Electron Velocity Overshoot Research Articles

Enhanced drift velocity of photoelectrons in a semiconductor with ultrafast carrier recombination

It is shown by Monte Carlo simulations that the electron velocity overshoot phenomenon in the transient response causes a significant enhancement of the steady-state electron drift velocity in semiconductors in which the electron recombination time is of the same order as the time of the electron transient response. The effect can take place in GaAs layers grown by molecular beam epitaxy at low temperatures.

Transient electron transport in wurtzite GaN, InN, and AlN

Transient electron transport and velocity overshoot in wurtzite GaN, InN, and AlN are examined and compared with that which occurs in GaAs. For all materials, we find that electron velocity overshoot only occurs when the electric field is increased to a value above a certain critical field, unique to each material. This critical field is strongly dependent on the material, about 4 kV/cm for the case of GaAs but much higher for the III–nitride semiconductors: 140 kV/cm for GaN, 65 kV/cm for InN, and 450 kV/cm for AlN. We find that InN exhibits the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distances when compared with GaN and AlN. Finally, using a one-dimensional energy–momentum balance approach, a simple model is used to estimate the cutoff frequency performance of nitride based heterojunction field effect transistors (HFETs) and a comparison is made to recently fabricated AlGaN/GaN HFETs.

Scaling of gate length in ultra-short channel heterostructure field effect transistors

Ultra-short channel AlGaAs/GaAs modulation doped field effect transistors (MODFETs) have been fabricated with gate lengths ranging from 25 to 115 nm, using an electron beam lithography process, in order to examine the fundamental limitation of transistor scaling. For gate lengths in the tens of nanometer range, where the gradual channel approximation is no longer valid, it is observed that the transconductance is improved by electron velocity overshoot. Velocity overshoot starts to occur in our devices with sub-70 nm gate lengths, which is smaller than that expected from other published experimental results. This is due to the suppression of the velocity overshoot by a parasitic gate-fringing effect in a reduced gate aspect ratio. However, we find that the enhancement in transconductance by the velocity overshoot is limited with further scaling and the transconductance starts to drop at a gate length below 40 nm. Short-channel electron tunneling is suggested experimentally to explain the degradation of transistor performance in the shortest channel devices.

Full Band Monte Carlo Comparison of Wurtzite and Zincblende Phase GaN MESFETs

AbstractThe output characteristics, cutoff frequency, breakdown voltage and the transconductance of wurtzite and zincblende phase GaN MESFETs have been calculated using a self-consistent, full band Monte Carlo simulation. It is found that the calculated breakdown voltage for the wurtzite device is considerably higher than that calculated for a comparable GaN zincblende phase device. The zincblende device is calculated to have a higher transconductance and cutoff frequency than the wurtzite device. The higher breakdown voltage of the wurtzite phase device is attributed to the higher density of electronic states for this phase compared to the zincblende phase. The higher cutoff frequency and transconductance of the zincblende phase GaN device is attributed to more appreciable electron velocity overshoot for this phase compared to that for the wurtzite phase. The maximum cutoff frequency and transconductance of a 0.1 μm gate-length zincblende GaN MESFET are calculated to be 220GHz and 210 mS/mm, respectively. The corresponding quantities for the wurtzite GaN device are calculated to be 160GHz and 158 mS/mm.

High-Frequency Response of Metal-Semiconductor-Metal Photodetectors Limited by Dynamic and Recombination Effects

A simplified quasi-two-dimensional analytical model of planar interdigitated metal-semiconductor-metal (MSM) photodetectors is presented. The model takes into account the transit-time and velocity overshoot effects of photogenerated electrons and holes and their recombination. The response is derived as a function of the signal frequency and the device structural parameters. It is shown that MSM photodetectors with relatively large contact spacing can exhibit rather high-frequency response in the terahertz range due to electron velocity overshoot. The obtained MSM photodetector response is in good agreement with the results of a Monte Carlo simulation. The response-frequency product reveals a high maximum corresponding to a frequency of about a few terahertz. The MSM photodetectors exploiting the velocity overshoot effect can be advantageous in comparison to the MSM photodetectors with ultrashort lifetime of the photogenerated electrons and holes.

Monte Carlo modeling of electron velocity overshoot effect in quantum well infrared photodetectors

Transient response of n-type AlGaAs/GaAs and InP/InGaAs quantum well infrared photodetectors (QWIPs) has been studied using an ensemble Monte Carlo particle method. It has been shown that the photocurrent initiated by an ultrashort pulse of infrared radiation reveals a sharp peak associated with the electron velocity overshoot effect. The existence of the photocurrent peak results in a plateaulike region in the QWIP frequency-dependent responsivity in the terahertz range. This effect can be used for heterodyne detection in QWIP-based mixers at terahertz frequencies and for generation of terahertz radiation. Larger intervalley separation and, consequently, more pronounced electron velocity overshoot in InP-based QWIPs in comparison with QWIPs with AlGaAs barriers lead to higher performance of the former in the terahertz range of spectrum.

