Electron Velocity Overshoot Research Articles

Extraction of Effective Carrier Velocity and Observation of Velocity Overshoot in Sub-40 nm MOSFETs

Carrier velocity in the MOSFET channel is the main driving force for improved transistor performance with scaling. We report measurements of the drift velocity of electrons and holes in silicon inversion layers. A technique for extracting effective carrier velocity which is a more accurate extraction method based on the actual inversion charge measurement is used. This method gives more accurate result over the whole range of V<SUB>ds</SUB>, because it does not assume a linear approximation to obtain the inversion charge and it does not limit the range of applicable V<SUB>ds</SUB>. For a very short channel length device, the electron velocity overshoot is observed at room temperature in 37 ㎚ MOSFETs while no hole velocity overshoot is observed down to 36 ㎚. The electron velocity of short channel device was found to be strongly dependent on the longitudinal field.

Hydrodynamic Simulations of Unitraveling-Carrier Photodiodes

We present simulated results of a unitraveling-carrier photodiode (UTC-PD) using the hydrodynamic carrier transportation model. A maximum responsivity of 0.25 A/W and a small-signal 3-dB bandwidth of 52 GHz were obtained for a 220-nm-thick InGaAs absorption layer. The physical properties of the UTC-PD have been investigated at different optical injection levels. Modulation of the energy-band profile due to the space charge effect has been observed at high injection level, and an electron velocity overshoot of 3 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s has been found to effectively delay the onset of space charge effects. Comparisons with reported simulated results using the drift-diffusion model as well as reported experimental results are presented. The results suggest the necessity of using the hydrodynamic transport equations to accurately model the UTC-PD. In addition, it has been corroborated that the photoresponse of the UTC-PD could be improved by incorporating a graded doping profile in the absorption layer.

COMPARISON OF HIGH FIELD STEADY STATE AND TRANSIENT ELECTRON TRANSPORT IN WURTZITE GaN, AlN AND InN

An ensemble Monte Carlo simulation is used to compare bulk electron transport in wurtzite phase GaN , AlN and InN materials. Electronic states within the conduction band valleys at the Γ1, U, M, Γ3 and K are represented by non-parabolic ellipsoidal valleys centered on important symmetry points of the Brillouin zone. For all materials, it is found 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 parameters. Transient velocity overshoot has also been simulated, with the sudden application of fields up to ~5 × 107 Vm -1, appropriate to the gate-drain fields expected within an operational field effect transistor. The electron drift velocity relaxes to the saturation value of ~1.4 × 105 ms -1 within 4 ps, for all crystal structures. The steady state and transient velocity overshoot characteristics are in fair agreement with other recent calculations.

A New Method to Extract Carrier Velocity in Sub-0.1-$mu$m MOSFETs Using RF Measurements

A novel RF method based on the accurate extraction of the gate–source channel capacitance and intrinsic transconductance from measured $S$ -parameters is proposed to determine the effective carrier velocity of sub-0.1- $mu$ m MOSFETs in the velocity saturation region. This method is developed to avoid the errors associated with underestimated charge in traditional ones. Using the RF technique, the electron velocity overshoot exceeding the bulk saturation velocity is observed along with the linear dependence of measured electron velocity on $1/L_ poly$ in bulk N-MOSFETs with a channel length of less than 0.085 $mu$ m. This extracted result is more accurate than that of previous methods, because $v_ eff$ is directly determined from the gate–source channel capacitance at high V $ ds$ instead of that at V $ ds=0$ .

Photonic generation of continuous THz wave using uni-traveling-carrier photodiode

A uni-traveling-carrier photodiode (UTC-PD) is monolithically integrated with a wideband log-periodic toothed antenna for generating millimeter and submillimeter waves at frequencies of up to terahertz range. A module with a quasi-optical output port fabricated for practical use operates up to 1.5 THz and generates an output power of 2.3 /spl mu/W at 1.04 THz with good linearity. The output power level and the operation frequency are records for wideband PD modules operating at 1.55 /spl mu/m. An investigation of the operational characteristics of the UTC-PD reveals that the effective use of the electron-velocity overshoot in the junction depletion layer is important for maximizing the output power in the terahertz range.

Velocity overshoot in ultrathin double-gate silicon-on-insulator transistors

Concerning electron velocity overshoot in ultrathin double-gate silicon-on-insulator transistors, as a function of the silicon thickness and the electron sheet density, it is proved that for very small silicon thicknesses (smaller than 5 nm), velocity overshoot behavior is dominated by the average conduction effective mass; that is, the lower the average conduction effective mass, the higher the velocity overshoot peak. Thus, the velocity overshoot peak increases as the silicon thickness decreases, unlike low-field electron mobility, which diminishes abruptly at silicon thicknesses below 5 nm. This fact enables further reduction in the device channel length, in contrast to what might be supposed from the low-field mobility behavior.

