Substrate coupling evaluation in BiCMOS technology

The magnitude of switching noise coupled through common substrate in BiCMOS technology is analyzed. Noise dependence on collector resistance and buried layer doping of the noisy bipolar junction transistor (BJT) is obtained by means of simulation. It is observed that trends are different depending on bipolar transistor biasing: in common-collector, a low collector resistance is desired, while in common-emitter biasing, large values of Rc make the transistor less noisy. A test chip is fabricated in 3-/spl mu/m BiCMOS technology to measure the substrate coupling produced by different BICMOS inverter gates. These experimental measurements show that noise increases with transistor size and collector resistance. Dependence on distance and speed of signal are also obtained, together with the effect of a guard ring.