Evaluating the graft base lateral diffusion depth of high-performance bipolar transistors by using test structures

A test structure is proposed for evaluating the graft base lateral diffusion depth of high-performance double-polysilicon bipolar transistors in order to achieve high cutoff frequencies. The test structure can evaluate the effective intrinsic base width by measuring the resistance between two electrodes for several devices with different size. Graft base depth can be obtained from the active base length and the effective intrinsic base width. The graft base depths of two kinds of transistors are evaluated using the test structure. Depths of 0.05 μm and 0.13 μm are obtained. The maximum f_T´s of these transistors are 50 GHz and 29 GHz. The feasibility of the test structure and the importance of controlling the graft base depth are confirmed through the evaluation of f_T for these transistors