Its common feature is that it has three electrodes, which is the origin of the abbreviation "transistor". Generally speaking, the interior of the transistor is a three-layer structure composed of P-type semiconductors and N-type semiconductors, which are divided into NPN type and PNP type according to the hierarchical order.
The above-mentioned three-layer structure is the three regions of the transistor. The thinner layer in the middle is the base region, and the other two layers are both N-type or P-type. Among them, the layer with relatively small size and relatively high majority carrier concentration The emitter area and the other layer is the collector area. This internal structural characteristic of the transistor is the internal condition for the transistor to be able to amplify.
Each of the three regions leads to three electrodes, which are base (b), emitter (e) and collector (c).
As shown in Figure b, the three-layer structure can form two PN junctions, which are called the emitter junction and the collector junction. The direction of the arrow in the transistor symbol indicates the direction of the emitter junction.
There are two PN junctions with unidirectional conductivity in the internal structure of the transistor, so of course it can be used as a switching element, but at the same time, the transistor is also an amplifying element. It is its appearance that promotes the rapid development of electronic technology.

2. The current amplification effect of the transistor
The DC voltage source Vcc should be greater than Vbb, so that the circuit meets the external conditions of amplification: the emitter junction is forward biased, and the collector is reverse biased. Changing the adjustable resistance Rb, the base current IB, the collector current Ic and the emitter current IE will all change, and the following conclusions can be drawn from the measurement results:
(1) IE = IB + IC (according to Kirchhoff's current theorem)
(2) IC ≈ IB ×? (? is called the current amplification factor, which can characterize the current amplification capability of the transistor)
(3) △ IC ≈ △ IB ×?
It can be seen from the above that the transistor is an analog device with current amplification.

3. Amplification principle of transistor
The following uses NPN transistor as an example to illustrate its internal carrier movement law and current amplificationprinciple,
1. Diffusion of electrons from the emitter region to the base region: Since the emitter junction is in forward bias, the majority of carriers (free electrons) in the emitter region continue to diffuse to the base region, and continuously replenish electrons from the power supply, forming the emitter current IE.
2. Diffusion and recombination of electrons in the base area: Since the base area is very thin, the majority carrier (hole) concentration is very low, so only a small part of the electrons diffused from the emitter can recombine with the base area holes to form The base current IB is relatively small, and most of the remaining electrons can diffuse to the edge of the collector junction.
3. The collector area collects electrons diffused from the emitter area: due to the reverse bias of the collector junction, electrons that diffuse from the emitter area to the base area and reach the edge of the collector area can be pulled into the collector area, thereby forming a larger The collector current IC.

4. Input and output characteristics of transistor
The input characteristic of the transistor refers to the relationship curve between the base current IB and the base-emitter voltage UBE when the collector-emitter voltage UCE is constant.
For silicon tubes, when UCE exceeds 1V, the collector junction has reached enough reverse bias to pull most of the electrons diffused from the emitter region to the base region into the collector region. If UCE is increased at this time, as long as UBE remains unchanged (the number of electrons emitted from the emission area to the base area is constant), IB will basically remain unchanged. That is to say, when UCE exceeds 1V, the input characteristic curves basically coincide.
It can be seen from the figure that, like the volt-ampere characteristic of the diode, the input characteristic of the transistor also has a dead zone. Only when UBE is greater than the dead zone voltage, the base current IB of the transistor will appear. Usually the dead zone voltage of the silicon tube is about 0.5V, and that of the germanium tube is about 0.1V. Under normal working conditions, the emitter junction voltage UBE of the NPN silicon tube is 0.6 to 0.7V, and the emitter junction voltage UBE of the PNP germanium tube is -0.2 to -0.3V.
The output characteristic of the transistor refers to the relationship curve between the collector current IC and the collector-emitter voltage UCE when the base current IB is constant. Under different IB, different curves can be obtained, so the output characteristics of the transistor are a set of curves. Usually the output characteristic curve is divided into three working areas:
1. Enlargement area: The nearly horizontal part of the output characteristic curve is the enlargement area. In the amplification area, IC = IB ×?, because the current amplification coefficients are approximately equal under different IBs, the amplification area is also called the linear area. For the transistor to work in the amplification area, the emitter junction must be in forward bias, and the collector junction should be in reverse bias. For silicon tubes, UBE>0 and UBC<0 should be made.
2. Cut-off area: The area below the curve of IB = 0 is called the cut-off area. In fact, for the NPN silicon tube, when UBE<0.5V, it starts to cut off, but in order to make the transistor cut off reliably, UBE≤0V is often used. At this time, both the emitter junction and the collector junction are in reverse bias.
3. Saturation area: The steep part of the output characteristic curve is the saturation area. At this time, the change of IB has little influence on IC, and the ? of the amplification area is no longer applicable to the saturation area. In the saturation region, UCE<UBE, both the emitter and collector junctions are forward biased.