Mastura Shafinaz Zainal Abidin

Open-Gated pH Sensor Fabricated on an Undoped-AlGaN/GaN HEMT Structure

The sensing responses in aqueous solution of an open-gated pH sensor fabricated on an AlGaN/GaN high-electron-mobility-transistor (HEMT) structure are investigated. Under air-exposed ambient conditions, the open-gated undoped AlGaN/GaN HEMT only shows the presence of a linear current region. This seems to show that very low Fermi level pinning by surface states exists in the undoped AlGaN/GaN sample. In aqueous solution, typical current-voltage (I-V) characteristics with reasonably good gate controllability are observed, showing that the potential of the AlGaN surface at the open-gated area is effectively controlled via aqueous solution by the Ag/AgCl gate electrode. The open-gated undoped AlGaN/GaN HEMT structure is capable of distinguishing pH level in aqueous electrolytes and exhibits linear sensitivity, where high sensitivity of 1.9 mA/pH or 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V is obtained. Due to the large leakage current where it increases with the negative gate voltage, Nernstian like sensitivity cannot be determined as commonly reported in the literature. This large leakage current may be caused by the technical factors rather than any characteristics of the devices. Surprisingly, although there are some imperfections in the device preparation and measurement, the fabricated devices work very well in distinguishing the pH levels. Suppression of current leakage by improving the device preparation is likely needed to improve the device performance. The fabricated device is expected to be suitable for pH sensing applications.

Concentration dependence of drift and magnetoresistance ballistic mobility in a scaled-down metal-oxide semiconductor field-effect transistor

The degradation of ballistic mobility in a metal-oxide semiconductor field-effect transistor is attributed to the nonstationary ballistic injection from the contacts as the length of a channel shrinks to the length smaller than the scattering-limited mean free path. Apparent contradiction between the rise of magnetoresistance mobility and fall of drift mobility with increasing channel concentration is attributed to scattering-dependent magnetoresistance factor. The ballistic mean free path of injected carriers is found to be substantially higher than the long-channel drift mean free path. Excellent agreement with the experimental data on length-limited ballistic mobility is obtained.

Crystallization of Electrodeposited Germanium Thin Film on Silicon (100)

We report the crystallization of electrodeposited germanium (Ge) thin films on n-silicon (Si) (100) by rapid melting process. The electrodeposition was carried out in germanium (IV) chloride: propylene glycol (GeCl4:C3H8O2) electrolyte with constant current of 50 mA for 30 min. The measured Raman spectra and electron backscattering diffraction (EBSD) images show that the as-deposited Ge thin film was amorphous. The crystallization of deposited Ge was achieved by rapid thermal annealing (RTA) at 980 °C for 1 s. The EBSD images confirm that the orientations of the annealed Ge are similar to that of the Si substrate. The highly intense peak of Raman spectra at 300 cm−1 corresponding to Ge-Ge vibration mode was observed, indicating good crystal quality of Ge. An additional sub peak near to 390 cm−1 corresponding to the Si-Ge vibration mode was also observed, indicating the Ge-Si mixing at Ge/Si interface. Auger electron spectroscopy (AES) reveals that the intermixing depth was around 60 nm. The calculated Si fraction from Raman spectra was found to be in good agreement with the value estimated from Ge-Si equilibrium phase diagram. The proposed technique is expected to be an effective way to crystallize Ge films for various device applications as well as to create strain at the Ge-Si interface for enhancement of mobility.

Temperature-dependent ballistic transport in a channel with length below the scattering-limited mean free path

The temperature-dependent ballistic transport, using nonequilibrium Arora distribution function (NEADF), is shown to result in mobility degradation with reduction in channel length, in direct contrast to expectation of a collision-free transport. The ballistic mean free path (mfp) is much higher than the scattering-limited long-channel mfp, yet the mobility is amazingly lower. High-field effects, converting stochastic velocity vectors to streamlined ones, are found to be negligible when the applied voltage is less than the critical voltage appropriate for a ballistic mfp, especially at cryogenic temperatures. Excellent agreement with the experimental data on a metal-oxide-semiconductor field-effect transistor is obtained. The applications of NEADF are shown to cover a wide spectrum, covering regimes from the scattering-limited to ballistic, from nondegenerate to degenerate, from nanowire to bulk, from low- to high-temperature, and from a low electric field to an extremely high electric field.

