A new optoelectronic integrated device consisting of an n-GaAs optical field-effect transistor (OPFET) and a p-AlGaAs/p-GaAs/n+-AlGaAs surface emitting double heterostructure light emitting diode (DH-LED) which are developed monolithically on a p+-GaAs subtrate and separated by a thin semi-insulating GaAs layer, is proposed in this paper. We call this device as Light Source Integrated-OPFET (LSI-OPFET). The proposed device structure is such that the optical radiation generated by the LED is fed into the OPFET as back illumination. The back radiation is used as the control signal to the OPFET which changes the transconductance of the OPFET. The intensity level of the back illumination to the OPFET can be controlled by changing the bias current of the LED. In this paper, analytical results have been presented for the I-V characteristics and transconductance of the OPFET as a function of the LED current. It has been shown that by changing the level of the back illumination to the OPFET, i.e. by changing the LED current, one can vary both the characteristics and transconductance, which makes the LSI-OPFET as the potential device for the optically controlled varying gain amplifier.
A new optoelectronic integrated device composed of an Optical Field Effect Transistor (OPFET) in series with a double heterojunction light emitting diode (LED) or a laser liode (LD) which can be used as a Light Amplifying Optical Switch (LAOS), is presented in this paper. We shall call this device as OPFET-LAOS since it is a new one in its category. Theoretical investigation has been carried out to develop the I-V characteristic of the proposed device. It is shown theoretically that the device changes its state form a low current (i.e. high impedance) state to a high current (i.e. low impedance) state through a region of negative differential resistance (NDR) when the applied voltage exceeds a certain limit, called the breakover voltage. Thus, the I-V characteristic of the device is similar to that of an existing LAOS composed of a heterojunction phototransistor (HPT) in series with a double heterojunction LED (or LD).
An enhancement mode MESFET (E-MESFET) is useful as a low voltage, low power device and plays an important role in VLSI designing .An E-MESFET behaves as a switch under optical illumination, it turns ON when light is ON and turns OFF when light is switched OFF. Studies have been made on optically controlled characteristics of an ion-implanted GaAs - MESFET which show that the drain source current can be enhanced with increasing photo voltage as well as radiation flux density. Effect of radiation becomes predominant over the impurity concentration. Sharp increase in drain to source current have been observed at flux density greater than or equal to 1020 / m2 .
The effect of illumination on the Schrodinger's wave function and related device characteristics has been studied in the quantum well of a n-AlGaAs/GaAs MODFET. Partial depletion of the active region of the MODFET has been considered. At the heterojunction interface, the quantum well has been considered as a modified triangular potential well of finite depth. The potential energy and the subband energy are calculated by solving the Poisson's equation. The wave function, sheet concentration and the I-V characteristics of the MODFET under dark and illuminated conditions have been calculated and discussed.
The transient behavior of a high electron mobility phototransistor has been calculated theoretically, where radiation is allowed to fall within the gaps between source, gate, and drain and the gate behaves as an opaque material. The parameters calculated are the threshold voltage, drain-source current, and transconductance of the device as functions of time. It is observed that when the light is turned on, the parameters reach the steady-state value at around 40 ps, and when the light is turned off, they reach their dark values at around 300 ps. In the turned-on case, it is the optical relaxation time that controls the behavior, and in the turned-off case it is the photovoltage across the heterointerface, which decreases linearly with time.
With the advancement of high bit rate optical communication system, there is a need for a high speed photo detector. Results have been presented for a n+-n-- n+ small geometry GaAs diode under optical illumination. The potential at the n+-nl-interface is found to be more negative due to the photovoltaic effect. The current increases with the increase in absorption coefficient and radiation flux density. The quantum efficiency is over 70% in the wavelength spectrum 0.6 - 0.8 um being more than 80% at 0.76 um. The device can be used in GaAs material for 1.55 um system.
An enhanced optical effect is observed in a high electron mobility transistor device when the radiation is allowed to fall on the gaps of the source, gate, and drain of a n-AlGaAs/GaAs system. A 2-D model is presented considering a realistic velocity field relation of carriers in the n-AlGaAs layer. Due to the incident radiation, a photovoltage is developed across the heterojunction in addition to band-edge discontinuity between the n-AlGaAs and GaAs layers. This photovoltage drags the electrons from the n-AlGaAs layer into the heterointerface, thus significantly enhancing the sheet concentration of 2-D electron gas. The drain current and transconductance of the device increase considerably. Plots have been made for the I-V characteristics and the transconductance versus gate voltage. Also, a ratio of drain current under illumination and drain current under a dark condition has been plotted against the input optical power density, which indicates that the overall gain of the device is enhanced by a factor of more than 10 compared to its dark case.
