For bipolar transistors, the time τB for minority carriers to cross the neutral base region is called the base region transit time.
For bipolar transistors, the time τB for minority carriers to travel across the neutral base region is called the base region transit time; it is directly related to the base region width W:
In = Aqnp(x)v(x), dx = v(x) dt,
Then we get τB = ∫o [1/v(x)] dx = ∫o [Aqnp(x)/In]dx ≈ QB / IC.
For a uniform base transistor τB = W/ (2 Dn ); for a slowly varying base transistor τB = W / (ηDn), η>2. For a transistor with a wide base region and a characteristic frequency fT that is not very high, τB is often the main factor that determines the frequency characteristics of the transistor. At this time, in order to improve the frequency performance, the width of the base region should be reduced (shallow junction process can be used to make thin Base region) and increase the electric field factor η (which can increase the doping concentration of the base region on the side of the emitter junction to enhance the drift electric field, and at the same time can increase the steepness of the impurity distribution in the emitter region to reduce the retarding electric field, but if doping If the concentration is too high, the diffusion coefficient of the carrier will be reduced, so η is generally controlled between 3 and 6).
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