The effects of annealing temperatures on composition and strain in SixGe1-x obtained by melting growth of electrodeposited Ge on Si (100)

The effects of annealing temperatures on composition and strain in SixGe1−x, obtained by rapid melting growth of electrodeposited Ge on Si (100) substrate were investigated. Here, a rapid melting process was performed at temperatures of 1000, 1050 and 1100°C for 1 s. All annealed samples show single crystalline structure in (100) orientation. A significant appearance of Si-Ge vibration mode peak at ~00 cm−1 confirms the existence of Si-Ge intermixing due to out-diffusion of Si into Ge region. On a rapid melting process, Ge melts and reaches the thermal equilibrium in short time. Si at Ge/Si interface begins to dissolve once in contact with the molten Ge to produce Si-Ge intermixing. The Si fraction in Si-Ge intermixing was calculated by taking into account the intensity ratio of Ge-Ge and Si-Ge vibration mode peaks and was found to increase with the annealing temperatures. It is found that the strain turns from tensile to compressive as the annealing temperature increases. The Si fraction dependent thermal expansion coefficient of SixGe1−x is a possible cause to generate such strain behavior. The understanding of compositional and strain characteristics is important in Ge/Si heterostructure as these properties seem to give significant effects in device performance.

Gateless-FET undoped AlGaN/GaN HEMT structure for liquid-phase sensor

A gateless field-effect-transistor (FET) device fabricated on undoped AlGaN/GaN high-electron-mobility-transistor (HEMT) structure is investigated as a liquid-phase sensor. Good gate controllability for typical current-voltage (I-V) characteristics of FET is observed. This result shows that an undoped-AlGaN surface at the open-gate area is effectively controlled by the isolated gate voltage via chemical solution. Stable pH sensing operation in aqueous solution is observed where this device exhibits a high linear sensitivity of 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V. Due to the occurrence of large leakage current, the Nernstians like sensitivity is not observed. It is also found that the device is sensitive to changes in electrostatic boundary conditions of the polar liquids. This indicates that the change in dipole moment in each liquid causes the potential change at AlGaN surface.

Fabrication and Characterization of GaN-Based Two Terminal Devices for Liquid Sensing

Gallium Nitride (GaN) based materials are highly suitable for liquid-phase sensor applications due to their chemical stability and high internal piezoelectric polarization. The sensitivity of GaN surfaces in aqueous solutions and polar liquids has been investigated. For this purpose, two terminal devices fabricated on bulk Si doped-GaN structures and undoped-AlGaN/GaN heterostructures with unpassivated open area are used to measure the responses to the changes of the H+ concentration in aqueous solutions and the dipole moment in polar liquids. The I–V characteristics show that the devices are able to distinguish the variations of pH. It is observed that the drain current decreases linearly with pH for both device structures. Evaluating the sensitivity in aqueous solutions at VDS = 2V, a quite large current change is obtained for both structures. For the response to polar liquids, it is also found that the drain current decreases with the dipole moments. The results indicate that both devices are capable of distinguishing molecules with different dipole moments.

Characterization of liquid-phase sensor utilizing GaN-based two-terminal device

GaN-based HEMT structures are versatile structures that may be used for a variety of sensing applications. Due to their low intrinsic carrier concentrations, wide band gap semiconductor sensors based on GaN can be operated at lower current levels than conventional Si-based devices and offer the capability of detection up to 600 °C [1–5]. The ability of electronic devices fabricated on these materials to operate in high temperature, high power and high flux/energy radiation conditions enable performance enhancements in a wide variety of spacecraft, satellite, homeland defense, mining, automobile, nuclear power, and radar applications.

Fabrication of open gate structure on GaN-based HEMT for pH sensing

In recent years, there has been demonstrated a rapid progress of the AlGaN/GaN system applications in optical and electronic device research due to its unique features. One of unique features of AlGaN/GaN system includes chemically stable where stability of surface is essentially important for liquid-phase sensor applications. Besides, AlGaN/GaN also allows highly sensitive detection of surface phenomena with the presence of a high-density two-dimensional electron gas (2DEG) in the high electron mobility transistor (HEMT) structure [1]. The material itself is found as environment-friendly and bio-friendly. Furthermore, the GaN material allows sensing operations at high temperatures due to large bandgap energies [2]. In future, realization of wireless sensor chips by on-chip co-integration of circuits for sensor signal processing and wireless communications also will be possible with the HFET technology. Regarding to these features, GaN-based HEMT motivates an interesting area to be studied particularly to be used in biochemical sensing applications.

Prototype Development of Paper Based Sensor for Ammonia Detection in Water

The presence of ammonia in water supply can be dangerous to human health. This is because the ammonia not easily detected and the cost of the existing ammonia detector is quite expensive. The existing method of detection and monitoring of ammonia with chemical approach is time consuming, thus, electrical-based is recommended. The sensor device was developed in this project utilizing bio-environmentally friendly material such as paper and graphite. The different type of paper and graphite was used in this project to evaluate the sensitivity and selectivity of sensor. A suitable pattern was defined as the sensing area for detected the presence of ammonia in water. The device was connected with a constant voltage to generate the current flow through electrode that made of graphite and the sample was tested by drop onto the sensing area. As the result, the current changes were detected with presence of ammonia in water. A zinc oxide layer was proposed to be added at the sensing area for sensor enhancement. It is found that the presence of zinc oxide can improve the sensitivity of sensor by faster detecting the sample drop on the sensing area compare to the graphite with shorter time response characteristics.