The structure and type of transistor
Transistor is one of the basic semiconductor devices. It has the function of current amplification and is the core component of electronic circuit. The transistor is the one which is made into two PN junctions with a very near distance on a piece of semiconductor substrate. The two PN junctions divide the entire semiconductor into three sections. The medium section is the base region, two lateral sections are emitter region and collector region. There are two kinds of arrangements, i.e. PNP and NPN. The corresponding electrodes are lead from the three regions as shown in the diagram, and are the base b, emitter e and collector c, respectively.
The PN junction between the emitter region and base region is called the emitter junction. The PN junction between the collector region and base region is called the collector junction. The base region is very thin, but the emitter region is thicker and is with much impurity. The emitter region of PNP transistor “Transmits” the cavity, and its moving direction is in accordance with the current direction. Hence, the emitter arrowhead is facing inward; The emitter region of NPN transistor “Transmits” the free electron, and its moving direction is inverted to the current direction. Hence, the emitter arrowhead is facing outward. The emitter arrowhead direction is also the conducting direction of PN junction under the forward voltage. The silicon transistor and the germanium transistor all have the PNP and the NPN types.
The packing form and pin identification of transistor
The packing form of common transistor includes two major categories, i.e. the metallic packing and the plastics packing. The pin layout follows a certain law. As shown in the diagram, for the small power metallic packing transistor, it is laid in bottom view as shown in the diagram, making the three pins constituting into the top points of isosceles triangle. They are arranged, from left to right, into e, b, c in sequence; For the medium and small power plastics packing transistor, it is laid with its plane face forwarding yourself as shown in the diagram, making the three pins laid downward. They are arranged, from left to right, into e, b, c in sequence.
Presently, there are many kinds of transistors in our country and their pin layouts are not just the same. During the application, if you do not confirm the pin layout of a transistor, it is necessary to carry on the test to determine the correct locations of the pins, or to consult with the Transistor User Manual to determine the characteristics and the corresponding technical parameters and information of the transistor.
The current amplification of transistor
The transistor has the current amplification function. The fact is that a transistor can control a larger variation amount of the collector current with a tiny variation amount of the base current. This are the most fundamental and important characteristics of the transistor. We call the ratio of Δ Ic/Δ Ib as the current amplification factor of a transistor, and use symbol “β” to represent. The current amplification factor is a definite value for a certain transistor, but it will be also changed with the transistor base working current.
Three kinds of operating status of transistor
Cut off status: When the voltage applied to the transistor emitter junction is less than the conductive voltage of PN junction, the base current is zero, the collector current and the emitter current are all zero. The transistor lost the current amplification function at this time. The collector and emitter are similar to the opening status of a switch, which we call the transistor at the cutoff status.
Amplification status: When the voltage applied to the transistor emitter junction is greater than the conductive voltage of PN junction, and when it is at a certain proper value, the transistor emitter junction is biased in forward direction, and the collector junction is biased in inverse direction. At this time, the base current has a control function with respect to the collector current, making the transistor have the current amplification function. Its current amplification factor β = Δ Ic/Δ Ib, and at this time the transistor is at the amplification status.
Saturated conductive status: When the voltage applied to the transistor emitter junction is greater than the conductive voltage of PN junction, and when the base current is increased to a certain extent, the collector current will no longer increase with the base current, but, instead, it will be just near a certain value and not change. In this case, the transistor loses the current amplification function, and the voltage across the collector and emitter is very small, the collector and emitter are just at a status equivalent to the conductive status of a switch. This kind of status of transistor is called the saturated conductive status.
The operating status of a transistor can be distinguished basing on each electrode level of the transistor in operation. Therefore, the electronic repair personnel usually needs to use a multimeter to test the voltage of each pin during repair so as to distinguish the working condition and the working status of the transistor.
Use multimeter to test a transistor
The discretion of transistor base: According to the structure diagram of transistor, we know the base of transistor is the common electrode of two PN junctions in the transistor. Therefore, while distinguishing the base of transistor, as long as we find out the common electrode of two PN junctions, it will the base of transistor. As a practical method, it is to adjust for R×1k resistance scale of the multimeter, using the red probe firstly to touch one of the transistor pins, using the black probe to touch other two pins. If it is conductive in both cases, the pin where the red probe touching is the base of the transistor. If it can not be found out once, the red probe is then changed to another pin and test for two more times; If it can not be found out, then change to use the black probe and touch the black probe to one of the pins of transistor, use the red probe to test for two times and see whether they are conductive. If it does not succeed, change again. Thus the maximum measurement times will not exceed 12, and you can find out the base finally.
The discretion of transistor type: There are only two types of transistor, namely, PNP and NPN. To distinguish, it is only needed to know whether the base is P type or N type material. While using the multimeter R×1k scale, the black probe represents the positive of power supply. If the black probe is connected to the base and it is conductive, it means that the base of transistor is of P material, and the transistor is of a NPN type. If the red probe is connected to the base and it is conductive, then it means that the base of transistor is of N material, and the transistor is of a PNP type.