Is it necessary to use bidirectional TVS diodes (ESD protection diodes) to protect against both positive and negative ESD events?

The static electrical characteristics of unidirectional and bidirectional TVS diodes are shown below.

Like typical Zener diodes, unidirectional TVS diodes are used with the reverse-biased condition. When the IC under protection is in normal operation, unidirectional TVS diodes are reverse biased, and the signal that can be applied to the protected line can be in the range of 0 V to V_RWM. However, TVS diodes absorb both positive and negative ESD events.

Therefore, when only a positive voltage is applied to the signal line to be protected during normal operation, unidirectional TVS diodes can be used to absorb both positive and negative ESD events.

When both positive and negative voltage are applied to the signal line to be protected, use bidirectional TVS diodes.