Optically controlled current-voltage characteristics of ion-implanted MESFETs

Optically controlled MESFETs are useful as optical devices for optical communications, and as photodetectors. In this paper, a theoretical model for the I–V characteristics of these MESFETs is presented. The model considers the nonuniform Gaussian doping for ion-implanted channels. It takes both the photogenerated carriers as well as the doping generated residual carriers into account. It is noted that the density of photogenerated carriers in the channel due to diffusion is much less than that due to drift. Treatment both under gradual channel approximation and saturation velocity approximation has been presented. The gradual channel and the velocity saturation approximations are applied to study the I–V characteristics of long-channel and short-channel MESFETs, respectively. Results for both long-channel and short-channel MESFETs indicate that drain saturation current and transconductance can be improved by properly fixing the optical flux, and the absorption coefficient of the material.     

Accurate modeling of an ion-implanted MESFET

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.     

A device model for an ion-implanted MESFET

A model is developed for an ion-implanted MESFET device in terms of the range and straggle parameters of the Gaussian profile. Reasonable approximations are introduced to obtain a closed-form solution for the linear and triode regions of operation. Simplified square-law approximations are obtained under certain conditions and the validity of these forms is discussed. The simplified expressions are compared with those of the MOSFET and parallels are drawn. Data are presented to confirm the dc results of the theory over a wide range of implant parameters and demonstrate the value of the model for computer-aided design applications.     

Microwave Characteristics of an Optically Controlled GaAs MESFET

This paper presents the results of an experimental investigation of microwave characteristics of a GaAs MESFET under optically direct-controlled conditions. The gain, drain current, and S-parameters were measured under various optical conditions in the frequency region from 3.0 GHz to 8.0 GHz, and it was found that they can be controlled by varying the incident light intensity in the same manner as when varying the gate bias voltage. As applications of this phenomenon, optical/microwave transformers and an optically switched amplifier were investigated.     

Optically-controlled ion-implanted GaAs MESFET characteristic withopaque 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     

Optically frequency modulated GaAs MESFET oscillator

A GaAs MESFET X-band oscillator has been frequency modulated by an amplitude modulated optical signal injected on the active device. A modulation index of 0.5 corresponding to MSK modulation has been achieved at 2 Mbit/s without parasitic amplitude modulation. It is shown that the FM modulation index variation against frequency is directly related to the drain current response of the FET considered as a photodetector. This responsivity has been measured to decrease above 5 MHz, which limits the oscillator FM capability.     

Effect of radiation and surface recombination on thecharacteristics 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     

Complementary silicon MESFET technology

Prototype complementary silicon MESFET inverters and rign oscillators were fabricated. N-channel transistors have platinum gates and erbium source and drain contacts, while for the p-channel devices the roles of the two metals are reversed. Silicon-on-sapphire substrates were used to provide good device isolation and realisation of normally-off operation.     

AC characteristics of optically controlled MESFET (OPFET)

A perturbation technique is used to develop a more accurate model of the time varying characteristics of the optically illuminated ion-implanted metal semiconductor field-effect transistor (MESFET) OPFET. In this model, the carrier life time is considered as a function of the carrier concentration in the channel. The carrier concentration in the active channel is altered due to the absorption of the incident light, which affects the carrier life time. The influences of this effect on the device parameters and characteristics is studied and analyzed. Based on this model the current voltage characteristics, the gate-source capacitance, and the threshold voltage of the OPFET have been evaluated for different incident light power intensities. This model shows comparable results to other reported models     

The gettering of boron by an ion-implanted antimony layer in silicon

Secondary ion mass spectrometry has been employed to reveal the gettering of implanted B by an annealed, implanted Sb layer. It is shown that the gettering of B is significant, and may be caused by electric-field-enhanced diffusion of the B as well as by solubility enhancement of the electrically-active Sb. These results emphasize the first-order importance of cooperative effects between donors and acceptors in diffusion profile calculations.     

High-efficiency ion-implanted silicon solar cells

The development of solar cells with AM1 coversion efficiency of 18 percent is reported. The cells comprise an n⁺-p-p⁺structure fabricated from float zone silicon having resistivity of 0.3 Ω . cm. The n⁺and p⁺regions are formed by low energy ion implantation and thermal annealing. An important feature of cell fabrication is the growth of SiO2passivation for reduction of surface recombination velocity. Details of both cell fabrication and testing are reported.     

Electron-beam annealing of ion-implanted silicon

A scanning-electron-beam zapping system for the annealing of ion-implanted semiconductors is described. Experiments have been conducted on silicon implanted with various doses of arsenic and boron. Results show that for a given beam power full annealing is achieved above a threshold exposure.     