When radiation is allowed to fall on the spacing between the source and gate and the gate and drain of a AlGaAs/GaAs high electron mobility transfer device, a photovoltage develops across the heterojunction. This significantly enhances the device characteristics.
For long wavelength single node
DFB type laser devices it Is required to
rrugate the surface of InP substrate in the
form of lines (Grating) with spatial period In
sub-half micron ng. Electron beam
lithography (E2L) can be used to make such high
resolutlon lines first in the resist and then
these lines can be transferred to the substrate
by suitable dry or wet etching process. In
delineating such closely spaced lines In the
resist which Is coated on a relatively high
average atctnic number substrate ( At.No.32),
the well known proximity exposure (PE) effect
associated with EBL requires cxrrection • The
paper discusses arid gives the simulation
results which depict the effect of PE on the
width and spacing of lines. A comparison Is
made between the lines on the lower At.No.
substrate Si, and high At.No. InP substrate. A
proximity exposure compensation (PEC) scheme
based on dose variation technique is found to
provide just adequate correction to PE effect.
The experimental results on the delineation of
lines before and after correction show good
agreement with the simulated results within
experimental limitations.
The effect of optical radiation on the electrical characteristics of a HENT device has been carried out. Both photovoltaic effect in the metalsemiconductor junction and photogeneration of carriers in the space charge layer of n-A1GaAs HENT device have been taken into account. Offset voltage is found to decrease with flux density and active layer concentration. The drain current is observed to increase with the increase in radiation flux density photogeneration of electronhole pairs. The photovoltaic effect develops a forward voltage across the metalsemiconductor junction. In a A1GaAs/GaAs system the electrons and holes generated in nAlGaAs layer are separated out from each other by the field across the'' metalsemiconductor junction and heterointerface. Thus due to the additional charge carriers the charge density in 2DEG at the heterointerface of the HENT structure will be modified. The theory is given in the next section. Theory The optical HENT structure is shown in Fig. 1 . The region between the Schottky gate and the heterojunction interface is assumed to be totally depleted and the electrostatic potential v2 at the end of the semiconductor (x d2) is obtained by solving Poisson''s eqn. 2 - '' [N2(x) + n(x)(1) dx2 alongwith appropriate boundary condition N2(x) and 2 are the doping concentration and permittivity of n-A1GaAs layer. n(x) is the Photonic devices find a lot of applications in optical communication integrated optics and optical computer. High speed devices
Optically tuned GaAs OPFET Is a potential device for integrated Optics Optical Communication and Computing. The transient analysis of an ion implanted GaAs OPFET shows that the carriers generated in the gate depletion region due to absorption of photons attains the steady state value after 40 ps. The D. C. analysis indicates that a analysis normally ON device becomes OFF at a higher negative gate voltage compared to the dark condition and a normally OFF device is opened even at zero gate bias. The drainsource current and the transconductance are significantly enhanced due to optical radiation. In the a. c. analysis the optical radiation is modulated by the signal and hence the number of electronhole pairs generated below the gate is also modulated. So the device properties are greatly influenced by the modulated frequency.
A quantum well injection transit time (QWITT) diode has been proposed recently to improve the performance of quantum well devices incorporating the effect of transit time of carriers. Earlier, transit time devices such as IMPATT, BARITT, DOVATT, TRAPATT, TUNETT show different types of performances with certain limitations. QWITT is a low noise device applicable to the sub-millimeter frequency range. Large signal simulations show that a QWITT device may be capable of producing approximately +5 dBm power at 200 GHz.
The effect of surface recombination and modulated frequency on the intrinsic parameters of an ion implanted GaAs optical field effect transistor have been analyzed. The study reveals that the gate-source capacitance increases with gate-source voltage, first slowly, and then sharply under normally OFF condition with the increase of modulated frequency. However, the surface recombination reduces these effects depending upon the trap center density. These variations are small in a normally ON device. Also, the drain-source resistance is found to increase with the increase of modulating frequency, but it reduces with the reduction of trap- center density at a fixed flux density and drain-source voltage.
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