Maneea Eizadi Sharifabad

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.

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.

Fabrication and Characterization of Liquid‐Phase Sensor utilizing GaN‐Based Two Terminal Devices

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 o...