Abstract:
Transit time of a Bipolar Junction Transistor (BJT) is an important parameter. It is
required in determining different performance figures of a BJT. In the present work
the current-continuity equations for electrons and holes; expressions for Shockley-
Reed-Hall and Auger recombination are used in obtaining first order differential
equations for voltage and current. These two equations are rearranged in a form
that is analogous to the well-known time-independent Telegraphers' equation of
transmission line analysis. In this thesis transmission line methods have been
employed to construct an iterative procedure to find minority carrier distribution
for a particular electron current density. Integration of this distribution over the
base width will give the total electron charge for a given electron current density.
The ratio of electron charge to current density gives the base transit time. Doping
dependence of mobility, velocity saturation effect and bandgap narrowing effect
within the base are also incorporated in the analysis. The present approach is easier
and conceptually straightforward, in that this work did not lump the RIG's of the
transmission line (base). Instead the base is segmented and classical TL analysis
has been followed for each segment considering RIG's as strictly distributed
element. Thus transmission line model has been employed in a more fundamental
way and this is for the first time minority carrier distribution and base transit time
of BJT's have been computed. Here both uniform and nonuniform base doping
have been considered. Finally many other useful profiles, such as base transit time
with base-emitter voltage (for both uniform and nonuniform doping), electron
charge with current density and base-emitter voltage etc are obtained. It is observed
that base transit time increases strongly with base width, but moderately with peak
base-doping. It decreases with increasing slope of base-doping.
