The structure, working principle and main parameters of power transistors

Giant transistor (GTR) is a high-voltage, high-current bipolar junction transistor (BJT), so it is also called Power BJT. GTR (BJT) has the characteristics of convenient control, short switching time, low on-state voltage, and good high-frequency characteristics. It is widely used in small and medium capacity systems.

The structure and working principle of GTR

GTR is a current-controlled device, and the commonly used NPN type is the same as that of ordinary bipolar junction transistors. It is in the on state when it works in forward bias (IB>0), and it is in the off state when it works in reverse bias (IB<0). The main characteristics of GTR are high withstand voltage, large current, and good switching characteristics. The low-power bipolar junction transistors used for information processing pay attention to performance parameters such as single-tube current amplification factor, linearity, frequency response, noise and temperature drift.

GTR usually adopts a unit structure composed of at least two transistors in Darlington connection. Like GTO, the integrated circuit it uses is composed of two PN junctions (collector junction and emitter junction) formed by three layers of semiconductors (leading out the collector, base and emitter respectively), mostly using an NPN structure. Figure 1 shows the structure, schematic and graphic symbols of the NPN GTR respectively. Note that the “+” in the upper right corner of the letter representing the semiconductor type indicates a high doping concentration, and “﹣” indicates a low doping concentration.

The structure, working principle and secondary breakdown of giant transistor
Figure 1 – GTR structure, schematic diagram and graphical symbols

In order to withstand high voltage and high current, GTR not only has to increase its size with the increase of capacity, but its internal structure and appearance are also different from ordinary bipolar junction transistors. It can be seen from Figure 1a that, compared with ordinary bipolar junction transistors in information electronic circuits, GTR has one more N-drift region (low-doped N region), which is used to withstand high voltage. A cell structure composed of at least two transistors according to Darlington connection is adopted to increase the current capacity. Figures 2a and 2b show the NPN and PNP Darlington structures, respectively, and Figure 2c shows the single-arm bridge module circuit composed of two three-stage Darlington GTR and its auxiliary components.

The structure, working principle and secondary breakdown of giant transistor
Figure 2 – NPN type, PNP type Darlington structure and module circuit

In application, GTR generally uses common emitter connection, as shown in Figure 1b, the ratio of collector current iC to base current iB is

The structure, working principle and secondary breakdown of giant transistor
(1-1)

β is called the current amplification factor of GTR, which reflects the ability of the base current to control the collector current. When considering the leakage current ICEO between the collector and the emitter, the relationship between IC and IB is

                                    iC=βiB+ICEO                             (1-2)

The GTR product manual usually gives the DC current gain hFE, which is the ratio of the collector current to the base current in the case of DC operation. Generally speaking, β≈hFE. The β value of the single-tube CTR is much smaller than that of the low-power transistors used for processing information, usually about 10, and the Darlington connection can effectively increase the current gain.

The secondary breakdown phenomenon of GTR and safe working area

Secondary breakdown is the main reason for the sudden damage of GTR and has become an important factor affecting the reliable use of GTR.

When the collector voltage UCE of the GTR rises to the breakdown voltage, the collector current IC increases rapidly, and the breakdown that occurs is an avalanche breakdown, which is called a breakdown. At this time, if there is a resistance in the circuit to limit the current growth, it will generally not deteriorate the GTR operating characteristics. However, if the current IC continues to increase without restriction, when IC increases to a certain critical point, it will suddenly rise sharply. At the same time, with the sudden drop in voltage, a negative resistance effect will appear, leading to a destructive GTR secondary breakdown.

There are many factors that cause the secondary breakdown of GTR. In order to ensure the reliable operation of the GTR, a safe working area of the GTR is set up.

GTR cannot exceed the maximum operating voltage UCEM, the maximum collector current ICM, the maximum power dissipation PCM and the secondary breakdown critical line PSB during operation. These restrictions stipulate the GTR’s safe operating area (SOA), as shown in the shaded area in Figure 3.

At present, the gate-off thyristor GTO and power transistor GTR have been gradually replaced by power field effect transistors and insulated gate bipolar transistors with better performance.

The secondary breakdown phenomenon of GTR and safe working area

Secondary breakdown is the main reason for the sudden damage of GTR and has become an important factor affecting the reliable use of GTR.

When the collector voltage UCE of the GTR rises to the breakdown voltage, the collector current IC increases rapidly, and the breakdown that occurs is an avalanche breakdown, which is called a breakdown. At this time, if there is a resistance in the circuit to limit the current growth, it will generally not deteriorate the GTR operating characteristics. However, if the current IC continues to increase without restriction, when IC increases to a certain critical point, it will suddenly rise sharply. At the same time, with the sudden drop in voltage, a negative resistance effect will appear, leading to a destructive GTR secondary breakdown.

There are many factors that cause the secondary breakdown of GTR. In order to ensure the reliable operation of the GTR, a safe working area of the GTR is set up.

GTR cannot exceed the maximum operating voltage UCEM, the maximum collector current ICM, the maximum power dissipation PCM and the secondary breakdown critical line PSB during operation. These restrictions stipulate the GTR’s safe operating area (SOA), as shown in the shaded area in Figure 3.

At present, the gate-off thyristor GTO and power transistor GTR have been gradually replaced by power field effect transistors and insulated gate bipolar transistors with better performance.

The structure, working principle and secondary breakdown of giant transistor
Figure 3 – Safe working area of GTR