Device Characterization

Question 012-07: When doing device characterization, we often test the linear threshold voltage (Vth) and saturation Vth, I know the condition for saturation is Vs = Vb = 0, Vd = Vdd, and sweep Vg to find Vth with Gm max method, but I want to confirm if the condition for linear Vth is Vds = 0.1V? And, would you please help to explain why we need to test those two items? What is their difference?

Answer 012-07: In order to extract the linear Vth (Vt,lin) by linear extrapolation method at maximum Gm, the drain voltage must be as small as possible to maintain a uniform charge density in the inversion channel from the source to drain (that is similar to resistor with constant sheet resistance from source to drain) and negligible bulk depletion charge effect due to Vd. Therefore, a small value of Vds = 50 mV is preferred for advanced technologies, though Vds = 100 mV is typically used in practice.

The saturation Vth (Vt,sat) extracted by the quadratic extrapolation method is different from Vt,lin because of the drain induced barrier lowering (DIBL) effect at high Vd, especially, for short channel devices. As a result, the values of Vt,lin and Vt,sat are different. And, the magnitude of (Vt.lin – Vt,sat) is the measure of DIBL effect that contributes to the off-state leakage current in MOSFETs. Therefore, both Vt,lin and Vt,sat need to be measured to check the DIBL effect and robustness of device architecture.

Question 016-08: I want to know, in order to find the on-resistance of a power VDMOSFET, what should be the values of gate voltage and drain voltage?

Answer 016-08: Typically, the on resistance (Rdson) of VDMOSFETs is measured at the bias point: drain voltage, Vds = 0.1 V (or 0.2 V) and gate voltage, Vgs = Vdd – 10% of Vdd, where Vdd is the supply voltage. Some people in the field have, also, used Vgs = Vdd.