5.5 Bulk Current

As mentioned earlier in Section 2.4 the bulk current resulting from hot carrier effects can be used as a metric for the lifetime of a MOS transistor, therefore, it is a major concern to keep the bulk current as low as possible.

Fig. 5.12 shows the $I_{\mathrm{b}}$- $V_{\mathrm{g}}$ curve in saturation (at $V_{\mathrm{d}}$ = 1.5 V) for the 0.25 $\mu$m PCD device in forward and reverse mode compared to the uniformly doped device. Table 5.4 lists the corresponding characteristic data extracted from bulk current simulations. The PCD device in forward mode has the lowest maximum bulk current, followed by the uniformly doped device and the PCD device in reverse mode. While the maximum bulk current of the PCD device in both modes appears at about the same gate voltage (at about 2/3 of the supply voltage), the uniformly doped device has its maximum very close to $V_{\mathrm{dd}}$.

Figure 5.12: The bulk currents of the PCD device in forward and reverse mode compared to the uniformly doped device.

Table 5.4: Bulk current simulation results.

	$I_{\mathrm{b,max}}$	$V_{\mathrm{g}}$ at $I_{\mathrm{b,max}}$	$I_{\mathrm{b}}$ at $V_{\mathrm{g}}$ =  $V_{\mathrm{dd}}$	$E_{\mathrm{max}}$ at $V_{\mathrm{g}}$ =  $V_{\mathrm{dd}}$
	($\mu$A)	(V)	(nA)	(kV/cm)
uniform	0.69	1.4	0.69	341
PCD forward	0.46	1.0	0.30	220
PCD reverse	1.05	1.0	0.75	309

The different levels of bulk current result from the fact that the drain electric field is much smaller for the PCD device in forward mode than that of the uniformly doped device as shown in Fig. 5.13 for the case where $V_{\mathrm{g}}$ =  $V_{\mathrm{d}}$ = 1.5 V.

The lightly doped channel at the drain side of the PCD device reduces the lateral electric field in the channel and, therefore, the hot carrier effects [12,34]. Due to the higher drain current of the PCD device in reverse mode compared to the uniformly doped device, the bulk current is slightly higher at this operating point even though the maximum electric field is lower.

Figure 5.13: The lateral electric field in the channel of the PCD device in forward and reverse mode compared to the uniformly doped device.