Editing MOSFET (section)

===Increased gate-oxide leakage===
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 1.2&nbsp;[[nanometer|nm]] (which in silicon is ~5&nbsp;[[atom]]s thick) the [[quantum mechanical]] phenomenon of [[electron tunneling]] occurs between the gate and channel, leading to increased power consumption. [[Silicon dioxide]] has traditionally been used as the gate insulator. Silicon dioxide however has a modest dielectric constant. Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance (capacitance is proportional to dielectric constant and inversely proportional to dielectric thickness). All else equal, a higher dielectric thickness reduces the [[quantum tunneling]] current through the dielectric between the gate and the channel.

Insulators that have a larger [[dielectric constant]] than silicon dioxide (referred to as [[high-κ dielectric]]s), such as group IVb metal silicates e.g. [[hafnium]] and [[zirconium]] silicates and oxides are being used to reduce the gate leakage from the 45 nanometer technology node onwards. On the other hand, the barrier height of the new gate insulator is an important consideration; the difference in [[conduction band]] energy between the semiconductor and the dielectric (and the corresponding difference in [[valence band]] energy) also affects leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 [[Electronvolt|eV]]. For many alternative dielectrics the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant. The maximum gate-source voltage is determined by the strength of the electric field able to be sustained by the gate dielectric before significant leakage occurs. As the insulating dielectric is made thinner, the electric field strength within it goes up for a fixed voltage. This necessitates using lower voltages with the thinner dielectric.