Effective capacitance in a single-electron transistor
M. A. Laakso, T. Ojanen, T. T. Heikkila
备注：
4 pages, 5 figures In the past few days we have noticed a serious sign error in the theory presented in this preprint, which essentially changes the sign of the capacitance correction. That is, otherwise the physics is as described, but the sign is incorrect. The new version reflects these changes
Starting from the Kubo formula for conductance, we calculate the frequency-dependent response of a single-electron transistor (SET) driven by an ac signal. Treating tunneling processes within the lowest order approximation, valid for a wide range of parameters, we discover a finite reactive part even under Coulomb blockade due to virtual processes. At low frequencies this can be described by an effective capacitance. This effect can be probed with microwave reflection measurements in radio-frequency (rf) SET provided that the capacitance of the surroundings does not completely mask that of the SET.
In 2008, Salahuddin and Datta proposed that a ferroelectric material operating in the negative capacitance region could act as a step-up converter of the surface potential in a MOS structure, opening a new route for the realization of transistors with steeper subthreshold characteristics (S<60 mV/decade). In this letter, a comprehensive physics-based surface potential and drain current model for the negative capacitance field-effect transistor is reported. The model is aimed to evaluate the potentiality of such transistors for low-power switching applications. Moreover it provides a model core…
Negative capacitance (NC) in ferroelectrics, which stems from the imperfect screening of polarization, is considered a viable approach to lower voltage operation in the field-effect transistors (FETs) used in logic switches. In this paper, we discuss the implications of the transient nature of negative capacitance for its practical application. It is suggested that the NC effect needs to be characterized at the proper time scale to identify the type of circuits where functional NC-FETs can be used effectively.
We have incorporated an aluminum single electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single electron capacitance peaks.
I present a compact physics-based model of the drain current, charge and capacitance of graphene field-effect transistors, of relevance for exploration of DC, AC and transient behavior of graphene based circuits. The physical framework is a field-effect model and drift-diffusion carrier transport incorporating saturation velocity effects. First, an explicit model has been derived for the drain current. Using it as a basis, explicit closed-form expressions for the charge and capacitances based on the Ward-Dutton partition scheme, covering continuosly all operation regions. The model is of speci…
A small-signal equivalent circuit of 2D-material based field-effect transistors is presented. Charge conservation and non-reciprocal capacitances have been assumed so the model can be used to make reliable predictions at both device and circuit levels. In this context, explicit and exact analytical expressions of the main radio-frequency figures of merit of these devices are given. Moreover, a direct parameter extraction methodology is provided based on S-parameter measurements. In addition to the intrinsic capacitances, transconductance and output conductance, our approach allows extracting t…
We study theoretically an influence of the near-interfacial insulator traps and electron-hole puddles on the small-signal capacitance and conductance characteristics of the gated graphene structures. Based on the self-consistent electrostatic consideration and taking into account the interface trap capacitance the explicit analytic expressions for charge carrier density and the quantum capacitance as functions of the gate voltage were obtained. This allows to extract the interface trap capacitance and density of interface states from the gate capacitance measurements. It has shown that self-co…
The analytical model of the small-signal current and capacitance characteristics of RF graphene FET is presented. The model is based on explicit distributions of chemical potential in graphene channels (including ambipolar conductivity at high source-drain bias) obtained in the framework of drift-diffusion current continuity equation solution. Small-signal transconductance and output conductance characteristics are modeled taking into account the two modes of drain current saturation including drift velocity saturation or electrostatic pinch-off. Analytical closed expression for the complex cu…
