Soft Error - Critical Charge

Critical Charge

Whether or not a circuit experiences a soft error depends on the energy of the incoming particle, the geometry of the impact, the location of the strike, and the design of the logic circuit. Logic circuits with higher capacitance and higher logic voltages are less likely to suffer an error. This combination of capacitance and voltage is described by the critical charge parameter, Q_crit, the minimum electron charge disturbance needed to change the logic level. A higher Q_crit means fewer soft errors. Unfortunately, a higher Q_crit also means a slower logic gate and a higher power dissipation. Reduction in chip feature size and supply voltage, desirable for many reasons, decreases Q_crit. Thus, the importance of soft errors increases as chip technology advances.

In a logic circuit, Q_crit is defined as the minimum amount of induced charge required at a circuit node to cause a voltage pulse to propagate from that node to the output and be of sufficient duration and magnitude to be reliably latched. Since a logic circuit contains many nodes that may be struck, and each node may be of unique capacitance and distance from output, Q_crit is typically characterized on a per-node basis.