FULLY RADIATIVE CURRENT PATH STRUCTURE (FRACS) FOR SUB-0.1 MU-M EMITTER TRANSISTOR

A new structure is proposed for bipolar transistor FRACS (Fully Radiat ive Current Path Structure). A FRACS transistor has a line emitter and a cylindrical base and collector or a point emitter and a spherical b ase and collector. Device parameters of the FRACS transistor is obtain ed by extending the conventional one-dimensional transistor model to t wo- or three-dimensional model. In this structure, base transit time i s reduced as the emitter size is reduced by radiative collector curren t flow. Using this model, a general bipolar transistor with a shallow link base is found to increase the cutoff frequency as the emitter siz e is reduced. The Kirk effect is suppressed in this structure because of the small collector current density at the collector-base junction. The effect was experimentally examined. Cylindrical base was fabricat ed by thermal diffusion of boron to achieve FRACS transistor. Cutoff f requency was observed to increase as the emitter was getting smaller. Maximum cut-off frequency of 64 GHz was achieved by this transistor wi th 25-nm thick base formed by Rapid Vapor-phase diffusion (RVD).