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The influence of passivation by silicon nitride (\SiNx) of different stress states (compressive, neutral, and tensile) is presented as a function of varying Al mole fraction (x). All types of \SiNx passivant induced, as expected, an increase in 2-D electron gas (2DEG) concentration. In addition, however, the 2DEG mobility increased after passivation for the low- x (0.15) sample, and the more tensile the film stress is, the greater the relative increase. This led to a very highly measured 2DEG mobility of 2360 \cm^2\V^-1\s^-1 at 300 K. In higher x samples, however, mobility was decreased after passivation, increasingly so as x increased, and to a varying extent with different stresses. It is apparent from the results that there is a complex relationship between the stress in the \SiNx layer, the mole fraction, and the transport properties of the 2DEG. Thus, tailoring of the passivant deposition conditions, and not simply passivant dielectric choice, to optimize transport properties, is critical for a given passivant, deposition tool, and Al mole fraction, to maximize device performance. [ABSTRACT FROM AUTHOR]

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface. [ABSTRACT FROM AUTHOR]

We describe the pH dependence of the electron mobility as a function of various structural and electrolyte factors for unfunctionalised AlGaN/GaN-based chemical sensors. The charge distribution across the sensor-electrolyte system is calculated self-consistently and the dependence of the Hall mobility of the two-dimensional electron gas on different scattering mechanisms is studied. Our calculations show that the pH of the electrolyte has a significant effect on the mobility and consequently the conductivity of the 2DEG channel electrons. A wide range of structures is examined to describe the correlation between the sensitivity and channel electron density. It is shown that the change of mobility with pH is a strong function of the structure of the sensor (AlGaN, GaN-cap and oxide-layer thicknesses), while it is almost independent of the ionic strength of the electrolyte. The observed dependence of the Hall-mobility to the thickness of AlGaN-barrier, GaN-cap and oxide layer can explain the discrepancy in the behaviour of the Hall-mobility, as reported by different groups. [ABSTRACT FROM AUTHOR]

Abstract: AlGaN/GaN heterostructure-based devices can be engineered through heterostructure design to have a high transconductance near zero gate–drain voltage, potentially enabling high sensitivity, reference electrode free, ion sensing. As a proof of concept, these devices were coated with a PVC-based membrane containing a plasticizer and an ionophore to detect nitrate ions in solution. The sensor response is measured as a change in conductivity across two contacts using a Kelvin probe (or four-contact) geometry, with the current between the two outer contacts kept constant. We show that this sensor for nitrate is sensitive and stable with a rapid response time (i.e. less than 60s). The detection limit remains consistently low over multiple runs/days. In a 0.1M KH2PO4 ion buffer, a detection limit of less than 1×10−6 M and a linear response range of 10−6–10−3 M were achieved. Furthermore, detection limits of approximately 10−6 M and 10−4 M in 0.1M K2SO4 and 0.1M KCl ion buffers, respectively, were demonstrated. In a 0.1M KH2PO4 ion buffer, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11. As a control, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to nitrate ion exposure. Furthermore, using transistor theory, we show that the apparent gate response is near-Nenrstian under a variety of conditions. The success of this study paves the way for extending this technology to selectively sensing multiple ions in water through incorporation of the appropriate polymer based membranes on arrays of devices. [Copyright &y& Elsevier]

Abstract: We report on the effect of implantation angle on contact resistance of non-alloyed ohmic contacts to selectively implanted source/drain regions in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures. Three different components of contact resistance are observed for such contacts: (i) contact resistance between the metal and the semiconductor, (ii) resistance of the implanted region and (iii) an additional resistance attributed to a transition region between implanted and non-implanted region. This third component varies strongly with implantation angle. The variation with implantation angle shows that the ratio of lateral implantation damage to penetration depth is critical for implantation of AlGaN/GaN HEMT source/drain contact regions. Our results also show that increasing the implantation angle in combination with reducing the implantation width can reduce contact resistance. [ABSTRACT FROM AUTHOR]