zero substrate bias, is the body effect parameter, is oxide thickness, is oxide permittivity, is the permittivity of silicon, is a doping concentration, is elementary charge. Dependence on oxide thickness In a given technology node, such as the 90-nm CMOS process, the 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. Although this may seem to be an improvement, it is not without cost; because the thinner the oxide thickness, the higher the subthreshold leakage current through the device will be. Consequently, the design specification for 90-nm gate-oxide thickness was set at 1 nm to control the leakage current. This kind of tunneling, called Fowler-Nordheim Tunneling. where; and are constants, is the electric field across the gate oxide. Before scaling the design features down to 90 nm, a dual-oxide approach for creating the oxide thickness was a common solution to this issue. With a 90 nm 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. Temperature dependence As with the case of oxide thickness affecting threshold voltage, temperature has an effect on the threshold voltage of a CMOS device. Expanding on part of the equation in the body effect section where; is half the contact potential, is the Boltzmann constant, is temperature, is the elementary charge, is a doping parameter, is the intrinsic doping parameter for the substrate. We see that the surface potential has a direct relationship with the temperature. Looking above, that the threshold voltage does not have a direct relationship but is not independent of the effects. This variation is typically between −4 mV/K and −2 mV/K depending on doping level. For a change of 30 °C this results in significant variation from the 500 mV design parameter commonly used for the 90-nm technology node. Dependence on random dopant fluctuation Random dopant fluctuation (RDF) is a form of process variation resulting from variation in the implanted impurity concentration. In MOSFET transistors, RDF in the channel region can alter the transistor's properties, especially threshold voltage.