a-b, Current versus voltage for NEM devices exhibiting non-volatile (a) and volatile (b) behaviour.
As the voltage is increased the current remains zero until the pull-in voltage is reached and the active element makes contact with the electrode (inset).
The current then increases linearly with the voltage. However, when the voltage is reduced below the pull-in voltage, adhesive forces between the active element and electrode hold them together and the current either decreases linearly with the voltage (a) or drops back to zero at a lower voltage when the stiffness of the active element overcomes the adhesive force (b).
The fact that there can be two stable states for a given input voltage (between pull-out and pull-in) can be exploited in memory applications.