The origin of the kink phenomenon of transistor scattering parameter S-22

A novel theory based on dual-feedback circuit methodology is proposed to ex plain the kink phenomenon of transistor scattering parameter S-22. Our resu lts show that the output impedance of all transistors intrinsically shows a series RC circuit at low frequencies and a parallel RC circuit at high fre quencies. It is this inherent ambivalent characteristic of the output imped ance that causes the appearance of kink phenomenon of S-22 in a Smith chart . It was found that an increase of transistor transconductance enhances the kink effect while an increase of drain-to-source (or collector-to-emitter) capacitance obscures it. This explains why it is much easier to see the ki nk phenomenon in bipolar transistors, especially heterojunction bipolar tra nsitors, rather than in field-effect transistors (FETs), It also explains w hy the kink phenomenon is seen in larger size FETs and not in smaller size FETs, Our model not only can predict the behavior of S-22, but also calcula te all S-parameters accurately. Experimental data of submicrometer gate Si MOSFETs and GaAs FETs are used to verify our theory. A simple method for ex tracting transistor equivalent-circuit parameters from measured S-parameter s is also proposed based on our theory. Compared with traditional Z-or Y-pa rameter methods, our theory shows another advantage of giving deep insight into the physical meaning of S-parameters.