitable for high-frequency and high-power applications such as radar communication equipment, ultrasonic power amplification, pulse power amplification, and high-frequency induction heating. 

However, SIT is turned on when no signal is applied to the gate, and it’s turned off when the gate is applied with a negative bias, which is not convenient to use. In addition, the large on-state resistance of the SIT will increase the loss, so it has not been widely used in most power electronic equipment.

 ● Single-Electron Transistors

Single-electron transistors can record signals with one or a few electrons. 

With the development of the semiconductor etching technique, the integration level of large-scale integrated circuits is getting higher and higher. At present, each memory cell of a general memory contains 200,000 electrons, while each memory cell of a single-electron transistor contains only one or a small number of electrons, which could greatly reduce power consumption and improve the integration level of integrated circuits. 

Schematic Diagram of a Single-Electron Transistor

In 1989, J.H. F. Scott Thomas and his partners discovered the Coulomb blockade during an experiment. On the test, they tried to make a metal electrode with a small area on the two-dimensional electron gas at the interface of modulation-doped hetero-junction, so that a quantum dot with small capacitance(10 ~ 15 faras) can be formed in the electron gas. When a voltage is applied, there will be no current flow through the device until the voltage is large enough to cause a change in an electron charge. Therefore, the current-voltage relationship is not linear but step-like. This experiment was the first time in history to manually control the movement of an electron, providing an experimental basis for manufacturing single-electron transistors. 

In order to increase the operating temperature of the single-electron transistor, the size of the quantum dot must be less than 10 nanometers, which is a pressing issue for laboratories all over the world.

III How to Test Transistors

The transistors in the circuit mainly include crystal diodes, crystal transistors, thyristors, and field-effect transistors, among which the crystal transistors and diodes are most commonly used. So how can we correctly judge the quality of diodes and transistors?

1. Detection of Crystal Diodes

 ● Performance: Good or Bad

Firstly, we should judge the material of the crystal diode is silicon or germanium. Use one multimeter to measure its forward resistance, and use another multimeter to measure the voltage drop. Generally, the forward voltage drop of the germanium tube is between 0.1-0.3V, and that of the silicon tube is usually between 0.6-0.7V. 

Besides, the difference between the forward and reverse resistance of the diodes should be as big as possible. If the forward resistance of a crystal diode is hundreds to thousands of ohms, and the reverse resistance is tens of thousands of ohms or more, then it can be regarded as a good diode. 

 ● Electrode: Positive or Negative

Also, the positive and negative electrodes of the diode can be determined at the same time. When the measured resistance is several hundred ohms or several thousand ohms, then it should be determined as the forward resistance of the diode. At this time, the negative test lead is connected to the negative electrode, and the positive test lead is connected to the positive electrode. In addition, if the forward and reverse resistance are infinite, it means that there is an internal disconnection; if the forward and reverse resistance are zero, indicating that it is short-circuited.

2. Crystal Transistors Testing Method

 ● Testing Amplifying Ability 

The crystal transistor is mainly used for amplification, so how do we judge its amplification ability? 

First, set the gear of the multimeter to R × 100 or R × 1K. When we measure the NPN tube, the positive test lead is connected to the emitter and the negative test lead is connected to the collector. The measured resistance should generally be more than several thousand ohms.

Then connect a 100 kΩ resistor in series between the base and the collector. At this time, the resistance value measured by the multimeter should be significantly reduced. The larger the change, the stronger the amplification ability of the transistor. If the change is small or even none, it means that the transistor has weak or no amplification capability. 

 ● Judging Electrodes

  ○ Find the base

First, connect the red test lead to any of the pins, and use the black test pen to respectively measure the other two pins. 

To see if two small resistance can be measured, if not, connect the black test to one pin and contact the red test lead with other pins to measure until two small resistance are obtained.

When the two small resistances are found, the fixed test lead used at that moment is the base. If the fixed test pen is black, the transistor is NPN-type; if the fixed test lead is the red one, the tube is a PNP-type transistor.

