BASE TRANSIT-TIME MINIMIZATION WITH FIXED BASE DOPANT DOSE

The problem of determining the doping profile minimizing base transit time tau(B) in a BJT is investigated for the case in which the net act ive dopant dose Q in the quasi-neutral base is held fixed, but the wid th W of the base is allowed to vary. It is found that the minimum tau( B) is obtained, for reasonable values of Q, by shrinking the base widt h until the profile becomes box-shaped with doping equal to the solid solubility. This occurs at so narrow a base width, however, that in an y practical device it will always be favourable to grade the base dopi ng profile. (C) 1998 Elsevier Science Ltd. All rights reserved.