Non-equilibrium Electron Dynamics Phenomena in Scaled Sub-100 nm Gate Length Metal Semiconductor Field Effect Transistors: Gate-fringing, Velocity Overshoot, and Short-channel Tunneling

Ultrashort channel GaAs metal semiconductor field effect transistors were fabricated with gate lengths ranging from 30 nm to 105 nm, by electron beam lithography, in order to examine the scaling characteristics of transconductance. For gate lengths in sub-100 nanometer range, where gradual channel approximation is no longer valid, it was observed that the transconductance varies with a variety of small-dimension-related, nonequilibrium electron dynamics phenomena such as gate-fringing effect, electron velocity overshoot, and short-channel tunneling. Short-channel tunneling was suggested experimentally for the first time to explain the degradation of transistor performance, overriding an enhancement due to electron velocity overshoot for a gate length smaller than the average inelastic mean free path of an electron.

I-V and small signal parameters modelling of ultrasubmicron MOSFETs including the significant electron-velocity overshoot effects, which are enhanced at low temperature

We have developed a new analytical ultra-short channel MOSFET model for circuit simulation including velocity overshoot effects. We have used a previous physically based model as the starting point and added a new term in order to account for electron velocity overshoot effects. We have been able to reproduce experimental I-V curves and conductances of MOSFETs down to 0.07 μm channel lengths. We have paid more attention to the modelling of velocity overshoot effects at low temperature, where ballistic transport is more likely due to the reduced energy relaxation produced by phonons.

Monte Carlo analysis of ultrafast electron transport in quantum well infrared photodetectors

A self-consistent ensemble Monte Carlo simulation is used to study ultrafast electron transport in quantum well infrared photodetectors (QWIPs). It is shown that transient photocurrent triggered by a short infrared radiation pulse reveals a sharp peak followed by a relatively slow decay. The photocurrent peak is associated with the electron velocity overshoot effect while the slow component of the photocurrent is determined by the electron transit time and capture effects. The velocity overshoot effect results in the existence of a plateau in the QWIP frequency-dependent responsivity in the range of terahertz frequencies.

Monte Carlo Simulation of a Submicron MOSFETIncluding Inversion Layer Quantization

A Monte Carlo simulator of the electron dynamics in the channel, coupled with a solution of the two‐dimensional Poisson equation including inversion‐layer quantization and drift‐diffusion equations has been developed. This simulator has been applied to the study of electron transport in normal operation conditions for different submicron channel length devices. Some interesting non‐local effects such as electron velocity overshoot can be observed.

Influence of Electron Velocity Overshoot Effect on High-Frequency Characteristics of Quantum Well Infrared Photodetectors

We have studied the transient response of AlxGa1-xAs/GaAs quantum well infrared photodetectors (QWIPs) and their high-frequency performance using a self-consistent ensemble Monte Carlo particle method. It has been shown that the photocurrent caused by the electron intersubband transitions under the influence of an ultra-short pulse of infrared radiation reveals a sharp peak followed by relatively slow decay. This is associated with the electron velocity overshoot and the electron transit time and capture effects. The velocity overshoot results in the existence of an additional plateau-like region in the QWIP frequency-dependent responsivity which can correspond to the terahertz range.

Study of Electron Velocity Overshoot in NMOS Inversion Layers

Non-local electron transport in nMOSFET inversion layers has been studied by Monte Carlo (MC) simulations. Inversion layer quantization has been explicitly included in the calculation of density of states and scattering rate for low-energy electrons while bulk band structure is used to describe the transport of more energetic electrons. For uniform, high-lateral field conditions, the effects of quantization are less pronounced due to the depopulation of electrons in the lower-lying subbands. On the other hand, Monte Carlo results for carrier transport in spatially varying lateral fields (such as those in the inversion layer of MOSFETs) clearly indicate that depopulation of the low-lying subbands is less evident in the non-local transport regime. Quasi-2D simulations have shown that, at high transverse effective field, the inclusion of a quantization domain does have an impact on the calculated spatial velocity transient.

A model for the drain current of deep submicrometer MOSFETs including electron-velocity overshoot

We have developed a new analytical ultra-short channel MOSFET model for circuit simulation including velocity overshoot effects. We have been able to reproduce experimental I-V curves and conductances of MOSFETs down to 0.07 /spl mu/m channel lengths both at low and room temperatures.

