A NEW TEST STRUCTURE FOR THE EVALUATION OF GRAFT-BASE LATERAL DIFFUSION DEPTH IN HIGH-PERFORMANCE BIPOLAR-TRANSISTORS

A new test structure allows the first electrical measurement of latera l diffusion depth of the graft base in high-performance bipolar transi stors. It is indispensable for realizing high cutoff frequency to cont rol the graft-base depth. The test structure has two independent base electrodes and no emitter region. It can evaluate the effective intrin sic-base length by measuring resistance between two electrodes. Graft- base depth can be derived from the active-base length and the effectiv e intrinsic-base width. The feasibility of this structure is confirmed by evaluating 50-GHz and 30-GHz transistors, with graft-base depths o f 0.05 mum and 0.13 mum, respectively. The new method is compared with conventional ones.