What is a bipolar transistor and what is its operating mode

• What is a bipolar transistor
• How bipolar transistors work
• Bipolar junction transistor four modes of operation
• Bipolar transistor development applications

What is a bipolar transistor

Bipolar transistor, the full name of bipolar junction transistor, commonly known as the transistor, is an electronic device with three terminals, made of three parts of different degrees of doping semiconductor, the charge flow in the transistor is mainly due to the carrier at the PN junction diffusion and drift motion.

The work of this transistor involves the flow of both electron and hole carriers, so it is called bipolar, so it is also called bipolar carrier transistor. This mode of operation is different from unipolar transistors such as field effect tubes, which only involve the drift of a single type of carrier. The boundary between the aggregation regions of two different dopants is formed by PN junction.

Bipolar transistors can amplify signals, and has good power control, high-speed work and durability, so it is often used to form amplifier circuits, or drive loudspeakers, motors and other equipment, and is widely used in aerospace engineering, medical devices and robots and other applications.

How bipolar transistors work

The NPN type bipolar transistor can be seen as two diodes joined together with a common anode. In the normal operating state of a bipolar transistor, the base-emitter junction (called the PN "emission junction") is in a forward biased state, and the base-collector (called the PN "collector junction") is in a reverse biased state. When there is no applied voltage, the concentration of electrons in the N region of the emitter junction (most of the carriers in this region) is greater than the concentration of electrons in the P region, and some electrons will diffuse to the P region. Similarly, some of the holes in the P region will also spread to the N region. In this way, the emission junction will form a space charge region (also known as the depletion layer), generating an internal electric field, its direction from the N region to the P region, this electric field will prevent the above diffusion process from further occurring, so as to achieve a dynamic equilibrium. At this time, if a forward voltage is applied to the emitter junction, the dynamic balance between the above carrier diffusion motion and the internal electric field in the depletion layer will be broken, which will allow the heat-excited electrons to be injected into the base region. In NPN transistors, the base region is P-type doping, where the holes are mostly doped substances, so the electrons in this region are called "minority carriers".

The electrons injected into the base region from the emitter, on the one hand, will recombine with most of the carrier holes here, on the other hand, due to the low doping degree of the base region, thin physical size, and the collector junction in the reverse bias state, most of the electrons will reach the collector region through drift motion, forming the collector current. To minimize recombination of electrons before they reach the collector junction, the base region of the transistor must be made so thin that the time required for carrier diffusion is shorter than the lifetime of a few carriers in a semiconductor, and the thickness of the base must be much smaller than the diffusion length of the electrons (see Fick's Law). In modern bipolar transistors, the typical value of base region thickness is a few tenths of a micron. It should be noted that although the collector and emitter are N-type doping, the degree of doping and physical properties of the two are not the same, so it is necessary to distinguish the bipolar transistor from the form of two diodes in series in opposite directions.

Bipolar junction transistor four modes of operation

1. Amplification mode
The amplification mode of bipolar junction transistors is the most common mode of operation. In amplification mode, a small change in the input signal can result in a larger output signal. The input of a bipolar junction transistor is the base, the output is the collector, and the emission is in the middle region. In amplification mode, the base is the input terminal, and its voltage change will cause the collector current change. By selecting the appropriate component parameters, the desired magnification can be obtained at different currents and voltages.
2.switch mode
The switching mode of bipolar junction transistor is also called digital mode, and its basic principle is that when the transistor is in a saturated state, its switching characteristics are similar to a digital switch. At this time, due to the strong conductivity of the device, it can withstand large currents and voltages. In switching mode, the input signal passes through the circuit to control the base current between black and saturation. When the input signal is "1", the output is high; When the input signal is "0", the output is low.
3.reverse saturation mode
The reverse saturation mode refers to the polarity reversal of the emitter of a bipolar junction transistor. In reverse saturation mode, a small current is used to control a large current, which is used in switching mode and digital systems. In reverse saturation mode, the polarization is the opposite of the amplifier, that is, the emission is extremely negative and the base is positive. In this mode, the base and collector operate in the same saturation region as in the switching mode, and the current is controlled between the base and emitter.
4. Positive saturation mode
The principle of the forward saturation mode is similar to that of the reverse saturation mode, but the polarity is the opposite of the reverse saturation mode. In the forward saturation mode, the emission is extremely positive, and the base and collector work between the saturated region. This method is widely used in digital systems and switching power supplies.
These are the four operating modes of bipolar junction transistors, each mode has different application scenarios, choosing the right mode can make the circuit performance better.

Bipolar transistor development applications

In December 1947, John Bardeen and Walter Hauser Bratton of Bell LABS, under the direction of William Shockley, invented the point-contact form of the bipolar transistor. In 1948, Shockley invented the bipolar transistor using the junction structure. For the next thirty years or so, this device was the best choice for the manufacture of discrete component circuits and integrated circuits.

The early transistors were made of germanium. In the 1950s and 1960s, germanium transistors were used more than silicon transistors. Compared to silicon transistors, germanium transistors have a smaller cut-off voltage, usually about 0.2 volts, which makes germanium transistors suitable for certain applications. In the early history of transistors, a variety of manufacturing methods for bipolar transistors were developed.

A major disadvantage of germanium transistors is that they are prone to thermal runaway. Because germanium has a narrow band gap and requires more stringent temperatures for stable operation than silicon semiconductors, most modern bipolar transistors are made of silicon. Another important reason for using silicon materials is that silicon is much more abundant on Earth than germanium (second only to oxygen).

Later, people also began to use compounds represented by gallium arsenide to make semiconductor transistors. The electron mobility of gallium arsenide is five times that of silicon, and transistors made with it can achieve higher operating frequencies. In addition, gallium arsenide has a low thermal conductivity, which is conducive to processing at high temperatures. Compound transistors can often be applied to high-speed devices.

Bipolar transistor can provide signal amplification, it is used in power control, analog signal processing and other fields. In addition, since the relationship between the base-emitter bias voltage and temperature and current is known, bipolar transistors can also be used to measure temperature. Based on the logarithmic relationship between the base-emitter voltage and the base-emitter and collector-emitter currents, bipolar transistors can also be used to calculate logarithms or to find power exponents of natural logarithms.

As people's understanding of energy issues continues to deepen, field-effect tube (such as CMOS) technology with lower power consumption, in the digital integrated circuit gradually become the mainstream, the use of bipolar transistors in integrated circuits gradually less. However, it should be noted that even in modern integrated circuits, bipolar transistors are still an important device, and there are still a large number of complete and inexpensive transistor products on the market to choose from. Compared with metal oxide semiconductor field effect transistors (MOSFETs, which are a type of field effect tubes, the other is a junction field effect tube), bipolar transistors can provide higher transconductance and output resistance, and have high-speed, durable characteristics, and outstanding ability in power control. Therefore, bipolar transistors are still an important part of analog circuits, especially circuits for very high frequency applications (such as radio frequency circuits in wireless communication systems). Bipolar transistors can be made on an integrated circuit with MOSFETs using BiCMOS technology, so that the advantages of both can be fully utilized (such as the current amplification capability of bipolar transistors and the low power consumption characteristics of Fets).