Md. Shofiqul Islam

Realization of in situ doped n-type and p-type Si-microprobe array by selective vapor-liquid-solid (VLS) growth method

In this paper we report the development of n-type and p-type Si-microprobe arrays fabricated by using in situ doping in vapor-liquid-solid (VLS) growth employing a gas-source molecular beam epitaxy (GS-MBE) system as the growth environment. VLS growth using Si2H6 only gives intrinsic Si microprobes with the resistivity of ~104 Ω-cm, which decreases to ~10-2 Ω-cm after phosphorous diffusion at 1100°C. However, by incorporating in situ doping into the VLS growth method, more conductive probes (resistivity ~10-3 Ω-cm) can be realized at a temperature less than 700°C. The site and diameter of the VLS-grown probe can be controlled and the growth rate is higher than that of a poly-Si or epitaxial Si crystal grown by the vapor-solid (VS) method. Due to the low processing temperature, in situ doping is effective for realizing highly conductive probe arrays with smart sensor devices by a standard IC process followed by VLS growth. The wide range of doping leads to the possibility of using these probes for the fabrication of vertical active devices such as diodes and transistors.

An improved analytical model of current in tunnel field effect transistor

In this paper gate on source tunnel field effect transistor structure has been considered which is an unconventional one. In analytical derivation of current, impact of field-induced quantum confinement under the gate has been successfully incorporated which has resulted in a significant modification in tunneling current as compared to semiclassical models. The model has also used a more accurate potential term at the Si/HfO2(Source/Gate oxide) interface in the source through some rigorous calculation of electron concentration in both strong and weak inversion regimes. Exponential integral function is introduced for the derivation of the analytical formula of the band to band tunneling current. The obtained analytical expressions are compared with results from the simulator and good agreement is found.

A compact analytical model of band-to-band tunneling in a nanoscale p-i-n diode

A semiclassical model of band-to-band tunneling current in a reverse biased silicon p-i-n diode is presented in this paper. Due to the absence of a simple analytical model for the nanoscale reverse biased p-i-n diode, the working principle is generally not well understood. Using Kanes model, an analytical expression for the current through the nanoscale p-i-n diode in reverse bias is derived. Device simulation has been performed using SILVACO ATLAS. The obtained analytical expressions are compared with results from the simulator and good agreement at different reverse voltages is found.

Properties of a pn junction developed with a Si microprobe by vapour-liquid-solid growth using in situ doping

This paper reports the development of a pn junction by growing n-type Si-microprobe arrays on a p-layered Si (1 1 1) substrate by Au-catalyzed vapour–liquid–solid (VLS) growth at 700 °C using the mixed gas of Si2H6 and PH3. The diameter of the probes was selectively controlled in the range of 1–5 µm by patterning Au dots with the desired size and thickness, whereas the length is dictated by the growth conditions and time. The growth rate, impurity concentration and resistivity of these probes were observed to vary with the gas ratio of PH3 to Si2H6. The pn junction formed at the interface of the n-probe and p-layer of the substrate exhibits standard diode characteristics with the built-in potential around 0.7 V and the reverse breakdown at about −11 V. This needle-like pn junction array may be applied as insertion electrodes, photodetectors, photosensitive emitter array devices, etc. Also low-temperature realization of a Si microprobe would be compatible to process this junction array with on-chip circuitry to develop smart chips for sensor applications.

Conduction band-valence band coupling effects on the band structure of In 0.28 Ga 0.72 N/GaN Quantum Well

A quantitative measure of how much the inclusion of the conduction band-valence band coupling effects influence the band structure was obtained. The numerical results of two formalisms were derived using Finite Difference Method (FDM) where one formalism ignores the coupling effects between conduction and valence bands. It was found that the conduction band- valence band coupling effects significantly affects the band structure of typical InGaN Quantum Wells (QW) especially the conduction subbands.

Analytical model of spatial distribution of potential along the channel of low band gap GNR-FET at high temperature

In this paper, an analytical model is developed for potential along the channel of low band gap graphene nanoribbon field effect transistor (GNR-FET) at high temperature. For the derivation of equation of potential, non-locality approximation of poissons equation has been used. The spatial variation of potential along the channel has been investigated with the variation of top gate voltage, drain voltage, top gate length and temperature. Since graphene nanoribbon with higher width has very narrow band-gap, due to inverse relationship between ribbon width and band-gap, this analytical model would be applicable for GNR with larger width.

Analytical modelling of current in higher width graphene nanoribbon field effect transistor

An analytical model for drain-source current of higher width graphene-nanoribbon field-effect-transistor (GNR-FET) with back and top gates is developed. This analytical model is based on the two-dimensional Poissons equation in the weak nonlocality approximation. Analytical formula for drain-source current is derived in terms of device parameters and applied voltages. Dependence of current on applied voltages is investigated. It is shown that drain-source current is controlled by applied voltages hence has applications in digital and analog circuits.

Off current modeling of a tunnel field effect transistor

A tunnel field effect transistor (TFET) is characterized specially by its low off current and high on current to off current ratio. For a nanoscale TFET, the off current is primarily band to band tunneling current. Off current expression in TFET is derived in this paper and device simulation has been performed using SILVACO ATLAS. The analytical expression matches closely with the simulation results for different drain voltages with zero gate voltage.

Design, fabrication and performance evaluation of a piezoresistive arsenic sensor

In this paper the design, analysis and testing of a cantilever based piezoresistive arsenic sensor has been described. The fabrication on a single crystal silicon substrate utilizes the technologies of anisotropic chemical etching. The arsenic sensor, which is of resistive type, changes its resistance when the sensing cantilever adsorbs arsenic on its surface coated with arsenic adsorbent. Using the COMSOL software, a model of the structure was established and the stress distribution of the cantilever was analyzed. The results show that the sensor has a high linearity and high sensitivity of 0.76mV/ppm at room temperature.

Peak emission wavelength tuning for Light Emitting Diodes and lasers for InGaN — deltaIn y Ga 1−y N Quantum Well by varying the composition of the delta well

Four InGaN-delta InyGa1-yN Quantum Wells (QW) with four different values of y (1, 0.9, 0.8, 0.7) were investigated through the numerical solution of a k.p Hamiltonian with the Finite Difference Method (FDM). The spontaneous emission spectra and optical gain spectra of the four structures have been found to have a definite pattern where increasing the value of y gradually shifted the spectra toward the higher energy regions. The electron-hole ground state overlaps were not found to change by a great amount. Increasing y brought about a predictable shift of the spectra toward the higher energy regions at the expense of electron-hole wave function overlap.