The structure of transistor and the effect of transistor current amplification
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The structure of transistor and the effect of transistor current amplification

Posted Date: 2024-01-17

There are two types of carriers with different polarity charges in the transistor that participate in conduction, so it is called a bipolar transistor (BJΓ), also called a semiconductor transistor, hereafter referred to as a transistor.


Figure 1.3.1 shows several common shapes of transistors.
Figures (a) and (b) show low-power tubes, Figure (c) shows medium-power tubes, and Figure (d) shows high-power tubes.
The shape shown in Figure (b) (c) facilitates the installation of a radiator to prevent the tube from being damaged due to excessive power consumption.

Collector (a) (b) (c) (d) Figure 1.3.1 Several common shapes of transistors (a) Low power tube (b) Low power tube (c) Medium power tube (d) High power tube 1.3. 1. The structure and type of transistor. According to different doping methods, three doping regions are created on the same silicon wafer and two PN junctions are formed to form a transistor.

The structure of an NPN silicon material transistor made by planar technology is shown in Figure 1.3.2(a). The P region in the middle is called the base region, which is very thin and has a low impurity concentration; the N region in the upper layer is the emitter. area, the doping concentration is very high; the N area located in the lower layer is the collector area and has a large area; the external characteristics of the transistor are closely related to the above characteristics of the three areas.

The three electrodes they lead to are base b, emitter e and collector c.

Figure (b) shows a schematic structural diagram of an NPN tube. The PN junction between the emitter region and the base is called the emitter junction, and the PN junction between the base region and the collector is called the collector junction.

Figure (c) shows the symbols for NPN type tubes and PNP type tubes. This section uses NPN silicon transistors as an example to describe the amplification effect, characteristic curves and main parameters of transistors.

Transistor current amplification

Amplification is the basic processing of analog signals. In actual production and scientific experiments, the electrical signals obtained from the sensors are very weak and can only be processed further after amplification, or have enough energy to drive the actuator. Transistors are components of amplifier circuits that can control the conversion of energy and amplify any small changes in input to the output without distortion.

Figure 1.3.3 shows the basic amplifier circuit. Δu is the input voltage signal, which is connected to the base-emitter loop, which is called the input loop; the amplified signal is in the collector-emitter loop, which is called the output loop. Since the emitter is the common terminal of the two loops, the circuit is called a common emitter amplifier circuit.

The external condition that makes the transistor operate in the amplified state is that the emitter junction is forward biased and the collector junction is reverse biased. Therefore, the base power supply V needs to be added to the input circuit; the collector power supply Vc needs to be added to the output circuit; the polarity of V0 and Vc should be as shown in Figure 1.3.3, and Vc should be greater than V. The amplification effect of the transistor is small. Base current can control large collector current.

Next, we will do further analysis from the relationship between the movement of internal carriers and the external current. 1. Movement of carriers inside the transistor When Δu=0 in the circuit shown in Figure 1.3.3, the schematic diagram of carrier movement inside the transistor is shown in Figure 1.3.4. 7./cIcaoRc+7, 7, R.RbuR1.+lAuVcc7.VVBBl.


Figure 1.3.3 Basic common-emitter amplifier circuit diagram 1.3.4 Internal carrier movement and external current of the transistor

1. Apply forward voltage to the emitter junction, and the diffusion movement forms the emitter current I.

Because a forward voltage is applied to the emitter junction and because the impurity concentration in the emitter region is high, a large number of free electrons cross the emitter junction and reach the base region due to diffusion movement.

At the same time, holes also diffuse from the base region to the emitter region. However, due to the low impurity concentration in the base region, the current formed by holes is very small and can be ignored in approximate analysis.

It can be seen that the diffusion movement forms the emitter current l2. The recombination movement of free electrons and holes diffused into the base region forms the base current I. Since the base region is very thin and the impurity concentration is very low, a reverse voltage is added to the collector junction. Therefore, only a very small part of the electrons diffused into the base region recombine with holes, and the rest reach the collector junction as non-equilibrium minority carriers in the base region.

And due to the power supply V, the recombination movement of electrons and holes will continue to form a base current. I. 3. When a reverse voltage is applied to the collector junction, the drift movement forms the collector current lc. Since the collector junction is applied with a reverse voltage and its junction area is large, the non-equilibrium minority carriers in the base area cross the collector junction and reach the collector area under the action of the external electric field. , forming a drift current.

At the same time, the balanced minority carriers in the collector area and the base area also participate in the drift motion, but their number is very small and can be ignored in the approximate analysis. It can be seen that under the action of collector power supply Vc, the drift motion forms the collector current, I.


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