Optically Controlled Silicon MESFET Modeling Considering Diffusion Process

Abstract

An analytical model is proposed for an optically controlled Metal Semiconductor Field Effect Transistor (MESFET), known as Optical Field Effect Transistor (OPFET) considering the diffusion fabrication process. The electrical parameters such as threshold voltage, drain-source current, gate capacitances and switching response have been determined for the dark and various illuminated conditions. The Photovoltaic effect due to photogenerated carriers under illumination is shown to modulate the channel cross-section, which in turn significantly changes the threshold voltage, drainsource current, the gate capacitances and the device switching speed. The threshold voltage V T is reduced under optical illumination condition, which leads the device to change the device property from enhancement mode to depletion mode depending on photon impurity flux density. The resulting I-V characteristics show that the drain-source current IDS for different gate-source voltage V gs is significantly increased with optical illumination for photon flux densities of Φ = 10 15 and 10 17 /cm 2 s compared to the dark condition. Further more, the drain-source current as a function of drain-source voltage V DS is evaluated to find the I-V characteristics for various pinch-off voltages V P for optimization of impurity flux density Q Diff by diffusion process. The resulting I-V characteristics also show that the diffusion process introduces less process-induced damage compared to ion implantation, which suffers from current reduction due to a large number of defects introduced by the

Optically Controlled Silicon MESFET Fabrication and Characterizations for Optical Modulator/Demodulator

“…1 shows a cross-sectional schematic diagram for 13 µm gate length. The theory for OPFET considering non-uniform doping has been described with the physics based analytical modeling [21]. When the device is illuminated by photons with energy E ph = hν (h = Planck's constant and ν = frequency of the light) greater than the energy bandgap of silicon (1.12 eV) but smaller than the energy bandgap of indium tin oxide (~3.5 eV), electron and hole pairs are generated.…”

“…The active area was formed by diffusion process, which is preferred over ion implantation due to diffusion having little or no process induced defects [21]. Ion implantation induced defects in silicon substrates have been characterized to study the dependence of substrate dopant species (phosphorous and boron) on defect formations [29].…”

“…Phosphorus ion implantation not only results in the transition of the crystalline fullerenes to an amorphous material phase, but also produces a significant defect level, which is evident from solar cell efficiency ranges of only 10-20% [20]. An effective loss of photo-generated carriers due to the ion implantation process induced defects in the active channel region of OPFET is a major issue of degradation of quantum efficiency and sensitivity [21]. A dc analytical model of an ion implanted GaAs based OPFET has been proposed by many researchers [22][23][24].…”

IV and Switching Characteristics of Back Illuminated OPFET using Finite Difference Methods

“…Thus under small signal condition [2] t j e     1 0   (11) where "zero" indicates the dc value and "one" indicates the ac value.…”

“…The switching time is computed for different active layer thickness expressed by the following equation [11] …”