P. Kordoš

Current Collapse and High-Electric-Field Reliability of Unpassivated GaN/AlGaN/GaN HEMTs

Long-term ON-state and OFF-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN high-electron-mobility transistors on SiC substrates. Because of the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite the fact that they are not passivated. This comes at the price of a relatively high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both ON-state and OFF-state stresses produce simultaneous current-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drain surface degradation and reduced gate electron injection. This study shows that although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation, and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, prepassivation surface treatments, and/or field-plate gate

Miniaturization of potentiometric sensors using porous silicon microtechnology

A new capacitive field-effect microsensor based on a porous EIS (electrolyte-insulator-semiconductor) structure is presented. The porous silicon sensor was prepared using standard techniques of semiconductor processing. A well-defined macroporous layer was formed on silicon by electrochemical etching and a SiO2Si3N4 sandwich was deposited as insulating and pH-sensitive layer. The porous sensor exhibits a high, near-Nernstian pH sensitivity of about 54 mV per decade in the concentration range from pH 4 to pH 8, similar to a planar non-porous EIS structure with the same layer sequence. The enlargement of the active sensor area (surface) due to the porous structure increases the measured capacitance and thus allows a scaling down of the sensor. The preparation of biosensors based on the same structure is demonstrated by immobilization of the enzyme penicillinase as biosensitive component.

AlGaN/GaN HEMTs on (111) silicon substrates

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE. Devices exhibit a saturation current of 0.91 A/mm, a good pinchoff and a peak extrinsic transconductance of 122 mS/mm. A unity current gain frequency of 12.5 GHz and f/sub max//f/sub T/=0.83 were obtained. The highest saturation current reported so far, static output characteristics of up to 20 V and breakdown voltage at pinchoff higher than 40 V demonstrate that the devices are capable of handling /spl sim/16 W/mm static heat dissipation.

Schottky barrier height enhancement on n‐In0.53Ga0.47As

Schottky barrier height enhancement on n‐InGaAs is studied on structures with thin surface layers of different compositions. Counter‐doped p+‐InGaAs layers, as well as layers of n‐ and p‐InP, n‐GaAs, and n‐InGaP of different thicknesses and dopant densities, respectively, were used to enhance the barrier. Titanium was used as a barrier metal to prepare Schottky diodes of different areas and the barrier height is analyzed by current‐voltage measurements. It is observed that the barrier height enhancement by p+‐InGaAs layers increases with the layer thickness and dopant density, respectively, and effective barrier heights up to 0.63–0.68 eV, i.e., higher values than previously reported, have been measured. The barrier height enhancement by counter‐doped p+‐InGaAs layers on n‐InGaAs can be described by the two‐carrier model. Schottky diodes with extremely low reverse current densities have been prepared, JR(1 V) =4.5×10−6 A/cm2. It is shown that lattice‐matched InP surface layers can be used as an alternativ...

An insect-based BioFET as a bioelectronic nose

Abstract Insects are able to perceive volatiles released by damaged plants in order to find food sources or mating partners. In order to use the highly developed olfactory sense of insects for analytical purposes, the “biological nose” of insects has to be combined with some electronic instrument via a bioelectronic interface to yield a “bioelectronic nose”. In order to combine a field-effect-transistor (FET) with an insect antenna of the Colorado potato beetle ( Leptinotarsa decemlineata ) in an electrically and mechanically stable way, the bioelectronic interface was adapted to the needs of the insect antenna. A mobile biosensor system basing on the pre-adaptation method containing a biologically sensitive field-effect-transistor (BioFET) as sensor head was used for measurements of plant damage in a glasshouse under real world conditions. First measurements with the biosensor showed for ( Z )-3-hexen-1-ol, a marker volatile for plant damage, a dynamic range of 6 orders of magnitude, a threshold of quantification of about 1 ppbv, a limit of detection (i.e., signal-to-noise ratio>3) of about 0.1 ppbv, and is able to detect down to 300 ag ( Z )-3-hexen-1-ol/ml air in a 500 ml sample within a few seconds (ca. 50 ms raise time to 90% signal amplitude, ca. 5 s adaptation time after signal maximum). These characteristics were sufficient to distinguish single mechanically or beetle-damaged plants in background emissions of 1000 undamaged plants in the glasshouse.

