Working principle and application scope of bipolar transistors

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What is a bipolar transistor

•Bipolar transistors are electronic components composed of three layers of precisely doped semiconductor materials. This device is often called a semiconductor transistor or transistor in the field of engineering technology because it has three independent electrode structures. To facilitate subsequent discussion and understanding, this article will uniformly use the professional term "transistor" to express it.

How transistors work

•The transmission process of carriers in transistors under amplified state，when the transistor operates in the amplification zone, its transmitting junction is in a forward biased state, while the current collecting junction is in a reverse biased state. In this operating mode, the motion rules of carriers inside the transistor and their current transmission mechanism can be explained in detail through the schematic diagram shown in the figure below.

①The electron current IEP injected into the base region of the emission region is much smaller than the emitter electron current IEN (IEP << IEN), so the emitter current IE is mainly determined by IEN, that is, IE≈IEN.

②Electrons recombine with holes during the diffusion of the base region to form the base region recombination current IBN, which forms the main component of the base current IB.

③Electrons are collected by the collector junction field under the action of the collector junction field to form the collector collector current ICN, which forms the main component of the collector current IC.

④Under the reverse bias condition, the drift motion of minority carriers leads to the generation of the reverse saturation current ICBO.

•In the transistor, the narrow base region tightly couples the transmitter junction and the current collector junction, so that almost all the forward current generated by the transmitter junction when forward bias is transmitted to the reverse-biased current collector junction loop. This characteristic is the core mechanism for the transistor to achieve the amplification function.

Compared with CMOS technology

•The bipolar transistor process has unique advantages in the field of high-performance analog circuits and can realize precise simulation functions that are difficult to achieve by MOS devices. It has high-speed, high-transconductance, excellent device matching characteristics, and accurate voltage with good temperature stability, which are the key indicators pursued by high-performance analog circuit design. However, the layout design complexity of bipolar transistors is significantly higher than that of MOS devices, and there are some failure mechanisms that do not exist in the CMOS process. Therefore, although bipolar transistors have advantages in analog performance, modern analog integrated circuit designs tend to adopt CMOS processes.

Emitter current edge collection effect

•The emitter current edge collecting effect results in a significant increase in the current density in the emitter edge region, thereby reducing the effective utilization area of the emitter junction.

•This effect is derived from the influence of the base resistance of the NPN transistor, including specifically the lateral diffusion resistance of the base region just below the transmitting region and the lateral resistance of the base region peripheral to the transmitting region. Since the base current flows laterally within the base region, a voltage drop will occur on the base resistance, resulting in uneven potential distribution of points in the base region directly below the emission area: the potential is higher near the edge of the base region, and the potential gradually decreases as the distance from the base region increases. This unevenness of potential distribution causes the injection current density of each point on the emission junction surface to be different, and the current density near the edge of the base region is relatively large, resulting in the emitter current mainly concentrated on the side of the emission junction near the base region.

•Solution

In the NPN transistor layout design, simply increasing the emission area cannot achieve a year-on-year increase in the emitter current. The design focus should be on optimizing the ratio of emitter circumference to area, that is, improving the current driving capability by increasing the circumference of the emission zone. To this end, a comb-shaped layout structure can be used, and each rack is designed to be an elongated shape. However, considering the resistance effect of the emitter metal wire, the size of the rack needs to be weighed: neither too long can lead to too large resistance, nor too thin can affect process achievability and reliability.

Application range of bipolar transistors

•In hi-fi sound system, television receiving device and broadcast receiving device

•In the fields of industrial robots, smart power tools and automotive electronic control systems

•In household appliances (such as air conditioners, refrigerators) that require precise temperature regulation and industrial automation control systems

•In the field of precision medical instruments, such as electrocardiogram monitoring equipment, anesthesia workstations and electrical nerve stimulation devices

•In the field of aerospace science and technology, including the research and development and application of aircraft, artificial satellites and launch vehicles, as well as the design and deployment of high-performance computing equipment and network communication systems.