Threshold Voltage - Dependence On Oxide Thickness

Dependence On Oxide Thickness

In a given technology node, such as the 90 nanometer CMOS process, threshold voltage depends on the choice of oxide and on oxide thickness. Using the body formulas above, is directly proportional to, and, which is the parameter for oxide thickness.

Thus, the thinner the oxide thickness, the lower the threshold voltage. While this may seem to be an improvement, it is not without cost; for the thinner the oxide thickness, the higher the subthreshold leakage current flowing through the device will be. Consequently, the design specification for 90 nanometer gate oxide thickness was set at 1 nanometer to control the leakage current. This kind of tunneling, called Fowler-Nordheim Tunneling.

where and are constants and is the electric field across the gate oxide.

Before scaling the design features down to 90 nanometers, a dual oxide approach for creating the oxide thickness was a common solution to this issue. With a 90 nanometer process technology, a triple oxide approach has been adopted in some cases. One standard thin oxide is used for most transistors, another for I/O driver cells, and a third for memory and pass transistor cells. These differences are based purely on the characteristics of oxide thickness on threshold voltage of CMOS technologies.