B.B. Pal

Effect of radiation and surface recombination on the characteristics of an ion-implanted GaAs MESFET

The effects of optical radiation and surface recombination on the characteristics of an ion-implanted GaAs MESFET are studied analytically in the below-pinchoff region. Results show that optical radiation significantly enhances the drain-source current of the GaAs MESFET when only electron-hole pair generation is considered. However, the surface recombination, which in turn results in a gate leakage current, reduces the drain-source current, with the reduction depending on the density of trap centers. The threshold voltage is found to decrease under the normally OFF condition and increase under the normally ON condition due to photogenerated carriers. The surface recombination reverses the effect, i.e. threshold voltage increases under the normally OFF condition and decreases under the normally ON condition, with an increase in the trap center density at a particular ion dose compared to those cases where the effect of recombination is not considered. >

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.

Optically-controlled ion-implanted GaAs MESFET characteristic with opaque gate

An analytical modelling has been carried out for an ion-implanted GaAs MESFET having a Schottky gate opaque to incident radiation. The radiation is absorbed in the device through the spacings of source, gate, and drain unlike the other model where gate is transparent/semitransparent. Continuity equations have been solved for the excess carriers generated in the neutral active region, the extended gate depletion region and the depletion region of active (n) and substrate (p) junction. The photovoltage across the channel and the p-layer junction and that across the Schottky junction due to generation in the arc region of the gate depletion layer are the two important controlling parameters. The I-V characteristics and the transconductance of the device have been evaluated and discussed.

A new 2-D model for the potential distribution and threshold voltage of fully depleted short-channel Si-SOI MESFETs

A new two-dimensional (2-D) analytical model for the threshold voltage of a fully depleted short-channel Si-MESFETs fabricated on the silicon-on-insulator (SOI) has been presented in this paper. The 2-D potential distribution functions in the active layer of the device is approximated as a parabolic function and the 2-D Poissons equation has been solved with suitable boundary conditions to obtain the bottom potential at the Si/oxide layer interface. The calculations have been carried out for both uniform and nonuniform doping profiles in two dimensions. The minimum bottom potential is used to monitor the drain-induced barrier lowering effect and consequently an analytical expression for the threshold voltage of the device has been derived. The numerical results for the bottom potential and threshold voltage considering a wide range of device parameters have also been presented. The model has been compared with the simulated results obtained by using the ATLAS Device Simulation Software to show the validity of the proposed model. For uniform doping profile, the numerical results have also been compared with the reported data in the literature and a good agreement is observed among the three. The proposed model is simple and easy to understand the behavior of the fully depleted short-channel SOI-MESFETs as compared to the other models reported in the literature.

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. >

On the scaling of an ion-implanted silicon MESFET

Abstract An ion-implanted silicon MESFET is scaled to smaller sizes assuming constant field within the device. Ion implantation is a leading techn.ology for VLSI and scaling is an important tool for device miniaturization. A one-dimensional and fairly accurate analysis is carried out considering the effect of side walls in the space-charge region below the gate in the below pinch-off region. Different device parameters such as drain-source current, threshold voltage, time delay, frequency, d.c. power dissipation and switching energy are plotted and discussed with respect to scaling factor for α. The results show how the ion-implanted silicon MESFET device can be optimized with the ad of scaling factor for better device performance.

A new optoelectronic integrated device for light-amplifying optical switch (LAOS)

A new optoelectronic integrated device is proposed for a light-amplifying optical switch (LAOS). The device is composed of an optical field-effect transistor (OPFET) in series with a light source which may be either a double heterostructure light-emitting diode (LED) or laser diode (LD). A quantitative circuit model for the proposed LAOS is presented and theoretical investigation is carried out for developing a current-voltage (I-V) relation for the device. It is shown analytically that switching action takes place from a low current state to a high current state through a region of negative differential resistance (NDR) when a voltage greater than the breakover voltage is applied.

Analysis of GaAs OPFET with improved optical absorption under back illumination

The effect of back illumination with improved optical absorption has been analyzed for an ion-implanted GaAs OPFET considering the Pearson IV distribution of impurities. Plots have been made for two photo voltages developed across the substrate active layer junction and the Schottky junction. The drain-source current is significantly enhanced for the device when a fiber is inserted up to the active layer substrate junction compared with the case where the finite substrate effect is taken into account.

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. >

Frequency dependent characteristics in an ion-implanted photo MESFET

Abstract The possibility of using a MESFET for high speed photodetection and switching has been investigated by various workers. In this paper an attempt is made to theoretically evaluate the effect of a signal modulated optical radiation incident on the transparent or semi-transparent Schottky gate of a MESFET device. It is observed that the modulating frequency changes both the I - V characteristics and the threshold voltage of the ion-implanted Si-MESFET. However, in the microwave frequency range the threshold voltage is found to be almost unchanged with frequency. The effect of the wavelength of radiation is the same as observed earlier [1].