Monte Carlo Simulation of Non-Local Transport Effects in Strained Si on Relaxed Si1 – xGex Heterostructures

Electron transport properties of strained-Si on relaxed Si1 – xGex channel MOSFETs have been studied using a Monte Carlo simulator. The steady- and non-steady-state high-longitudinal field transport regimes have been described in detail. Electronvelocity- overshoot effects are studied in deep-submicron strained-Si MOSFETs, where they show an improvement over the performance of their normal silicon counterparts. The impact of the Si layer strain on the performance enhancement are described in depth in terms of microscopic magnitudes.

Experimental study of nonstationary electron transport in sub-0.1 μm metal–oxide–silicon devices: Velocity overshoot and its degradation mechanism

We have experimentally studied the electron velocity overshoot and the mechanism of its degradation in the inversion layer of sub-0.1 μm metal–oxide–silicon (MOS) field-effect transistors. Both silicon-on-insulator (SOI) and bulk structures were studied. At low transverse electric fields, that is, for low carrier densities in SOI devices under low gate drive conditions, it is possible to achieve electron velocity overshoot due to nonstationary transport in the sub-0.1 μm region. However, it is very difficult in MOS structures to improve electron velocity at high surface electron densities because of the reduced electron mobility in high transverse fields. Moreover, the surface electron density of MOS structures is reduced when a low channel impurity concentration is chosen to improve low field mobility; this results from the expanded inversion layer width. These results indicate the physical limitations of scaled MOS structures with regards to the realization of higher current capabilities.

Velocity overshoot of electrons and holes in Si inversion layers

Velocity overshoot of inversion layer electrons and holes is studied experimentally and analytically in special test structures with nominally uniform electric field. The data were used to calibrate energy relaxation parameters in a commercial simulator MEDICI ver. 2.0. We propose an analytical model for velocity overshoot and show that it agrees well with experimental data. The amount of hole velocity overshoot is small.

Modeling effects of electron-velocity overshoot in a MOSFET

A simple analytical expression to account for electron-velocity overshoot effects on the performance of very short-channel MOSFETs has been obtained. This new model can be easily included in circuit simulators of systems with a huge number of components. The influence of temperature and low-field mobility on the increase of MOSFET transconductance produced by electron-velocity overshoot as channel lengths are reduced can be easily taken into account in our model. The accuracy of this model has been verified by reproducing experimental and simulated data reported by other authors.

High-field transport of inversion-layer electrons and holes including velocity overshoot

In this paper, we experimentally address the effect of a wide range of parameters on the high-field transport of inversion-layer electrons and holes. The studied parameters include substrate doping level, surface micro-roughness, vertical field strength, nitridation of the gate oxide, and device channel length. We employ special test structures built on Silicon-On-Insulator (SOI) and bulk wafers to accurately measure the high-field drift velocity of inversion-layer carriers. Our findings point to electron velocity overshoot at room temperature, dependence of electron and hole saturation velocities on nitridation of the gate oxide, dependence of the high-field drift velocity on the effective vertical field, and relative insensitivity of electron and hole mobility and saturation velocity to moderate surface roughness.

Electron transport properties of quantized silicon carbide inversion layers

Electron transport properties in SiC quantized inversion layers have been studied by means of a Monte Carlo procedure. It has been observed that the contribution of polar-optical phonon scattering produces a significant influence of the effective-electric field on the high longitudinal field transport regime, this being the main difference of SiC with respect to standard Si inversion layers. The energy- and momentum-relaxation times have been calculated and the results suggest that electron velocity overshoot effects are less important than in Si metal-oxide semiconductor field effect transistors. The electron mobility is not very different from their silicon counterparts, but the saturation velocity is higher.

The dependence of the electron mobility on the longitudinal electric field in MOSFETs

The dependence of the electron mobility on the longitudinal electric field in MOSFETs has been studied in detail. To do so, a Monte Carlo simulation of the electron dynamics in the channel, coupled with a solution of the two-dimensional Poisson equation including inversion-layer quantization and drift-diffusion equations, has been developed. A simplified description of the silicon band structure in the effective-mass approximation including non-parabolicity has been considered. Different-channel-length MOSFETs and different biases have been taken into account. It has been shown that in order to accurately describe electron-mobility behaviour in short-channel MOSFETs it is necessary to take into account the electron-velocity overshoot. An analytical expression, easy to include in device simulators, is provided to account for the dependence of the electron mobility on the high values of the longitudinal electric field and of its gradient found in state-of-the-art MOSFETs.