We present a circuit-compatible compact model of the intrinsic capacitances of graphene field-effect transistors (GFETs). Together with a compact drain current model, a large-signal model of GFETs is developed combining both models as a tool for simulating the electrical behavior of graphene-based integrated circuits, dealing with the DC, transient behavior, and frequency response of the circuit. The drain current model is based in a drift-diffusion mechanism for the carrier transport coupled with an appropriate field-effect approach. The intrinsic capacitance model consists of a 16-capacitanc…
Integrating ferroelectric negative capacitance (NC) into the field-effect transistor (FET) promises to break fundamental limits of power dissipation known as Boltzmann tyranny. However, realizing the stable static negative capacitance in the non-transient non-hysteretic regime remains a daunting task. The problem stems from the lack of understanding of how the fundamental origin of the NC due to the emergence of the domain state can be put in use for implementing the NC FET. Here we put forth an ingenious design for the ferroelectric domain-based field-effect transistor with the stable reversi…
An evaluation of the gate capacitance of a field-effect transitor (FET) whose channel length and width are several ten nanometer, is a key point for sensors applications. However, experimental and precise evaluation of capacitance in the aF range or less has been extremely difficult. Here, we report an extraction of the capacitance down to 0.55 aF for a silicon FET with a nanoscale wire channel whose width and length are 15 and 50 nm, respectively. The extraction can be achieved by using a combination of four kinds of measurements: current characteristics modulated by double gates, random-tele…
Atomistic density functional theory (DFT) calculations of the capacitance between a metallic cylindric gate and a carbon nanotube (CNT) are reported. Results stressing the predominant effect of quantum capacitance in limiting or even enhancing screening properties of the CNT are shown. Other contributions to the quantum capacitance beyond the electronic density of state (DOS) are pointed out. Negative values of the quantum capacitance are obtained for low-density systems, which correspondingly over-screen the gate field. This unconventional behavior of the quantum capacitance is related to the…
A Metal-dielectric-topological insulator capacitor device based on hBN-encapsulated CVD grown Bi2Se3 is realized and investigated in the radio frequency regime. The RF quantum capacitance and device resistance are extracted for frequencies as a high as 10 GHz, and studied as a function of the applied gate voltage. The combination of the superior quality hBN dielectric gate with the optimized transport characteristics of CVD grown Bi2Se3 (n~10^18cm-3 in 8 nm) allow us to attain a bulk depleted regime by dielectric gating. A quantum capacitance minimum is observed revealing a purely Dirac regime…
We have used a superconducting single-electron transistor as a DC-electrometer that is strongly coupled to the metal island of another transistor. With this set-up, it is possible to directly measure the charge distribution on this island. The strong capacitive coupling was achieved by a multilayer fabrication technique that allowed us to make the coupling capacitance bigger than the junction capacitances. Simulations of this system were done using orthodox theory of single-electron tunnelling and showed excellent agreement with the measurements.
We have decomposed the modeling of the field-effect transistors into the two independent parts: the current continuity based kinetics and the charge neutrality based electrostatics. The former part, that is universal for all FETs, leads to an explicit and closed form of I-V characteristics as a function of the total channel charge. The latter part, which is specific for a particular material and geometric configurations can be considered as an independent engineering task. The quantum capacitance and the interface trap density are consistently incorporated into the solution of the current cont…
We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm of about 20 microSiemens is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/micrometer is typical of top gates on a conventional oxide with a dielectric constant equal to 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according t…
The field-effect mobility of graphene devices is discussed. We argue that the graphene ballistic mean free path can only be extracted by taking into account both, the electrical characteristics and the channel length dependent mobility. In doing so we find a ballistic mean free path of 300nm at room-temperature for a carrier concentration of ~1e12/cm2 and that a substantial series resistance of around 300ohmum has to be taken into account. Furthermore, we demonstrate first quantum capacitance measurements on single-layer graphene devices.
