Method for automatic measurement of failure in subthreshold region of metal-oxide-semiconductor transistor

1. A method for detecting an abnormal condition of a MOS transistor in a subthreshold region, the method comprising:

measuring a variation in a drain current while varying a gate voltage of the MOS transistor to obtain a characteristics curve;

calculating, with reference to the obtained characteristics curve, a variation of transconductance with respect to each of the gate voltages to obtain a transconductance variable curve;

differentiating the transconductance variable curve; and

determining whether the MOS transistor has an abnormal condition or not on the basis of computing a number of inflection points in a curve obtained by the differentiation.

2. The method of claim 1 , wherein the determining step comprises determining that the MOS transistor has the abnormal condition when a sign of the curve obtained by differentiating changes two times or when the curve obtained by differentiation becomes equal to zero, over a predetermined range.

3. The method of claim 1 , wherein the determining step comprises:

determining a number of times that a sign of the curve obtained by differentiating changes;

determining a number of times that the curve obtained by differentiation becomes equal to zero, over a predetermined range;

computing a value by adding two to the number of times that the sign changes each time the curve obtained by differentiation becomes equal to zero; and wherein

determining that the MOS transistor has an abnormal condition is carried out when the computed value is at least two.

4. The method of claim 3 , wherein the computed value equals the number of inflection points.