High-frequency base-transport factor and transit time of graded-base transistors

Closed expressions are derived for the modulus and argument, as well as for the real and imaginary parts of the complex base-transport factor (β) of diffusion and uniform drift-field transistors. Since these expressions involve only trigonometric, exponential, and hyperbolic functions, they are suitable for straightforward numerical computation as well as theoretical analysis. The variation of computed β-cutoff parameters as a function of impurity concentration is illustrated graphically and shown to be in good agreement with an approximate relation between the β-cutoff frequency and transit time due to Moll and Ross.