Physical phenomena underlying operation of ferroelectric field-effect transistors (FeFETs) is treated within a unified simulation framework. The framework incorporates the Landau mean-field treatment of free energy of a ferroelectric and the polarization dynamics according to Landau-Khalatnikov (LK) equation. These equations are self-consistently solved with the one-dimensional metal-oxide-semiconductor (MOS) structure electrostatics and the drift-diffusion solution for the current in the semiconductor channel. Numerical simulations demonstrate, depending on the ferroelectric (FE) thickness, b…
This work explores the consequences of introducing a piezoelectric gate barrier in a normal field-effect transistor. Because of the positive feedback of strain and piezoelectric charge, internal charge amplification occurs in such an electromechanical capacitor resulting in a negative capacitance. The first consequence of this amplification is a boost in the on-current of the transistor. As a second consequence, employing the Lagrangian method, we find that by using the negative capacitance of a highly compliant piezoelectric barrier, one can potentially reduce the subthreshold slope of a tran…
We incorporate an Al-AlO_x-Al single-electron transistor as the gate of a narrow (~100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET). Near the MOSFET channel conductance threshold, we observe oscillations in the conductance associated with Coulomb blockade in the channel, revealing the formation of a Si single-electron transistor. Abrupt steps present in sweeps of the Al transistor conductance versus gate voltage are correlated with single-electron charging events in the Si transistor, and vice versa. Analysis of these correlations using a simple electrostatic model demonstrat…
We presents a data-calibrated compact model of carbon nanotube (CNT) field-effect transistors (CNFETs) based on the virtual-source (VS) approach, describing the intrinsic current-voltage and charge-voltage characteristics. The features of the model include: (i) carrier VS velocity extracted from experimental devices with gate lengths down to 15 nm; (ii) carrier effective mobility and velocity depending on the CNT diameter; (iii) short channel effect such as inverse subthreshold slope degradation and drain-induced barrier lowering depending on the device dimensions; (iv) small-signal capacitanc…
When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance ($C_\textrm{G}$) becomes smaller than the oxide capacitance ($C_\textrm{ox}$) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS$_\textrm{2}$ as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS$_\textrm{2}$ capacitance is smaller than its quantum capacitance, limiting the single-gated $C_\text…
We have fabricated a Cooper-pair transistor (CPT) with parameters such that for appropriate voltage biases, the sub-gap charge transport takes place via slow tunneling of quasiparticles that link two Josephson-coupled charge manifolds. In between the quasiparticle tunneling events, the CPT behaves essentially like a single Cooper-pair box (SCB). The effective capacitance of a SCB can be defined as the derivative of the induced charge with respect to gate voltage. This capacitance has two parts, the geometric capacitance, C_geom, and the quantum capacitance C_Q. The latter is due to the level a…
The elegant simplicity of the device concept and the urgent need for a new "transistor" at the twilight of Moore's law have inspired many researchers in industry and academia to explore the physics and technology of negative capacitance field effect transistor (NC-FET). Although hundreds of papers have been published, the validity of quasi-static NC and the frequency-reliability limits of NC-FET are still being debated. The concept of NC - if conclusively demonstrated - will have broad impacts on device physics and technology development. Here, the authors provide a critical review of recent p…
The so-called Boltzmann Tyranny defines the fundamental thermionic limit of the subthreshold slope (SS) of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV/dec at room temperature and, therefore, precludes the lowering of the supply voltage and the overall power consumption. Adding a ferroelectric negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier. Meanwhile, two-dimensional (2D) semiconductors, such as atomically thin transition metal dichalcogenides (TMDs) due to their low dielectric constant, and ease of…
We have measured both the current-voltage ($I_\mathrm{SD}$-$V_\mathrm{GS}$) and capacitance-voltage ($C$-$V_\mathrm{GS}$) characteristics of a $\mathrm{MoS_2-LiNbO_3}$ field effect transistor. From the measured capacitance we calculate the electron surface density and show that its gate voltage dependence follows the theoretical prediction resulting from the two-dimensional free electron model. This model allows us to fit the measured $I_\mathrm{SD}$-$V_\mathrm{GS}$ characteristics over the \emph{entire range} of $V_\mathrm{GS}$. Combining this experimental result with the measured current-vol…
