5.2.2 Vertical Doping Peak Length

If the gate oxide of a MOS transistor becomes thinner, raising the bulk doping is necessary to keep the threshold voltage at a reasonably high level. As a result, the subthreshold swing will get worse due to the increased bulk effect, as described earlier in Section 2.1. The subthreshold swing deterioration can be omitted by a lower bulk doping together with a doping layer near the surface to set the threshold voltage. The bulk depletion layer will then be deeper which improves the subthreshold swing according to (2.6)-(2.8). The maximum doping should not be placed at the surface because the carrier mobility due to impurity scattering [66] would be decreased. A steep subthreshold slope is indispensable for ultra low-power devices as they have a very pronounced subthreshold region in contrast to devices for high performance desktop applications.

The optimized PCD structure shows similar properties: The doping peak has a finite depth and its maximum is slightly below the surface (see Fig. 4.8). Fig. 5.5 demonstrates the deterioration of the subthreshold swing of a PCD device with an infinite peak depth shown in Fig. 5.6 compared to the original device. The subthreshold swings are 88 mV/dec for the PCD device with infinite peak depth and 74 mV/dec for the original PCD device. The infinite peak depth is achieved by extending the peak in the vertical direction down to the bulk bottom. The peak doping concentration is decreased by a factor of 0.83 at the same time to adjust the leakage current to 1 pA. Due to the different subthreshold swings and the constant leakage current a threshold voltage shift between the conventional and infinite PCD devices can be observed.

Figure 5.5: Subthreshold characteristics of the PCD device: (a) finite and (b) infinite peak depth.

Figure 5.6: The PCD device with infinite peak depth.