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We examined various electrostatic discharge (ESD) protection devices for input pins in the case of floating body substrates to discuss optimal structures effective to both of the machine model (MM) and human body model (HBRM) ESD stresses. Because of a small series resistance, we observed oscillation alternating from positive to negative in the current waveforms during MM stress, leading to weak polarity dependence of ESD performance which is inconsistent with the results for HEM tests. As a result, protection devices effective to both HEM and MM should be robust to bipolar stress. As one of the candidates, we propose a combination of PN diode and thyristor.