Steep-slope $β$-Ga$_2$O$_3$ nano-membrane negative capacitance field-effect transistors (NC-FETs) are demonstrated with ferroelectric hafnium zirconium oxide in gate dielectric stack. Subthreshold slope less than 60 mV/dec at room temperature is obtained for both forward and reverse gate voltage sweeps with a minimum value of 34.3 mV/dec at reverse gate voltage sweep and 53.1 mV/dec at forward gate voltage sweep at $V_{DS}$=0.5 V. Enhancement-mode operation with threshold voltage ~0.4 V is achieved by tuning the thickness of $β$-Ga$_2$O$_3$ membrane. Low hysteresis of less than 0.1 V is obtain…
We show simultaneous p and n type carrier injection in bilayer graphene channel by varying the longitudinal bias across the channel and the top gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 $μF/cm^2$, a value about 125 times higher than the conventional SiO$_2$ back gate capacitance. Unlike the single layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top and back gate geometry is estimate…
In this letter, we describe operation of a radio-frequency superconducting single electron transistor (RF-SSET) with an on-chip superconducting LC matching network consisting of a spiral inductor L and its capacitance to ground. The superconducting network has a lower parasitic capacitance and gives a better matching for the RF-SSET than does a commercial chip inductor. Moreover, the superconducting network has negligibly low dissipation, leading to sensitive response to changes in the RF-SSET impedance. The charge sensitivity 2.4*10^-6 e/(Hz)^1/2 in the sub-gap region and energy sensitivity o…
High frequency performance limits of graphene field-effect transistors (FETs) down to a channel length of 20nm are examined by using self-consistent quantum simulations. The results indicate that although Klein band-to-band tunneling is significant for sub-100nm graphene FET, it is possible to achieve a good transconductance and ballistic on-off ratio larger than 3 even at a channel length of 20nm. At a channel length of 20nm, the intrinsic cut-off frequency remains at a couple of THz for various gate insulator thickness values, but a thin gate insulator is necessary for a good transconductanc…
We report a comprehensive micro-Raman scattering study of electrochemically-gated graphene field-effect transistors. The geometrical capacitance of the electrochemical top-gates is accurately determined from dual-gated Raman measurements, allowing a quantitative analysis of the frequency, linewidth and integrated intensity of the main Raman features of graphene. The anomalous behavior observed for the G-mode phonon is in very good agreement with theoretical predictions and provides a measurement of the electron-phonon coupling constant for zone-center ($Γ$ point) optical phonons. In addition, …
We analyze the distributions of electric potential and field, polarization and charge, and the differential capacitance of a silicon metal-oxide-ferroelectric field effect transistor (MOSFET), in which a gate insulator consists of thin layers of dielectric SiO2 and weak ferroelectric HfO2. It appeared possible to achieve a quasi-steady-state negative capacitance of the HfO2 layer, C_(HfO_2 )<0, if the layer thickness is close to the critical thickness of the size-induced ferroelectric-paraelectric phase transition. The quasi-steady-state negative capacitance, being a very slow-varying transien…
The operation of resistively-coupled single-electron transistor (R-SET) is studied quantitatively. Due to the Nyquist noise of the coupling resistance, degradation of the R-SET performance is considerable at temperatures $T$ as small as $10^{-3} e^2/C$ (where $C$ is the junction capacitance) while the voltage gain becomes impossible at $T\agt 10^{-2}e^2/C$.
An Al-AlO_x-Al single-electron transistor (SET) acting as the gate of a narrow (~ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET) can induce a vertically aligned Si SET at the Si/SiO_2 interface near the MOSFET channel conductance threshold. By using such a vertically coupled Al and Si SET system, we have detected a single-charge defect which is tunnel-coupled to the Si SET. By solving a simple electrostatic model, the fractions of each coupling capacitance associated with the defect are extracted. The results reveal that the defect is not a large puddle or metal island, but…
Understanding the origin of hysteresis in the channel resistance from top gated graphene transistors is important for transistor applications. Capacitance - voltage measurements across the gate oxide on top gated bilayer graphene show hysteresis with a charging and discharging time constant of ~100 μs. However, the measured capacitance across the graphene channel does not show any hysteresis, but shows an abrupt jump at a high channel voltage due to the emergence of an order, indicating that the origin of hysteresis between gate and source is due to charge traps present in the gate oxide and g…