Note: The germanium tube is measured with R×100, and the silicon tube is measured with R ×1k.

  ○ Determine the emitter and the collector

Use a multimeter to measure the resistance of the two poles apart from the base electrode. Exchange the test lead and measure it again. 

If it is a germanium tube, the smaller resistance is used for judgment. When the smaller resistance is obtained, for a PNP transistor, the black test lead is connected to the emitter, and the red one is connected to the collector. If it is an NPN type, the black test lead is linked to the collector, and the red test lead is linked to the emitter.

If it's a silicon transistor, the larger resistance is used. For the PNP type, the black lead is connected to the emitter, while the red test lead is connected to the collector. And as for the NPN transistor, the black and red test lead is respectively connected to the collector and the emitter.

Besides, we could also measure the forward resistance of the two PN junctions separately. The one with the larger forward resistance is the emitter and the other is the collector.

IV Darlington Transistor Testing Method

1. Detection of Ordinary Darlington Transistor

In the internal structure of an ordinary Darlington transistor, two or more collectors of transistors are connected together, and there are multiple emitter junctions between the base and the emitter.

 ● Testing of Forward and Reverse Resistance

The R × 1 kΩ or R × 10 kΩ of the multimeter is used for measurement.

Normally, the forward resistance between the collector and the base is similar to the value of ordinary silicon transistors collectors, which is 3-10 kΩ, and the reverse resistance value is infinite. The forward resistance value between the emitter and the base is 2 to 3 times that between the collector and the base, and the reverse resistance value is also infinite. 

Theoretically, the positive and negative resistance between the collector and the emitter should be close to infinity. If the positive and reverse resistance value between the collector and the emitter of the Darlington transistor is near zero, or the value between the base and the emitter or between the base and the collector is zero, it indicates the tube has broken. And if the forward and reverse resistance between the base and the emitter or between the base and the collector is measured to be infinite, it denotes that there is an open circuit.

Note: when we measure the NPN tube, black test lead is connected to the base; when a PNP tube is detected, the black test lead is linked to the collector.

Basic Darlington Transistor Configuration

2. Detection of High-Power Darlington Transistor

Based on ordinary Darlington transistors, the high-power Darlington has a protection circuit composed of a freewheel diode and a bleeder resistor, which may influence the measurement data.

 ● Detection Method 1

Use the R ×1 kΩ or R × 10 kΩ range of the multimeter to measure the forward and reverse resistance of the Darlington collector junction(between the collector and the base). Under normal conditions, when the base of the NPN tube is connected to the black test lead, the forward resistance value should be small, ranging from 1 to 10 kΩ, and the reverse resistance should be close to infinity. If the measured forward and reverse resistance values are both very small or infinite, it means that the tube has been short-circuited or damaged by an open circuit.

 ● Detection Method 2

Use the R × 100 Ω gear of multimeter to measure the forward and reverse resistance between the emitter and the base. The normal values are several hundred ohms to several thousand ohms. if measured resistance is 0 or infinite, the tube under test is damaged.

 ● Detection Method 3

R × l kΩ or R × 10 kΩof the multimeter is used to measure the forward and reverse resistance between the emitter and the collector. Normally, the forward resistance value should be 5- 15 kΩ, and the reverse resistance value should be infinite, if not, the collector and the emitter (or diodes) are broken or there is an open circuit.

Note: when we measure the NPN tube, black test lead is connected to the emitter, and the red test lead is connected to the collector; when we measure the PNP tube, black test lead is connected to the collector, and the red test lead is connected to the emitter.

Conclusion

In this passage, first, we’ve learned about the general classification method and the main typical types of transistors. Then, the testing method for crystal diodes and crystal transistors were introduced, which includes the means to judge the performance and determine the electrodes. And in the end, we discuss the detection methods for ordinary and high-power Darlington transistors. Hope this article could be useful to you!