Basic knowledge of electronic components -- semiconductor triode

BJT is through a certain process, the two devices together with PN junction, due to the mutual influence between the PN, BJT showed different characteristics from a single junction with current amplification, so that the application of PN junction has undergone a qualitative leap. This section will focus on why BJT has the current amplification effect. This paper discusses the structure of BJT, the process of internal carrier motion, and its characteristic curves and parameters. One, the structure of BJT brief introduction, BJT is often called transistor, it has many kinds. According to the frequency division, there are high frequency tubes, low-frequency tubes; according to power points, there are small, medium and large power pipes; in accordance with semiconductor materials, there are silicon tube, germanium tube; according to different structures, can be divided into NPN and PNP type, and so on. But in their shape, BJT has three electrodes.
It is made of two PN junctions with three layers of semiconductor. In the center is a very thin P type semiconductor (several microns to tens of microns), each side is a N type semiconductor. One of the three wires from each of the three semiconductors is called the emitter e, the base B and the collector C, and each corresponding semiconductor is called the emission region, the base region, and the collector region of the BJT. Although the emission region and collector region are all N type semiconductors, the emitter region is much more doped than the collector region. In geometry, the collector area is larger than the emission region, which can be seen from Fig. 3.1, so they are not symmetrical. Two, the current distribution and amplification of BJT, the transport process of carrier in 1 and BJT, the basic conditions of BJT working in the amplification state: the emission junction is positive and the collector is biased. The role of the applied voltage, the transmission process of BJT carrier is: (1) the emitter injection electron emitter due to the applied positive voltage of VEE, so the space charge region of the emitter is narrowed when the majority carrier electron emitter continuously through the emitter diffusion into the base region, the formation of emitter current IE, and its direction the electrons flow in the opposite direction. (2) in the base region of the electron diffusion and electron emission from the composite area injected into the base region, close to the emitter in the base region boundary accumulated, right in the base region formed a certain concentration gradient near the emitter junction, near the highest concentration, the farther away from the emitter concentration is small. Therefore, the electronic will spread to the collector's direction, in the process of diffusion and composite and holes in the base region, and it is connected with the power source of VEE in the base region is continuously from the base away like a continuous supply of electrons, the base hole. The number of electrons is equal to the number of electrons that the power source takes away from the base so that the hole concentration in the base region remains essentially constant. Thus, the base current IB is formed, so that the base current is the current in which the electrons are combined with the holes in the base. That is to say, there is a part of the electron in the injected base region that has not reached the collector junction. If the recombination is more, the less electrons will arrive at the collector, which will be detrimental to amplification. So in order to reduce the base compound, often made very thin (a few microns), and the concentration of base impurity doping is very low, so in the process of diffusion and electron hole recombination in a small number, most of them can reach the collector junction. (3) collecting electric collector and reverse voltage in collector area, the internal electric field of collector is very strong, and the direction of electric field is directed from C to B. The electron collector region and the base region of the hole is very difficult by the collector, but on the base diffusion to electron collector junction edge has a very strong appeal, make the electronic quickly drift over the collector collected in the collector region, the formation of the collector current IC. At the same time, the hole in the collector region will drift to the base region under the action of the electric field to form a very small reverse saturation current ICB0. The diode current 2, current distribution relationship and positive bias is similar to that of the emitter current iE and vBE exponential relationship: the collector current iC is part of iE, which is called beta type characteristic curve of BJT current amplification factor of three and BJT of 1 common emitter circuit (1) input characteristics VCE=0V when the forward voltage is applied between E and B, then the emitter and the collector are partial, characteristics of the equivalent of two parallel diode. VCE is not less than 1V, then the collector reverse bias, injected from the emitter base region of the vast majority of electron drift to the collector, only a small part of the formation of IB composite with hole. After vCE>1V, IC increases little, so the amount of IB change is very small, and the influence of vCE on IB can be neglected, that is, the input characteristic curves coincide.
Note: vBE:0.6V~0.7V began transmitting voltage conduction (silicon tube), 0.1~0.3V (GE tube) (2) output characteristic curve for a fixed value of iB, iC with VCE changes in the formation of a curve, given a number of different iB value, produces a family of curves. As shown in figure 3.6. IB=0V, IC=ICEOBJT cutoff, no amplification, so the region corresponding to the IB=0's output characteristic curve is called the cut-off area, as shown in figure 3.6. The laws of IB > 0, VCE<1V and iC do not follow the change of IB, and the variation of iC with VCE is also nonlinear, so this region is called saturation zone. The IB > 0, VCE = 1V, iC changes with iB: or in the region of IC almost does not change with the change of VCE, the characteristic curve corresponding to each IB value are almost parallel to the horizontal axis, so the area is called linear region or amplification region. Four, the main parameter of BJT, BJT parameter is used to characterize the performance of the pipe to adapt to the scope, it is the basis for the selection of BJT. To understand the significance of these parameters is necessary for the rational use and full use of BJT to achieve the economical and reliable design of the circuit. The amplification factor of 1. current amplification factor BJT in common emitter method, according to the working state