S.N. Chattopadhyay

Optically controlled characteristics in an ion-implanted silicon MESFET

Abstract OPFETs (optical FETs) are useful as compatible IC transducers in optical communication systems, although APDs (avalanche photo diodes) have higher multiplication gain and speed of response. Studies have been made on the optically controlled characteristics of an ion-implanted Si MESFET which show that drain-source current can be enhanced with increasing radiation flux intensity and lower wavelength of operation. Furthermore, the threshold voltage is found to be reduced under normally OFF conditions and increased under normally ON conditions for higher flux density and lower wavelength. The effect of radiation becomes predominant over the impurity concentration at flux densities greater than or equal to 1018/m2 and wavelengths less than or equal to 0.78 μm.

GaAs OPFET characteristics considering the effect of gate depletion with modulation due to incident radiation

A realistic analytical model of an ion-implanted GaAs OPFET has been presented. Both the photogeneration and photovoltaic effect and the voltage dependence of the depletion layer widths in the active region have been considered. The threshold voltage decreases in the enhancement device and increases in the depletion device at a particular dose, flux density, and trap center density when both the photovoltaic effect and photogeneration are taken into account compared to the case where the photovoltaic effect is ignored. At higher flux density and trap density, the threshold voltage shows a nonlinear effect at a lower value of the implanted dose, which is mainly due to the recombination term. The drain-source current significantly increases due to the photovoltaic effect because of the widening of the channel region. The device is pinched off at a higher drain-source voltage compared to the photogeneration case only. >

Simulation of 4H-SiC MESFET for High Power and High Frequency Response

In this paper, we report an analytical mo- deling and 2-D Synopsys Sentaurus TCAD simulation of ion implanted silicon carbide MESFETs. The model has been developed to obtain the threshold voltage, drain-source current, intrinsic parameters such as, gate capacitance, drain-source resistance and transconductance considering different fabrication parameters such as ion dose, ion energy, ion range and annealing effect parameters. The model is useful in determining the ion implantation fabrication parameters from the optimization of the active implanted channel thickness for different ion doses resulting in the desired pinch off voltage needed for high drain current and high breakdown voltage. The drain current of approximately 10 A obtained from the analytical model agrees well with that of the Synopsys Sentaurus TCAD simulation and the breakdown voltage approximately 85 V obtained from the TCAD simulation agrees well with published experimental results. The gate-to-source capacitance and gate-to- drain capacitance, drain-source resistance and trans- conductance were studied to understand the device frequency response. Cut off and maximum frequencies of approximately 10 GHz and 29 GHz respectively were obtained from Sentaurus TCAD and verified by the Smiths chart.

Analytical modeling of an ion-implanted silicon MESFET in post-anneal condition

Electrical parameters such as threshold voltage, drain-source current, and transconductance are studied. The channel charge is reduced with the increase in the diffusion coefficient of the implanted ions. Inclusion of a diffusion effect due to annealing shows an increase in the threshold voltage under normally ON condition and a reduction under normally OFF condition. At a fixed implant dose, anneal temperature can change the device from normally ON to OFF and vice versa, depending on the nature of the dopant and the anneal temperature. Drain-source current and transconductance also get reduced compared to the case where diffusion of the implanted ions due to annealing is not considered. >

Optically Controlled Silicon MESFET Modeling Considering Diffusion Process

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 VT 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 Vgs is significantly increased with optical illumination for photon flux densities of Φ = 10 15 and 10 17 /㎠s compared to the dark condition. Further more, the drain-source current as a function of drain-source voltage VDS is evaluated to find the I-V characteristics for various pinch-off voltages V P for optimization of impurity flux density QDiff 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 ion implantation process. Further the results show significant increase in gate-source capacitance C gs and gate-drain capacitance C gd for optical illuminations, where the photo-induced voltage has a significant role on gate capacitances. The switching time τ of the OPFET device is computed for dark and illumination conditions. The switching time τ is greatly reduced by optical illumination and is also a function of device active layer thickness and corresponding impurity flux density Q Diff . Thus it is shown that the diffusion process shows great potential for improvement of optoelectronic devices in quantum efficiency and other performance areas.

Time dependent analysis of an ion-implanted GaAs OPFET

A time-dependent analysis of the electrical characteristics of an ion implanted GaAs optical field effect transistor (OPFET) has been carried out. Both the cases of light turning on and off at a reference time t=0 have been considered. The photovoltaic effect across the Schottky junction and the depletion width modulation in the active layer have been taken into account. The threshold voltage, channel charge, channel conductance, drain-source current, transconductance, and gate-source capacitance of the device under light turning on and off conditions have been evaluated. When light is turned on, all the parameters increase with time before reaching the steady-state value and when light is turned off, these parameters decrease with time and reach their respective values corresponding to dark condition. The time under on condition is less than that under off condition. >

The effects of annealing on the switching characteristics of an ion-implanted silicon MESFET

The gate-source and gate-drain capacitances and the drain-source resistance are calculated in the region below pinchoff in the postimplanted annealed condition. It is observed that the capacitances decrease and the resistance increases compared to the case where diffusion of impurity ions due to annealing is not considered. P, B, As, Sb, Ga, and Al dopants are used for the calculation. The delay time is found to be mostly unaffected by annealing. The capacitances in the region above pinchoff show an increase as a result of annealing in the enhancement device compared to the case when diffusion is not taken into account. These capacitances are mainly due to the sidewalls of the space-charge region and are dependent on the threshold voltage, which decreases at higher anneal temperatures in the enhancement mode. >

Accurate modeling of an ion-implanted MESFET

Abstract An accurate modeling of an ion-implanted silicon MESFET device has been carried out considering a Gaussian-exponential distribution of impurities. Although two-dimensional analysis is essential for the characterization of the short-channel and submicrometer FET devices, one-dimensional analysis is fairly accurate for a moderately long-channel MESFETs fabricated with a shallow implanted channel where the concentration is highly nonlinear with depth. Potential distribution, channel charge, threshold voltage, I–V characteristics, mobility and transconductance of the Si MESFET device have been evaluated. The results show closer agreement with the experimentally obtained values compared to those obtained considering only Gaussian distribution of impurity profile.

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

An optically controlled silicon MESFET (OPFET) was fabricated by diffusion process to enhance the quantum efficiency, which is the most important optoelectronic device performance usually affected by ion implantation process due to large number of process induced defects. The desired impurity distribution profile and the junction depth were obtained solely with diffusion, and etching processes monitored by atomic force microscope, spreading resistance profiling and C-V measurements. With this approach fabrication induced defects are reduced, leading to significantly improved performance. The fabricated OPFET devices showed proper I-V characteristics with desired pinch-off voltage and threshold voltage for normally-on devices. The peak photoresponsivity was obtained at 620 ㎚ wavelength and the extracted external quantum efficiency from the photoresponse plot was found to be approximately 87.9%. This result is evidence of enhancement of device quantum efficiency fabricated by the diffusion process. It also supports the fact that the diffusion process is an extremely suitable process for fabrication of high performance optoelectronic devices. The maximum gain of OPFET at optical modulated signal was obtained at the frequency of 1 MHz with rise time and fall time approximately of 480 nS. The extracted transconductance shows the possible potential of device speed performance improvements for shorter gate length. The results support the use of a diffusion process for fabrication of high performance optoelectronic devices.

Scaling rule for OPFET

A scaling rule has been suggested to miniaturize the ion implanted GaAs optical field effect transistor (OPFET). The absorption coefficient and the generation rate have been scaled along with electrical parameters and device dimensions. Plots have been made for drain-source current, cut off frequency, and the DC power dissipation of the device.