A simple and highly reproducible single electron transistor (SET) has been fabricated using gated silicon nanowires. The structure is a metal-oxide-semiconductor field-effect transistor made on silicon-on-insulator thin films. The channel of the transistor is the Coulomb island at low temperature. Two silicon nitride spacers deposited on each side of the gate create a modulation of doping along the nanowire that creates tunnel barriers. Such barriers are fixed and controlled, like in metallic SETs. The period of the Coulomb oscillations is set by the gate capacitance of the transistor and ther…
A physics-based model for the surface potential and drain current for monolayer transition metal dichalcogenide (TMD) field-effect transistor (FET) is presented. Taking into account the 2D density-of-states of the atomic layer thick TMD and its impact on the quantum capacitance, a model for the surface potential is presented. Next, considering a drift-diffusion mechanism for the carrier transport along the monolayer TMD, an explicit expression for the drain current has been derived. The model has been benchmarked with a measured prototype transistor. Based on the proposed model, the device des…
We propose a set-up to realize two-channel Kondo physics using quantum dots. We discuss how the charge fluctuations on a small dot can be accessed by using a system of two single electron transistors arranged in parallel. We derive a microscopic Hamiltonian description of the set-up that allows us to make connection with the two-channel Anderson model (of extended use in the context of heavy-Fermion systems) and in turn make detailed predictions for the differential capacitance of the dot. We find that its lineshape, which we determined precisely, shows a robust behavior that should be experim…
Using 2-D self-consistent ballistic quantum transport simulations, we investigate the short-channel behavior of graphene field-effect transistors and its impact on the device transconductance and subsequently the intrinsic cut-off frequency (fT). Although with thin oxides, fT expectedly scales inversely with the gate length, significant band-to-band tunneling at OFF state leads to a departure from this trend in case of thick oxides. We also examine the effect of achieving better electrostatics at the cost of increased gate capacitance and illustrate that this can indeed degrade the fT. These c…
We experimentally characterise the impedance of a single electron transistor (SET) at an excitation frequency comparable to the electron tunnel rate. Differently from usual rf-SET operations, the excitation signal is applied to the gate of the device. At zero source-drain bias the single electron transistor displays both resistive (Sisyphus resistance) and reactive (tunnelling capacitance) components to its impedance. We study the bias dependence of the complex impedance, investigating its response as the electron tunnel rate becomes large with respect to the driving frequency. The experimenta…
Low-capacitance Josephson junction systems as well as coupled quantum dots, in a parameter range where single charges can be controlled, provide physical realizations of quantum bits, discussed in connection with quantum computing. The necessary manipulation of the quantum states can be controlled by applied gate voltages. In addition, the state of the system has to be read out. Here we suggest to measure the quantum state by coupling a single-electron transistor to the q-bit. As long as no transport voltage is applied, the transistor influences the quantum dynamics of the q-bit only weakly. W…
Few-layer phosphorene MOSFETs with 0.3-um-long gate and 15-nm-thick Al2O3 gate insulator was found to exhibit a forward-current cutoff frequency of 2 GHz and a maximum oscillation frequency of 8 GHz after de-embedding for the parasitic capacitance associated mainly with the relatively large probe pads. The gate lag and drain lag of the transistor was found to be on the order of 1 us or less, which is consistent with the lack of hysteresis, carrier freeze-out or persistent photoconductivity in DC characteristics. These results confirm that the phosphorene MOSFET can be a viable microwave transi…
We have suspended an Al based single-electron transistor whose island can resonate freely between the source and drain leads forming the clamps. In addition to the regular side gate, a bottom gate with a larger capacitance to the SET island is placed underneath to increase the SET coupling to mechanical motion. The device can be considered as a doubly clamped Al beam that can transduce mechanical vibrations into variations of the SET current. Our simulations based on the orthodox model, with the SET parameters estimated from the experiment, reproduce the observed transport characteristics in d…
A model of a superconducting tunnel junction which refrigerates a nearby metallic island without any particle exchange is presented. Heat extraction is mediated by charge fluctuations in the coupling capacitance of the two systems. The interplay of Coulomb interaction and the superconducting gap reduces the power consumption of the refrigerator. The island is predicted to be cooled from lattice temperatures of 200~mK down to close to 50~mK, for realistic parameters. The results emphasize the role of non-equilibrium correlations in bipartite mesoscopic conductors. This mechanism can be applied …
