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Working principle of bipolar transistor

24/03/2022

What is a bipolar transistor

Bipolar transistor, full name bipolar junction transistor (BJT), commonly known as triode, is an electronic device with three terminals, made of three parts of semiconductors with different levels of doping. The charge flow in the transistor is mainly due to the carrier in the PN Diffusion and drifting motion at the junction.

The work of this kind of transistor involves the flow of both electron and hole carriers at the same time, 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 transistors, which only involve the drift of a single type of carrier. The boundary between the two different dopant accumulation regions is formed by a PN junction.

Bipolar transistors can amplify signals, and have good power control, high-speed operation and durability, so they are often used to form amplifier circuits, or drive speakers, motors and other equipment, and are widely used in aerospace engineering , Medical equipment and robots and other application products.

The working principle of bipolar transistor

NPN type bipolar transistor can be regarded as two diodes with a common anode joined together. In the normal working state of a bipolar transistor, the base-emitter junction (called this PN junction as the "emitter junction") is in a forward biased state, and the base-collector (called this PN junction as the "collector End”) is in a reverse biased state. When there is no applied voltage, the electron concentration in the N region of the emitter junction (the majority of carriers in this region) is greater than the electron concentration in the P region, and part of the electrons will diffuse to the P region. In the same way, part of the holes in the P area will also diffuse to the N area. In this way, a space charge region (also known as a depletion layer) will be formed on the emitter junction, generating an internal electric field whose direction is from the N region to the P region. This electric field will hinder the further occurrence of the aforementioned diffusion process, thereby achieving a dynamic balance. At this time, if a forward voltage is applied to the emission junction, the dynamic balance between the aforementioned carrier diffusion and the internal electric field in the depletion layer will be broken, which will cause thermally excited electrons to be injected into the base region. In NPN transistors, the base region is P-type doped, where holes are the majority doped material, so electrons in this region are called "minority carriers".

The electrons injected from the emitter into the base region will recombine with the majority carrier holes here. On the other hand, due to the low level of doping in the base region, the thin physical size, and the reverse of the collector junction In the bias state, most of the electrons will reach the collector area through drifting motion, forming a collector current. In order to minimize the recombination of electrons before they reach the collector junction, the base region of the transistor must be made thin enough that the time required for carrier diffusion is shorter than the lifetime of semiconductor minority carriers. At the same time, the base area The thickness must be much smaller than the diffusion length of the electron (see Fick's law). In modern bipolar transistors, the thickness of the base region is typically a few tenths of a micrometer. It should be noted that although the collector and emitter are both N-type doped, the degree of doping and physical properties of the two are not the same. Therefore, it is necessary to distinguish between the bipolar transistor and the two diodes in opposite directions in series. Open.

Development and Application of Bipolar Transistor

In December 1947, John Barding and Walter Hauser Braton of Bell Laboratories jointly invented the point-contact bipolar transistor under the guidance of William Shockley. In 1948, Shockley invented a bipolar transistor with a junction structure. In the next thirty years, this device is the best choice for manufacturing discrete component circuits and integrated circuits.

Early transistors were made of germanium. In the 1950s and 1960s, germanium transistors were used more than silicon transistors. Compared with 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 bipolar transistor manufacturing methods were developed.

One of the main disadvantages of germanium transistor is that it is prone to thermal runaway. Because germanium has a narrower band gap, and the temperature required for stable operation is stricter than that of silicon semiconductors, most modern bipolar transistors are made of silicon. Another important reason for using silicon materials is that the reserves of silicon on the earth are much more abundant than germanium (second only to oxygen).

Later, people also began to use compounds represented by gallium arsenide to manufacture semiconductor transistors. The electron mobility of gallium arsenide is 5 times that of silicon, and transistors made with it can reach higher operating frequencies. In addition, gallium arsenide has a low thermal conductivity, which is good for processing at high temperatures. Compound transistors can generally be applied to high-speed devices.

Bipolar transistors can provide signal amplification, and it has applications 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. According to the logarithmic relationship between the base-emitter voltage and the base-emitter and collector-emitter currents, bipolar transistors can also be used to calculate the logarithm or find the power exponent of the natural logarithm.

As people’s awareness of energy issues continues to deepen, FET (such as CMOS) technology has gradually become the mainstream in digital integrated circuits with lower power consumption, and the use of bipolar transistors in integrated circuits has gradually decreased. . 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 low-cost transistor products available on the market. Compared with metal oxide semiconductor field effect transistors (MOSFET, which is a kind of field effect transistor and the other is a junction field effect transistor), bipolar transistors can provide higher transconductance and output resistance, and have High-speed, durable characteristics, outstanding ability in power control. Therefore, bipolar transistors are still an important part of analog circuits, especially VHF application circuits (such as radio frequency circuits in wireless communication systems). Bipolar transistors can be fabricated on an integrated circuit with BiCMOS technology and MOSFETs, 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 field effect transistors)

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