D. Landheer

Study of the electrolyte-insulator-semiconductor field-effect transistor (EISFET) with applications in biosensor design.

This paper presents a comprehensive review of the ion-sensitive field-effect transistor (ISFET) and its applications in biomolecular sensing and characterization of electrochemical interfaces. An introduction to the physics of field-effect transistors is presented, followed by a study of the properties of electrolytic solutions and electrolyte interface surface effects. Full modeling of the ion-sensitive transistor is given, followed by a survey of the different uses of the ISFET in biomedical and environmental applications. Particular attention is given to the use of the ion-sensitive transistors as replacements for microarrays in DNA gene expression analysis.

Microfabricated Reference Electrodes and their Biosensing Applications

Over the past two decades, there has been an increasing trend towards miniaturization of both biological and chemical sensors and their integration with miniaturized sample pre-processing and analysis systems. These miniaturized lab-on-chip devices have several functional advantages including low cost, their ability to analyze smaller samples, faster analysis time, suitability for automation, and increased reliability and repeatability. Electrical based sensing methods that transduce biological or chemical signals into the electrical domain are a dominant part of the lab-on-chip devices. A vital part of any electrochemical sensing system is the reference electrode, which is a probe that is capable of measuring the potential on the solution side of an electrochemical interface. Research on miniaturization of this crucial component and analysis of the parameters that affect its performance, stability and lifetime, is sparse. In this paper, we present the basic electrochemistry and thermodynamics of these reference electrodes and illustrate the uses of reference electrodes in electrochemical and biological measurements. Different electrochemical systems that are used as reference electrodes will be presented, and an overview of some contemporary advances in electrode miniaturization and their performance will be provided.

Model for the field effect from layers of biological macromolecules on the gates of metal-oxide-semiconductor transistors

The potential diagram for field-effect transistors used to detect charged biological macromolecules in an electrolyte is presented for the case where an insulating cover layer is used over a conventional eletrolyte-insulator metal-oxide-semiconductor (EIMOS) structure to tether or bind the biological molecules to a floating gate. The layer of macromolecules is modeled using the Poisson-Boltzmann equation for an ion-permeable membrane. Expressions are derived for the charges and potentials in the EIMOS and electrolyte-insulator-semiconductor structures, including the membrane and electrolyte. Exact solutions for the potentials and charges are calculated using numerical algorithms. Simple expressions for the response are presented for low solution potentials when the Donnan potential is approached in the bulk of the membrane. The implications of the model for the small-signal equivalent circuit and the noise analysis of these structures are discussed.

Resonant tunneling in Si/Si1−xGex double‐barrier structures

Resonant tunneling of holes has been observed for the first time in double‐barrier diodes with strained Si1−xGex quantum wells formed between unstrained Si barriers. Negative differential resistance with a peak‐to‐valley ratio in current of 1.8 at 77 K and of 2.2 at 4.2 K has been exhibited by a sample with a 3.3‐nm‐wide Si0.79Ge0.21 well between 6.0 nm Si barriers. The positions of the current peaks are in reasonable agreement with calculations of the positions of heavy‐hole levels in the quantum well.

Gadolinium silicate gate dielectric films with sub-1.5 nm equivalent oxide thickness

GdSixOy gate dielectric films were deposited on Si(001) substrates using ultra-high-vacuum electron-beam evaporation from pressed-powder targets. Transmission electron microscopy showed that the films were amorphous as deposited and remained amorphous when annealed to temperatures up to 900 °C. Capacitance–voltage measurements indicate an equivalent oxide thickness (EOT) of 13.4 A for a film with composition GdSi0.56O2.59 determined by in situ x-ray photoelectron emission spectroscopy. After forming gas annealing at 500 °C the EOT was reduced to 11.0 A, at a physical thickness of 45 A. The same film has a low leakage current of approximately 5.7×10−3 A cm−2 at +1 V, a reduction of 8.7×104 compared to current density estimates of SiO2 films with the same specific capacitance.

