Effects of pad and interconnection parasitics on forward transit time in HBTs

A new analytical expression using Y-parameters is derived from the full heterojunction bipolar transistor (HBT) equivalent circuit including probe-pattern parasitics, in order to extract the forward transit time accurately without a conventional de-embedding procedure. The correction procedure of this Y-parameter method does not require the measurement of extra test patterns such as “open” or “short” structures, thus being free of the possible de-embedding errors attributed to the inaccuracy of test structures. To compare the de-embedding effect on τ_F in the Y-parameter method with that in the previous f_T method, C_π/g_mo versus 1/I_c curves are plotted with 1/2πf_T versus 1/I_c in the cases of de-embedding and no de-embedding. This comparison clearly exhibits that the contribution of the probe-pattern parasitics to τ_F in the Y-parameter method is much smaller than that in the conventional f _T method. The smaller contribution to τ_F may result in the enhancement of the de-embedding accuracy in the Y-parameter method