Beatriz G. Vasallo

Ballistic nanodevices for terahertz data processing: Monte Carlo simulations

By using a semi-classical two-dimensional Monte Carlo simulation, simple devices (T-branch junctions (TBJs) and rectifying diodes) based on AlInAs/InGaAs ballistic channels are analysed. Initially, the model is validated by means of Hall-effect measurements of mobility and electron concentration in long (diffusive) channels. Then, quasi-ballistic transport at room temperature is confirmed in a 100 nm channel. Our simulations qualitatively reproduce the experimental results of electric potential measured in a TBJ appearing as a result of electron ballistic transport, and in close relation with the presence of space charge inside the structure. As examples of devices exploiting the ballistic transport of electrons, preliminary simulations of a multiplexor/demultiplexor and a rectifying diode are presented, demonstrating their capability for terahertz operation.

Microscopic modeling of nonlinear transport in ballistic nanodevices

By using a semi-classical two-dimensional (2-D) Monte Carlo simulation, simple ballistic devices based on AlInAs/InGaAs channels are analyzed. Our simulations qualitatively reproduce the experimental results in T- and Y-branch junctions as well as in a ballistic rectifier appearing as a result of electron ballistic transport. We show that a quantum description of electron transport is not essential for the physical explanation of these results since phase coherence plays no significant role. On the contrary, its origin can be purely classical: the presence of classical electron transport and space charge inside the structures.

Operation and high-frequency performance of nanoscale unipolar rectifying diodes

By means of the microscopic transport description supplied by a semiclassical two-dimensional Monte Carlo simulator, we provide an in depth explanation of the operation (based on electrostatic effects) of the nanoscale unipolar rectifying diode, so called self-switching diode, recently proposed in A. M. Song, M. Missous, P. Omling, A. R. Peaker, L. Samuelson, and W. Seifert, Appl. Phys. Lett. 83, 1881 (2003). The simple downscaling of this device and the intrinsically high electron velocity of InGaAs channels allows one to envisaging the fabrication of structures working in the THz range. We analyze the high-frequency performance of the diodes and provide design considerations for the optimization of the downscaling process.

Comparison Between the Dynamic Performance of Double- and Single-Gate AlInAs/InGaAs HEMTs

The static and dynamic behavior of InAlAs/InGaAs double-gate high-electron mobility transistors (DG-HEMTs) is studied by means of an ensemble 2-D Monte Carlo simulator. The model allows us to satisfactorily reproduce the experimental performance of this novel device and to go deeply into its physical behavior. A complete comparison between DG and similar standard HEMTs has been performed, and devices with different gate lengths have been analyzed in order to check the attenuation of short-channel effects expected in the DG-structures. We have confirmed that, for very small gate lengths, short-channel effects are less significant in the DG-HEMTs, leading to a better intrinsic dynamic performance. Moreover, the higher values of the transconductance over drain conductance ratio gm /gd, and, especially, the lower gate resistance Rg also provide a significant improvement of the extrinsic fmax.

Nonlinear effects in T-branch junctions

The negative potential appearing at the central branch of T-branch junctions (TBJs) when biasing left and right contacts in push-pull fashion has been found to appear under high biasing not only for short (ballistic) TBJs but also for long (diffusive) ones. By means of a microscopic Monte Carlo simulation we are able to explain this nonlinear effect as a consequence of intervalley scattering mechanisms leading to the emergence of an accumulation domain that modifies the electronic potential profile within the devices.

Monte Carlo study of kink effect in short-channel InAlAs/InGaAs high electron mobility transistors

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical microscopic analysis of the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs). Due to the small band gap of InGaAs, these devices are very susceptible to suffer impact ionization processes, with the subsequent hole transport in the channel, both supposedly implicated in the kink effect and easy to be implemented in a Monte Carlo simulation. The results indicate that for high enough VDS, holes, generated by impact ionization, tend to pile up in the channel under the source side of the gate due to the attracting potential caused by the surface charge at the recess and, mostly, by the gate potential. Due to this pile up of positive charge, the potential barrier controlling the current through the channel is lowered, so that the channel is further opened and ID increases, leading to the well known kink effect in the current–voltage characteristics. The microscopic un...

Kink-effect related noise in short-channel InAlAs/InGaAs high electron mobility transistors

We perform a microscopic analysis of the noise associated with the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs) by using a semiclassical two-dimensional (2-D) ensemble Monte Carlo simulator. The kink effect in HEMTs has its origin in the pile up of holes (generated by impact ionization) taking place under the source side of the gate, that leads to a reduction of the gate-induced channel depletion and results in a drain current enhancement. Our results indicate that the generation of holes by impact ionization and their further recombination lead to fluctuations in the charge of the hole pile up which provoke an important increase of the drain-current noise, even when the kink effect is hardly perceptible in the output characteristics. In addition, shot noise related to the hole gate leakage current is found at the gate terminal, which can further degrade the global noise performance of HEMTs.

Comparison Between the Noise Performance of Double- and Single-Gate InP-Based HEMTs

The noise performance of InAlAs/InGaAs double-gate (DG) and standard high-electron-mobility transistors (HEMTs) is analyzed by means of an ensemble 2-D Monte Carlo simulator. The DG-HEMT is found to have a better noise behavior than the single-gate (SG) device. The results show a moderate decrease of the and noise parameters for the DG HEMT with respect to that of the SG device, since current fluctuations due to electrons injected into the buffer are eliminated. Moreover, the DG HEMT reveals a significantly lower extrinsic minimum noise figure and a higher associated gain , not only due to the better intrinsic performance but also to the lower contact resistances.

Monte Carlo analysis of four-terminal ballistic rectifiers

We present a Monte Carlo study of an InGaAs based four-terminal ballistic rectifier operating at different temperatures. The rectifying effect is due to the vertical asymmetry of the electron concentration originated, in the presence of ballistic transport, by the action of an obstacle located in the centre of a ballistic cross junction. An increase of temperature degrades the efficiency of the device, since transport becomes more diffusive. However, it shows an intrinsic capability for rectification up to a frequency of 1.0 THz almost independently of the temperature.

Monte Carlo study of kink effect in Isolated-Gate InAs/AlSb High Electron Mobility Transistors

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical analysis of the kink effect in InAs/AlSb high electron mobility transistors (HEMTs). Kink effect, this is, an anomalous increase in the drain current I-D when increasing the drain-to-source voltage V-DS, leads to a reduction in the gain and a rise in the level of noise, thus limiting the utility of these devices for microwave applications. Due to the small band gap of InAs, InAs/AlSb HEMTs are very susceptible to suffer from impact ionization processes, with the subsequent hole transport through the structure, both implicated in the kink effect. The results indicate that, when V-DS is high enough for the onset of impact ionization, holes thus generated tend to pile up in the buffer (at the gate-drain side) due to the valence-band energy barrier between the buffer and the channel. Due to this accumulation of positive charge the channel is further opened and I-D increases, leading to the kink effect in the I-V characteristics and eventually to the device electrical breakdown. The understanding of this phenomenon provides useful information for the development of kink-effect-free InAs/AlSb HEMTs.