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Typical test procedure. Devices were first stressed under high electrical field. gives an electrical field across the interfacial layer of . The electron trapping during the stress leads to a positive gate voltage shift . After the stress, a relatively low negative field of was applied to detrap electrons from the dielectric and allow the positive charges being measured from the negative . The was measured from the gate voltage shift in the subthreshold region of the transfer characteristics.
Impact of gate materials on positive charge formation under positive gate bias stresses. The is the electron fluency during the stress. The positive charge formation with the metal gate is significantly higher than that with poly-Si gate.
Schematic energy band diagrams for metal-gated samples under positive (a) and negative (b) gate bias stresses. It is assumed that hydrogen released from the anode dominates the positive charge formation and the hydrogen density near the TaN gate interface is higher than that near the substrate interface.
Dependence of positive charge formation on gate bias polarity and gate materials. When stressed under negative gate bias of , positive charge formation with the metal gate is similar to that with the poly-Si gate. For the metal gate, positive charge formation under is significantly less than that under .
Behavior of positive charges formed in metal-gated stack under alternating gate bias polarities . When was applied to a fresh device, the symbol “◇” shows that charging is negligible under , but some electron trapping occurs under , leading to a positive . After a stress under for an electron fluency of , a large part of the formed positive charges can be repeatedly discharged under and recharged under . The symbol “엯” is the directly measured . The symbol “▴” represents positive charges after correcting the electron trapping effect.
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