<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter reports the engineering of effective work function (EWF) for tantalum carbide (TaC) metal gate on high-<formula formulatype="inline"><tex>$k$</tex></formula> gate dielectrics. The dependence of EWF on Si concentration in HfSiO as well as nitridation techniques is revealed. The EWF was extracted by both terraced oxide and terraced high-<formula formulatype="inline"><tex>$k$</tex></formula> techniques with the bulk and interface charges taken into account. The incorporation of Si in Hf-based dielectrics results in an increase of EWF, while the presence of N tends to decrease the EWF. Plasma nitridation is found to be more effective in lowering the EWF than a thermal nitridation. The phenomena can be explained by the modification of TaC/high-<formula formulatype="inline"><tex>$k$</tex></formula> interface dipole moment, which arises from the electronegativity difference for various interface bonds. Based on the above findings, we proposed a novel approach to reduce the EWF of TaC on HfSiON by using a thin <formula formulatype="inline"><tex>$\hbox{HfO}_{2}$</tex></formula> cap layer after optimizing the nitridation. The MOSFET results show that this technique is able to achieve a lower <formula formulatype="inline"><tex>$V_{t}$</tex></formula> without degrading the device performance. </para>