4.2.3 Bias Dependence

In this subsection the relative sensitivity as a function of the gate bias and the polarization of the drains is investigated. Figure 4.7 shows the relative sensitivity as a function of the gate voltage for different bias at the drains.

Figure 4.7: Simulated $S_r$ as a function of the gate voltage at 300 K and -50 mT.

As the drain bias is increased the relative sensitivity decreases, because the thickness of the inversion layer is modulated, and also the longitudinal electric field increases, which reduces the carrier deflection. For example, at a gate voltage of 5.0 V the relative sensitivity decreases from 3.24 % T$^{-1}$ down to 2.62 % T$^{-1}$ showing that this modulation exists, but it is not as strong as with the gate voltage, where the relative sensitivity decreases from 5.50 % T$^{-1}$ down to 2.62 % T$^{-1}$.

As the gate voltage is increased, the differential current increases too. Figure 4.8 shows how this differential current increases for a voltage of 1.0 V at the drains. However, that does not mean that the relative sensitivity increases. As can be seen from the same figure, the relative sensitivity decreases, because the differential current only increases a few microamperes whereas the total source current is larger at least two orders of magnitude.

Figure 4.8: Simulated $S_r$ and $\Delta$ for a gate swept. $V_{D1}$ and $V_{D2}$ are set to 1.0 V.

The 's' shape of the relative sensitivity as a function of the gate voltage can be explained in terms of how carriers in the inversion layer move. For low drain to source voltages, most of the carriers move by diffusion. Once the drain to source voltage is high enough, the carriers are swept by the lateral electric field, that is, the carriers move by drift.

This behavior is more visible, if the relative sensitivity is plotted as a function of the drain voltage. Figure 4.9 shows this plot for different gate voltages. The minima of the relative sensitivity versus the drain voltage indicate the transition between diffusion and drift carrier transport, showing the complexity of the electro-magnetic interaction. In the low temperature analysis some surprising results are found for the same bias conditions, cf. Section 4.3.

Figure 4.9 also shows that the relative sensitivity is high at low gate voltages, because the drain currents are comparable with the differential current. Large differential currents can be reached at higher gate or drain voltages, but at expenses of larger drain currents. However, the relative sensibility decreases according to (4.1).

Figure 4.9: Simulated $S_r$ as a function of the drain voltage at 300 K and -50 mT.