From the definition of softness factor, it can be seen that it reflects how long the base minority disappears due to recombination in the reverse recovery TB process of the diode. Therefore, the softness factor is closely related to minority carrier lifetime control method, base width and diffusion concentration distribution, component structure and structural parameters. The softness factor will be improved if more residual charges are stationed in the residual base area after the expansion of the space charge area and longer time is stationed.

2: Ways to Improve Performance

Although PIN transistor has good reverse voltage withstanding ability, it produces large reverse peak voltage during reverse recovery because of its poor reverse recovery characteristic, which affects the normal operation of the whole circuit. Secondly, a practical PIN rectifier has an order of magnitude higher forward voltage drop at the instant of switching on than the steady-state voltage drop. The peak voltage can exceed 30V, which is mainly caused by the limited diffusion velocity of minority carriers. It is related to the resistivity and width of N-base material. In order to improve the working characteristics of the diode, related technologies are used in the design and fabrication of the device.

For example, minority carrier lifetime control technology. Because the change of minority carrier lifetime and the control method of minority carrier lifetime affect the frequency characteristics and reverse recovery softness of fast recovery devices, the control technology of minority carrier lifetime is very important. The minority carrier lifetime control technology can be divided into three types according to its characteristics: conventional type, heavy metal doping type and electron radiation type.

1) Conventional type: Conventional type is to control minority carrier lifetime by adjusting the structure parameters of switch tube, including several methods: reducing material resistivity, controlling impurity distribution, and thinning base thickness.