Calculations of high-speed performance for submicrometer ion-implanted GaAs MESFET devices

Interest in high-speed logic devices has motivated a study of various device geometries in order to potentially improve the cutoff frequency of GaAs MESFET devices. In this paper, a two-dimensional model is used as a tool to investigate the effect of modifications of device dimensions on device performance. Modifying the device geometries by reducing the spacing between the gate and the contacts was found to improve the cutoff frequency slightly. Calculations are also presented for a device scaled to a channel length of 0.2 µm, where an fTof about 60 GHz is predicted.     

The effects of annealing on the switching characteristics of anion-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     

Excess noise measurements in ion-implanted silicon resistors

The low-frequency noise spectra of partially annealed boron-implanted silicon resistors with various geometries are measured. The implantation energies are 50, 80 and 110 keV and the doses are 2·5 × 10¹² cm^?2, 1·0 × 10¹³ cm^?2 and 1·0 × 10¹⁴ cm^?2. The spectra exhibit thermal noise and $\text{?}{}^{\mathrm{?n}}$ (excess) noise exclusively. Investigations indicate that the contracts from the implanted layer to the electrode generally contribute small amounts to the total excess noise observed. The excess noise exhibits a strong dependence on the sheet resistance of the layers, while the dependence on substrate bias, implantation energy, and on temperature is relatively weak. A discussion of the results is given in terms of a volume effect. Noise measurements on implanted layers, produced under carefully controlled conditions, show promise as a tool to investigate excess noise.     

A p-channel MESFET on silicon using an erbium gate

A p-channel MESFET (metal semiconductor field-effect transistor) has been fabricated using erbium as gate material and iridium as source and drain contacts. The results show that it is possible to achieve p-type devices with characteristics comparable to n-type devices. As substrate silicon on sapphire (SOS) was used since it gives a well-defined channel thickness. The thickness of the silicon was 0.6 μm; after processing this was reduced to about 0.5 μm, which was, thus, the ultimate channel thickness.     

High-speed low-power silicon MESFET parallel multipliers

The design of fast low-power silicon LSI MESFET parallel multipliers is studied. The architecture of the multipliers and the designs of the functional blocks are discussed. The overall performance of the multipliers is estimated from the simulated performances of the functional blocks and from system simulations with a logic simulator. The actual performance of 8/spl times/8 and 10/spl times/10 bit TTL-compatible multipliers, fabricated with a 2.5 /spl mu/m silicon MESFET technology (1.5-2 /spl mu/m effective dimensions) is compared to the simulations.     

Half-micrometer gate-length ion-implanted GaAs MESFET with 0.8-dBnoise figure at 16 GHz

Ion-implanted GaAs MESFETs with half-micrometer gate length have been fabricated on 3-in-diameter GaAs substrates. At 16 GHz, a minimum noise figure of 0.8 dB with an associated gain of 6.3 dB has been measured. This noise figure is believed to be the lowest ever reported for 0.5- and 0.25-μm ion-implanted MESFETs, and is comparable to that for 0.25-μm HEMTs at this frequency. By using the Fukui equation and the fitted equivalent circuit model, a K_f factor of 1.4 has been obtained. These results clearly demonstrate the potential of ion-implanted MESFET technology for K-band low-noise integrated circuit applications     

Enhanced microwave performance of ion-implanted MESFET with graded GaAs/AlGaAs heterojunctions

Ion-implanted MESFETs have been fabricated on an inverted GaAs/AlGaAs heterostructure. The aluminium concentration in the AlGaAs is graded from 0% at the substrate to 30% at the heterointerface. A maximum extrinsic transconductance of 410 mS/mm is achieved with 0.5 mu m gate devices. This heterojunction ion-implanted FET (HIFET) also exhibits enhanced microwave performance, especially at low drain current, when compared to conventional ion-implanted GaAs MESFETs. At 20% of I/sub dss/, the current gain cutoff frequency f/sub t/ is 40 GHz, which increases up to a maximum value of 47 GHz as the drain current rises. These characteristics of high f/sub t/ and high gain at low current are advantageous for low-noise applications.<>     

Optically controlled contradirectional coupler

A multiple-quantum-well optically controlled contradirectional coupler has been realized following the design criteria discussed here and elucidated by the simulations. Our room-temperature experiments show a power-dependent contradirectional coupling condition, allowing optically controlled switching between two output channels with control energies on the order of 1 pJ, and at a wavelength around 1.55 μm. The device is treated theoretically by a coupled mode analysis in order to compute the linear and nonlinear transmission spectra at the two output ports. The effects induced by the propagation of intense control beams are modeled using a nonlinear two-dimensional beam propagation method