Helena Rodilla

High-mobility heterostructures based on InAs and InSb: A Monte Carlo study

In this work, by means of Monte Carlo simulations, two different narrow band gap semiconductors, InAs and InSb, and their associated heterostructures, AlSb/InAs and AlInSb/InSb, have been studied. The parameters for the bulk simulations have been optimized in order to correctly reproduce the experimental mobility values. For the correct simulation of the heterostructures, roughness scattering has been included in the model, and its strength has been adjusted to achieve a good agreement with the experimentally measured mobility.

Characterization and Modeling of Cryogenic Ultralow-Noise InP HEMTs

Detailed S-parameter and noise characterization and modeling of ultralow-noise InP/InAlAs/InGaAs high-electron mobility transistors (InP HEMTs) optimized for operation at 10 K are presented. At the optimum low-noise bias at 10 K, the InP HEMT exhibited a 60% improvement in cutoff frequency fT and a 100% improvement in dc transconductance gm compared with 300 K. A small-signal noise model was evaluated at different bias conditions at 10 and 300 K. The bias dependence of the minimum noise temperature at low-noise operation was modeled at 10 K. The temperature dependence of the threshold voltage VT, gm, and gate-source and gate-drain capacitances Cgs and Cgd indicated that the excellent cryogenic noise performance of optimized InP HEMTs is due to a higher degree of confinement in the carrier concentration closest to the gate at 10 K compared with 300 K. As a result, a fast depletion of the HEMT channel with respect to drain current Id occurs under cryogenic operation.

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.

Terahertz tunable detection in self-switching diodes based on high mobility semiconductors: InGaAs, InAs and InSb

In this work we report on the use of high mobility materials in the channel of self-switching diodes as potential candidates for terahertz operation. By means of Monte Carlo simulations we envisage the feasibility of tuneable-by-geometry detection in the terahertz range. The low effective mass of InAs and InSb in relation to InGaAs enhances ballistic transport inside the diode, thus improving the amplitude and quality factor of the resonance found in the detection spectra of self-switching diodes. The frequency of the resonant peak is also increased with the use of these narrow band gap semiconductors. The analysis of the noise spectra provides useful information about the origin of the resonance. By decreasing temperature below 300 K, a clear improvement in detection sensitivity is also achieved.

Kink effect and noise performance in isolated-gate InAs/AlSb high electron mobility transistors

The kink effect can spoil the otherwise excellent low noise performance of InAs/AlSb high electron mobility transistors. It has its origin in the pile-up of holes (generated by impact ionization) taking place mainly at the drain side of the buffer, which 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 in the drain current noise, even when the kink effect is hardly perceptible in the output characteristics.

Dynamic Monte Carlo study of isolated-gate InAs/AlSb HEMTs

In this work, by means of Monte Carlo simulations, the static and dynamic behavior of isolated-gate InAs/AlSb high electron mobility transistors (Sb-HEMTs) has been studied and compared with experimental results. The influence of the existence of a native oxide under the gate, the value of the surface charges in the gate recess and the possible variation of electron sheet carrier density, n(s), have been studied. A decrease in the gate-source capacitance, transconductance and intrinsic cutoff frequency is observed because of the presence of the native oxide, while changes in the value of the surface charges in the recess only introduce a threshold voltage shift. The increase of n(s) shifts the maximum of the transconductance and intrinsic cutoff frequency to higher values of drain current and improves the agreement with the experimental results.

Cryogenic Performance of Low-Noise InP HEMTs: A Monte Carlo Study

In this paper, we present a study of the cryogenic performance of InP high electron mobility transistors (HEMTs) in the low-noise region by means of Monte Carlo simulations. A decrease of the contact resistances and an increase in the electron velocity in the channel together with enhanced channel electron confinement upon cooling of the device are observed, and considered to be the reason for the excellent low-noise behavior of cryogenic InP HEMTs. These findings are supported by a good agreement between simulated and experimental DC, RF, and noise figure data of a 130-nm gate length InP HEMT at 300 and 77 K. An increase of the transconductance gm and gate-to-source capacitance Cgs is observed when cooling from 300 to 77 K as a consequence of electron velocity increase and improved channel confinement.

Cryogenic Kink Effect in InP pHEMTs: A Pulsed Measurements Study

We present a study based on pulsed measurement results of the kink effect observed on the I-V output characteristics in InGaAs/InAlAs/InP pseudomorphic high-electron mobility transistors (InP pHEMTs) at cryogenic temperatures. Pulsed measurements were performed at 300 and 10 K. Gate and drain lags were observed at both temperatures with a strong increase upon cooling for the drain lag. To study the influence of surface traps in the kink, pulsed measurements of devices passivated by either atomic layer deposited Al2O3 or plasma enhanced chemical vapor deposited Si3N4 were compared with no significant differences at 10 K. The influence on the kink effect from the buffer was studied by comparing pulsed measurement data from an InP pHEMT with measurements on a GaAs metamorphic HEMT (GaAs mHEMT). For the GaAs mHEMT, an increase of the drain lag at 10 K was observed when compared with the InP pHEMT. Contrary to the InP HEMT, for the GaAs mHEMT the 0.1 μs pulses were short enough to eliminate the kink when using a quiescent point with VDS = 0. The quality of the pinchoff was sensitive to pulse length and quiescent point for the InP pHEMT but not for the GaAs mHEMT.

Cryogenic ultra-low noise amplification - InP PHEMT vs. GaAs MHEMT

We present a comparative study of 130 nm high electron mobility transistors (HEMTs) fabricated on pseudomorphic InGaAs/InAlAs/InP (InP PHEMT) and InGaAs/InAlAs/GaAs (GaAs MHEMT) intended for ultra-low noise amplifiers (LNAs). The epitaxial growth, as well as the HEMT process, was performed simultaneously. When integrated in a 4-8 GHz 3-stage LNA at 300 K, the measured average noise temperature was 34 K for the GaAs MHEMT and 27 K for the InP PHEMT. When cooled down to 10 K, the InP PHEMT LNA was improved to 1.6 K, while the GaAs MHEMT LNA was only reduced to 5 K. The reason for the superior cryogenic noise performance of the InP PHEMT compared to the GaAs MHEMT in this study, was found to be a higher quality of pinch-off when cooled down.

Monte Carlo simulation of ballistic transport in high-mobility channels

By means of Monte Carlo simulations coupled with a two-dimensional Poisson solver, we evaluate directly the possibility to use high mobility materials in ultra fast devices exploiting ballistic transport. To this purpose, we have calculated specific physical quantities such as the transit time, the transit velocity, the free flight time and the mean free path as functions of applied voltage in InAs channels with different lengths, from 2000 nm down to 50 nm. In this way the transition from diffusive to ballistic transport is carefully described. We remark a high value of the mean transit velocity with a maximum of 14×105 m/s for a 50 nm-long channel and a transit time shorter than 0.1 ps, corresponding to a cutoff frequency in the terahertz domain. The percentage of ballistic electrons and the number of scatterings as functions of distance are also reported, showing the strong influence of quasi-ballistic transport in the shorter channels.