We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In our samples with relatively large charging energy values Ec > EJ, where EJ is the energy of the Josephson coupling, two transport mechanisms were found to be dominating, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island (in the open state of the transistor around the points Qg = CgVg = e mod…
A capacitance standard based on the definition of capacitance C = Ne/U is realized when a capacitor is charged with a known number N of electrons (e is the elementary charge) and the voltage U across the capacitor is measured. If U is measured by means of a Josephson voltage standard, C is solely determined by quantum numbers and fundamental constants. Presently, PTB is setting up this experiment using a special type of single-electron pump, the so-called r-pump, for transferring electrons one by one onto a cryogenic vacuum capacitor with the decadic capacitance C = 1 pF. We report on the prog…
Two-dimensional materials and their heterostructures have emerged as a new class of materials for not only fundamental physics but also for electronic and optoelectronic applications. Black phosphorus (BP) is a relatively new addition to this class of materials. Its strong in plane anisotropy makes BP a unique material to make conceptually new type of electronic devices. However, the global density of states (DOS) of BP in device geometry has not been measured experimentally. Here we report the quantum capacitance measurements together with conductance measurements on a hBN protected few layer…
The weak localization correction and the mesoscopic fluctuations of the polarizability and the capacitance of a small disordered sample are studied systematically in 2D and 3D geometries. While the grand canonical ensemble calculation gives the positive magnetopolarizability, in the canonical ensemble (appropriate for isolated samples) the sign of the effect is reversed. The magnitude of mesoscopic fluctuations for a single sample exceeds considerably the value of the weak localization correction.
We have developed a fabrication process for nanoscale tunnel junctions which includes focused-ion-beam etching from different directions. By applying the process to a Nb/(Al-)Al_2O_3/Nb trilayer, we have fabricated a Nb single-electron transistor (SET), and characterized the SET at low temperatures, T=0.04-40 K. The superconducting gap energy and the transition temperature of the Nb SET agree with the bulk values, which suggests high quality Nb junctions. The single-electron charging energy of the SET is estimated to be larger than 1 K.
Solid-state modulation of 2-dimensional electron gases (2DEGs) with extreme (~3.3 x 1014 cm-2) densities corresponding to 1/2 electron per interface unit cell at complex oxide heterointerfaces (such as SrTiO3/GdTiO3 or SrTiO3/SmTiO3) is challenging because it requires enormous gate capacitances. One way to achieve large gate capacitances is by geometrical capacitance enhancement in fin structures. In this work, we fabricate both Au-gated planar field effect transistors (FETs) and Fin-FETs with varying fin-widths on 60nm SrTiO3/5nm SmTiO3 thin films grown by hybrid molecular beam epitaxy (hMBE)…
We present a novel measurement approach which combines the electrical characterization of solution-gated field effect transistors based on epitaxial bilayer graphene on 4H-SiC (0001) with simultaneous Raman spectroscopy. By changing the gate voltage, we observed Raman signatures related to the resonant electron-phonon coupling. An analysis of these Raman bands enabled the extraction of the geometrical capacitance of the system and an accurate calculation of the Fermi levels for bilayer graphene. An intentional application of higher gate voltages allowed us to trigger electrochemical reactions,…
This work reports the design and analysis of an n-type tunneling field effect transistor based on InN. The tunneling current is evaluated from the fundamental principles of quantum mechanical tunneling and semiclassical carrier transport. We investigate the RF performance of the device. High transconductance of 2 mS/um and current gain cut-off frequency of around 460 GHz makes the device suitable for THz applications. A significant reduction in gate to drain capacitance is observed under relatively higher drain bias. In this regard, the avalanche breakdown phenomenon in highly doped InN juncti…
Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of 1.35 meV. The characteristics of the transistors and arrays exhibit some features in the…
We study the surface conductivity of a field-effect transistor (FET) made of periodic array of spherical semiconductor nanocrystals (NCs). We show that electrons introduced to NCs by the gate voltage occupy one or two layers of the array. Computer simulations and analytical theory are used to study the array screening and corresponding evolution of electron concentrations of the first and second layers with growing gate voltage. When first layer NCs have two electrons per NC the quantization energy gap between its 1S and 1P levels induces occupation of 1S levels of second layer NCs. Only at a …
