5.4 Short Channel Effect

Due to variations during the device fabrication process the geometry and doping parameters of a MOS transistor are subject to uncertainties. Therefore, the electrical parameters of the transistor vary, as well. Devices should have electrical parameters that are rather insensitive against geometry fluctuations.

Traditionally, the relationship between threshold voltage and gate length, also called `` $V_{\mathrm{th}}$ roll-off'', is used to measure the short channel effect of a MOS transistor, as shown in Fig. 5.11 for the PCD device and the uniformly doped device. The high-drain bias threshold voltages are extracted using the constant current method which means that the gate-source voltage is taken at $I_{\mathrm{d}}$ = 100 nA.

For larger gate lengths the PCD device has a rather constant $V_{\mathrm{th}}$ compared to the uniformly doped device because the channel length of the effective transistor region, that is the lateral length of the highly doped peak (high- $V_{\mathrm{th}}$ region), stays unchanged and only the length of the lightly doped region at the drain side (low- $V_{\mathrm{th}}$ region) increases [12,52].

The PCD device also shows a much better $V_{\mathrm{th}}$ roll-off for shorter channel lengths for the same reason. At the nominal gate length of 0.25 $\mu$m the slope of the $V_{\mathrm{th}}$- $L_{\mathrm{g}}$ curve is approximately 0.20 V/$\mu$m for the PCD device and 0.56 V/$\mu$m for the uniformly doped device.

Figure 5.11: The Vth roll-off of the PCD device compared to the uniformly doped device.