The working principle of semiconductor transistor

The basic working principles of PNP semiconductor transistor and NPN semiconductor transistor are exactly the same. Taking NPN semiconductor transistor as an example, we will explain its internal current transmission process and then introduce its working principle.

The current transmission in semiconductor transistors can be divided into three stages.

1. Emission area emits electrons to the base area

After the power is turned on, the transmitting junction is connected in the forward direction. Under the action of a positive electric field, the diffusion motion of the majority carriers (electrons) in the emission region is strengthened. Therefore, electrons in the emission region can easily cross the emission junction and enter the base region under the action of an external electric field, forming an electron flow IEN (note that the direction of the current is opposite to the direction of electron motion). Of course, the majority carriers (holes) in the base region will also flow towards the emission region under the action of an external electric field, forming a hole current IEP. However, due to the low impurity concentration in the base region, compared to the electron flow from the emission region, IEP can be ignored.

2. Diffusion and recombination of electrons in the base region

After the electrons diffuse from the emission region to the base region, the electron concentration on the side of the base region closer to the emission region is higher, while the electron concentration on the side closer to the collector region is lower So the electrons continue to diffuse towards the collector region. During the diffusion process, electrons may meet and recombine with the empty space in the base region, and the base power supply and EB continuously provide holes, forming the base current IBN. Due to the thin base region and low hole concentration, there are few opportunities for electron hole recombination, and most electrons continue to diffuse towards the collector region. In addition, during the operation of a semiconductor transistor, the collector junction is connected in reverse. Under the action of a reverse electric field, the drift motion of minority carriers between the base region and the collector region is strengthened because there are very few carriers in the base region There are fewer electrons, so the drift motion is mainly due to the flow of holes from the collector region to the base region. The value of the current ICBO formed by drift motion is very small and has little relationship with the magnitude of the external electric field. It is called collector reverse saturation current.

3. Formation of collector current

Due to the reverse electric field applied to the collector junction, the electrons that continue to diffuse towards the collector area through the base region are in the direction of the reverse electric field, and therefore experience tensile force, accelerating their flow towards the collector area Form an electron flow ICN. If we consider the influence of collector saturation current ICBO.

From the current transmission process of the semiconductor transistor, it can be seen that the collector current IC is large, while the base current IB is small. In addition, due to the predetermined structure of the transistor itself, there is always a fixed proportional relationship between IC and IB within a considerable range.

Among them, β represents the relationship between IC and IB The DC amplification factor, known as the common emitter, has a beta greater than 1 and is generally between 20 and 200.

Due to the proportional relationship between IC and IB, and IE=lC+IB, the semiconductor transistor acts as a current divider, dividing the emitter current IE into IC and IB according to a certain distribution relationship. IC is much larger than IB. Due to this allocation relationship, as long as IB is slightly increased, The IC will increase significantly, which has an amplifying effect.