Abstract-Simulation of a MOS or MIS structured Spatial Light Modulator for Terahertz (THz) Imaging

Alam, Md Shahanur

https://etd.ohiolink.edu/pg_10?0::NO:10:P10_ETD_SUBID:166100

A large area, square-gate Metal-Oxide/Insulator-Semiconductor (MOS or MIS) structured `Spatial Light Modulator’ has been proposed for the Terahertz (THz) imaging at room temperature. This theoretical study has been done entirely by using the MATLAB platform. The 1D Poisson’s equation has been solved iteratively by Euler’s method with the exact Fermi-Dirac integral for n-doped Si-MOS and GaAs- MIS capacitors in deep accumulation mode. Free carrier density and the potential profile have been found for the degenerate case. The results have been compared with the analytical method, which is approximated by Boltzmann statistics, and clearly, show the difference between the degenerate case and non-degenerate Boltzmann approximation at higher bias. Then the sheet carrier density was calculated by performing the numerical integration over the depth of accumulation. The mobility was calculated for various scattering mechanisms at room temperature, and the total mobility was found by using Matthiessen’s rule. The mobility and free carrier density were then used for calculating the bulk conductivity of the device. The sheet conductance and sheet resistance were found by the numerical integration of the bulk conductivity over the depth of accumulation. The sheet resistance is the DC resistance and was scaled to THz frequencies by using the Drude model. 

Then a lossless two-port transmission line model was adopted for studying the transmittances of the THz signal through the MOS or MIS devices for different bias and doping concentration and calculated the depth of modulation was calculated Vs frequency between 0.1 and 1 THz. In contrast to the GaAs-MIS device, it has been found that the depth of modulation of the Si-MOS device is much higher.
The switching time has been calculated as the 10% to 90% charging time of the interface capacitor by solving the equivalent large-signal RC circuit. The GaAs-MIS displays lower switching time than Si-MOS because GaAs-MIS offers lower interface capacitance than that of the Si-MOS at the same interface bias. The 3dB bandwidth of the modulator has been estimated by assuming the linear RC circuit relations. Although the GaAs-MIS exhibited higher bandwidth than that of Si-MOS, both devices are still promising for the possible fabrication of spatial light modulator for THz imaging.