4.4 Geometric Parameters

From Figure 4.1 three geometric parameters for the two-drain MAGFET can be identified: the length of the device, the width of the device, and the distance between the drains. From (4.2) it is clear that longer devices will give higher differential currents, because there is more ``path'' for the carriers to be deflected before they reach the drains. Wider devices will give higher differential currents, because there are more current lines to be deflected. Finally, the smaller the separation between the drains, the higher the differential current will be, because any minor deflection will be detected by the drains [42].

In order to verify these statements, some simulations are carried out with the following bias: a gate voltage of 4.95 V, 1.0 V at the drains, and 0.0 V at the source and substrate. The applied magnetic field is -50 mT and the simulations are carried out at both 300 K and 77 K. The Hall scattering factor for electrons has been set to a constant value of 0.8 for all the simulation results in this section. The geometry of the two-drain MAGFET is as follows: in the simulation results presented in Figure 4.93 the width and length of the two-drain MAGFET are set to $100\,\mu$m and $125\,\mu$m respectively. In the simulations results presented in Figure 4.94 the width of the two-drain MAGFET is set to $100\,\mu$m and the distance between the drains is set to $10\,\mu$m. In the simulation results presented in Figure 4.95 the length of the two-drain MAGFET is set to $125\,\mu$m and the distance between the drains is $10\,\mu$m.

Figure 4.93 shows simulation results for different distances between the drains. As it can be seen, the separation distance does not play a dominant role in the general behavior of the two-drain MAGFET. At 300 K, the relative sensitivity increases from 3.51 % T$^{-1}$ at a separation distance between drains of 10 $\mu$m, to 3.66 % T$^{-1}$ at a separation distance between drains of 2 $\mu$m. Even at 77 K little improvement is obtained, where the relative sensitivity increases from 10.97 % T$^{-1}$ at a separation distance between drains of 10 $\mu$m, to a maximum of 11.94 % T$^{-1}$ at a separation distance between drains of 4 $\mu$m, and afterwards the relative sensitivity decreases.

Figure 4.93: Simulated $S_r$ for different distances between the drains.

Figure 4.94 shows how the relative sensitivity is improved as the length of the device is increased, as it is predicted by (4.2). At 300 K, the relative sensitivity increases from 2.10 % T$^{-1}$ to 3.64 % T$^{-1}$. However, at 77 K the improvement of the relative sensitivity is very high, going from 7.07 % T$^{-1}$ to 11.11 % T$^{-1}$.

Figure 4.94: Simulated relative sensitivity
for different lengths.

Finally, Figure 4.95 shows the relative sensitivity for different widths. As is also reported in [42], the relative sensitivity shows a maximum as the width is increased. At 300 K, the relative sensitivity increases from 2.75 % T$^{-1}$ to a maximum of 3.56 % T$^{-1}$ at a width of 120 $\mu$m. However, at 77 K, the relative sensitivity increases from 6.45 % T$^{-1}$ to a maximum of 13.67 % T$^{-1}$ at a width of 190 $\mu$m. The fact that both maxima are not at the same width can be explained in terms of the cryogenic operation of the MAGFET: at room temperature operation, the scattering mechanisms are so strong that degrade the sensitivity of the device. At low temperature operation, those scattering mechanisms are reduced.

Figure 4.95: Simulated relative sensitivity
for different widths.