Pinch-off effect

This pinched-off effect appears to be due to narrowing of the catheter as it passes over the first rib and beneath the clavicle, when using the Seldinger technique, but it is usually only observed after long-term use. The authors recommended that the cephalic cut-down (Seldinger) technique is best avoided. 

In the saturation region, at and above the pinch-off point, neglecting the effective channel length change due to the Vds value, due to the condition OIds/OVds = 0(thusVDs = Vgs—Vt), this equation becomes ... 

Fig. 12. NMOSFET operated (a) in the linear (low drain voltage) region, where S is the grounded source contact, j is the distance from source to drain, and dis differential increase and (b) at the onset of the saturation region, VD = VD. The point Yindicates the channel pinch-off point. For VD > VD point T moves to the left, reducing the effective channel length, but the drain current remains nearly constant. See text (26).

Effective mobility, peff, is obtained by considering the linear portion of the pre-pinch-off regime, defined by Yds Vgs - Vx, in which case. 

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