Noise considerations in field-effect biosensors

Field-effect sensors used to detect and identify biological species have been proposed as alternatives to other methods such as fluorescence deoxyribonucleic acid (DNA) microarrays. Sensors fabricated using commercial complementary metal-oxide-semiconductor technology would enable low-cost and highly integrated biological detection systems. In this paper, the small-signal and noise modeling of biosensors implemented with electrolyte-insulator-semiconductor structures is studied, with emphasis on design guidelines for low-noise performance. In doing so, a modified form of the general charge sheet metal-oxide-semiconductor field-effect transistor model that better fits the electrolyte-insulator-semiconductor structure is used. It is discussed how if the reference electrode and the insulator-electrolyte generate no noise associated with charge transport, then the main noise mechanisms are the resistive losses of the electrolyte and the low-frequency noise of the field-effect transistor. It is also found that...

Characterization of Gd2 O 3 Films Deposited on Si(100) by Electron-Beam Evaporation

Gadolinium oxide films were deposited on Si(100) substrates from a rod-fed electron beam evaporator using a pressed-powder Gd 2 O 3 target. Films 25 nm thick were shown to he stoichiometric Gd 2 O 3 by Rutherford backscattering and had a dielectric constant at 100 kHz of 16.0 ± 0.3. Transmission electron microscopy and X-ray reflectivity measurements showed that films 7-13 nm thick annealed in oxygen consisted of three distinct layers, an interfacial silicon dioxide layer next to the substrate, a second amorphous oxide layer containing silicon, gadolinium, and oxygen above this, and a polycrystalline Gd 2 O 3 layer on top. Annealing in oxygen reduced the leakage currents, increased the thickness of the silicon dioxide layer, and increased the grain size of the top Gd 2 O 3 layer. The characteristics of the leakage currents through the gadolinium oxide were consistent with a Frenkel-Poole conduction mechanism with a silicon-Gd 2 O 3 band offset of 1.8 V. Interfaces with excellent electrical properties, Characteristic of good SiO 2 , were obtained after annealing in oxygen Annealing of the films in vacuum prior to oxygen annealing reduced the thickness of the interfacial silicon dioxide.

Tunneling through AlAs barriers: Γ-X transfer current

We have investigated Γ–X intervalley transfer tunneling by measuring currents at low temperature in a series of single AlAs barrier samples grown on GaAs substrates. The transfer occurs at AlAs‐GaAs interfaces and is intrinsic because the translational symmetry is broken in the growth direction of the epitaxial layered structures. Calculated current‐voltage characteristics employing a simple effective mass model which includes Γ–X transfer at heterointerfaces agree reasonably well with experiments, while calculations assuming only the Γ‐band potential predict currents several orders of magnitude lower than experiments.

Effect of circuit oscillations on the dc current‐voltage characteristics of double barrier resonant tunneling structures

The influence of external circuit parameters on the measured current‐voltage behavior of double barrier resonant tunneling structures is studied both experimentally and through modeling. It is demonstrated that characteristic ‘‘plateaulike’’ structures, which often appear in the region of negative differential resistance, can be accounted for in terms of the oscillating nature of the circuit, given that only average currents and voltages are monitored.

Interfacial growth in HfOxNy gate dielectrics deposited using [(C2H5)2N]4Hf with O2 and NO

The interface growth by oxygen diffusion has been investigated for 5 nm thick HfOxNy gate-quality dielectric films deposited on Si(100) by low-pressure pulsed metalorganic chemical vapor deposition. Analysis by x-ray photoelectron spectroscopy of the films deposited using the precursor tetrakis (diethylamido) hafnium with O2 showed that the films contained 4 at. % nitrogen. This increased to 11 at. % N when NO was used as the oxidant. Significant growth of the interface layer was observed for films exposed to air at ambient temperature and lower rates of growth were observed for vacuum annealed films and those with the higher N content. For films annealed in O2 at temperatures in the range 600–900 °C, the activation energies of the interfacial growth were 0.36 and 0.25 eV for N concentrations of 11 and 4 at. %, respectively. The results were interpreted in terms of atomic oxygen formation in the bulk and reaction at the interface. The increase in N incorporation from 4 to 11 at. % increases the crystalliz...