A highly long-term stable silicon-based pH sensor fabricated by pulsed laser deposition technique

Abstract A highly long-term stable pH sensor based on a capacitive electrolyte insulator semiconductor heterostructure has been developed. The pH-sensitive gate insulator material Al2O3 has been deposited by means of the pulsed laser deposition technique. The basic characteristics of the sensor device, such as pH sensitivity, stability, selectivity, sensor drift and response time have been investigated using capacitance/voltage measurements. Furthermore the physical structure and the stoichiometric composition of the deposited Al2O3 layers have been studied by Rutherford backscattering spectrometry, ion channeling and transmission electron microscopy. According to the results obtained during a measurement period of more than 600 days, the sensor possesses a high pH sensitivity of about 56 mV/pH including a small baseline drift of the sensor signal of less than 1 mV per day.

Effect of surface passivation on performance of AlGaN/GaN/Si HEMTs

Abstract Performance of intentionally undoped and doped AlGaN/GaN/Si high electron mobility transistors (HEMTs) before and after passivation with SiO 2 and Si 3 N 4 is investigated. Hall effect measurements show higher impact of Si 3 N 4 than SiO 2 passivation on the carrier concentration increase in the channel. Improvements in DC performance of HEMTs after passivation with SiO 2 and Si 3 N 4 correspond to the changes in sheet carrier concentration. Small signal microwave characterisation shows a decrease (from 18.6 to 9 GHz) and an increase (from 18.4 to 28.8 GHz) of the current gain cut off frequency after SiO 2 and Si 3 N 4 passivation, respectively. Similar effect of passivation is found in microwave power changes––only about a half of the power is obtained after SiO 2 passivation but more than doubled power results from Si 3 N 4 passivation, measured at 2 GHz. Higher density of interface states for SiO 2 than Si 3 N 4 passivation is supposed to be responsible for these effects. However, for an optimal design of GaN-based power devices additional studies related to the interface between a passivation layer and GaN are needed.

Enzyme immobilisation on planar and porous silicon substrates for biosensor applications

Abstract Two methods for the immobilisation of enzymes on silicon-based so-called electrolyte–insulator–semiconductor (EIS) structures are suggested. These EIS structures are used as a basis for potentiometric biosensors. In the first method, heterobifunctional cross-linker molecules are employed to covalently bind enzymes to these capacitive layer structures which possess a planar surface that contains amine groups. Porous EIS sensors which, in comparison to planar sensors, exhibit an enlarged surface area, are used in the second method. For the first time, pH-sensitive Si3N4 was deposited on the walls and bottoms of the SiO2-covered pores. Here, a large amount of enzyme molecules can adsorptively be bound inside the porous structure. Penicillinase is used as a model enzyme. Capacitance–Voltage and Constant Capacitance measurements are performed in order to examine the respective penicillin sensor responses and thus to validate both immobilisation methods. Whereas the sensitivity of the sensors prepared by both methods is nearly identical for low penicillin concentrations up to around 0.25 mM, a difference of the calibration curves in the higher concentration range indicates a larger amount of immobilised enzyme in the case of the porous structures.

A BioFET on the basis of intact insect antennae

Abstract A novel biosensor device on the basis of a FET (field-effect transistor)–insect antenna junction is presented. This biosensor allows the quantitative detection of specific components of host plant odours. The sensor was operated at a fixed working point in the constant voltage mode. As a test odour the gas concentration of Z-3-hexen-1-ol was determined in the concentration range from 0.01 to 100 ppm with short response time of less than 1 s and a high reversibility of the sensor signal in ambient air. Two types of preparation of the biocomponent were examined yielding a whole-beetle-BioFET and an isolated-antenna-BioFET.

A novel silicon-based sensor array with capacitive EIS structures

Abstract A modified C / V (capacitance/voltage) measuring set-up for the synchronous operation of several EIS (electrolyte–insulator–semiconductor) sensors was developed. Therewith, the simultaneous detection of different substances in aqueous analytes is possible by using capacitive sensors that can easily be manufactured. This novel method is exemplarily demonstrated for a sensor array based on a pH-sensitive EIS structure (Al/Si/SiO 2 /Si 3 N 4 ) and a penicillin-sensitive EIS structure (Al/Si/SiO 2 /Si 3 N 4 /penicillinase). Both sensors are connected in parallel with each other. By using two additional d.c. voltage sources the single C / V curves are shifted against each other along the voltage axis. The resulting `step-like C / V curve allows the distinct assignment of each step fraction to the corresponding sensor, and thus the simultaneous detection of the pH and penicillin concentration in a single measurement.