A Theoretical Comparison of Strained-Si-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistors and Conventional Si-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistors Using a Drift-Diffusion-Based Simulator

Strained-Si-on-insulator (strained-SOI) metal–oxide–semiconductor field-effect transistors (MOSFETs) were compared to conventional SOI MOSFETs in terms of short-channel effects and delay time using a drift-diffusion-based device simulator. In the case of 90 nm gate devices, threshold voltage Vth for strained-SOI nMOSFETs decreased with decreasing conduction-band edge discontinuity, while Vth dependence on conduction-band edge and valence-band edge discontinuities for strained-SOI pMOSFETs were complicated. An analytical expression for Vth in strained-SOI pMOSFETs could explain the simulated results. The Vth shift for strained-SOI nMOSFETs with shortening the gate length was slightly smaller than that for conventional SOI nMOSFETs, while Vth shift for strained-SOI pMOSFETs was larger than that for conventional SOI nMOSFETs. The Vth shift for strained-SOI nMOSFETs with a strain-Si thickness tSi greater than the source and drain junction depth xj was smaller than that for strained-SOI nMOSFETs with tSi<xj. The strained-SOI MOSFETs were approximately 15% faster than conventional SOI MOSFETs when the saturation velocities were assumed to be 107 cm/s for both strained- and conventional SOI MOSFETs. Strained-SOI nMOSFETs are 1.9 times faster than conventional SOI nMOSFETs when the electron velocity overshoot is developed.

Direction-dependent band nonparabolicity effects on high-field transient electron transport in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p–i–n diode provide evidence of electron velocity overshoot at fields as low as ∼130 kV/cm for transport in the c-direction of wurtzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ∼300 kV/cm, this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass rather than intervalley transfer. Similar calculations of transport in the basal plane indicate that in this case, both a higher threshold field for velocity overshoot and a lower steady-state velocity at a given field are expected.

Experimental observation of electron velocity overshoot in AlN

The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on 6H-SiC was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through 80-nm-thick layers indicated the onset of quasiballistic transport for fields greater than 510 kV/cm. This was evidenced by a symmetric energy distribution centered at energies above the conduction band minimum. Drifted Fermi–Dirac energy distribution was fitted to the measured energy distribution, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations.

Experimental and Theoretical Studies of Transient Electron Velocity Overshoot in GaN

We employ an optically detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in an AlGaN/GaN heterojunction p-i-n diode to measure the electron velocity overshoot in GaN at room temperature. It has been found that electron velocity overshoot occurs at electric fields as low as 105 kV/cm, with the peak transient velocity increasing with E up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. At higher fields, the increase in transit time with increasing field suggests the onset of negative differential resistance due to intervalley transfer. The existence of transient velocity overshoot at fields lower than the calculated peak steady-state velocity suggests that it occurs while the electrons are primarily in the r valley. Full zone Monte Carlo calculations imply that the overshoot is associated more with band nonparabolicity in the Γ valley than with intervalley transfer at fields less than 325 kV/cm, and, in conjunction with theoretical calculations employing a semiclassical transport model, confirm the importance of this nonparabolicity for the determination of the temporal shape of the transient velocity overshoot curves.

Electron transport in strained Si inversion layers grown on SiGe-on-insulator substrates

We show by simulation that electron mobility and velocity overshoot are greater when strained inversion layers are grown on SiGe-On-insulator substrates (strained Si/SiGe-OI) than when unstrained silicon-on-insulator (SOI) devices are employed. In addition, mobility in these strained inversion layers is only slightly degraded compared with strained bulk Si/SiGe inversion layers, due to the phonon scattering increase produced by greater carrier confinement. Poisson and Schroedinger equations are self-consistently solved to evaluate the carrier distribution in this structure. A Monte Carlo simulator is used to solve the Boltzmann transport equation. Electron mobility in these devices is compared to that in SOI inversion layers and in bulk Si/SiGe inversion layers. The effect of the germanium mole fraction x, the strained-silicon layer thickness, TSi, and the total width of semiconductor (Si+SiGe) slab sandwiched between the two oxide layers, Tw were carefully analyzed. We observed strong dependence of the electron mobility on TSi, due to the increase in the phonon scattering rate as the silicon layer thickness is reduced, a consequence of the greater confinement of the carriers. This effect is less important as the germanium mole fraction, x, is reduced, and as the value of TSi increases. For TSi&amp;gt;20 nm, mobility does not depend on TSi, and maximum mobility values are obtained.

RF Noise in a Short-Channel n-MOSFET: A Monte Carlo Study

We present a complete Monte Carlo analysis of transport and RF noise in a short-gate n-MOSFET. Short-channel effects are observed in the static characteristics. Velocity overshoot of electrons in the channel, as well as the appearance of hot carriers are detected. P, R and C noise parameters show an important increase in their values with respect to the long-channel theory predictions due to the presence of hot electrons. NF min is also calculated; results confirm the importance of induced gate noise on the behaviour of this parameter.

Observations of electron velocity overshoot during high-field transport in AlN

ABSTRACTThe energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through 80 nm thick layers indicated the onset of quasi-ballistic transport. This was evidenced by symmetric energy distributions centered at energies above the conduction band minimum for fields greater than 530 kV/cm. Drifted Fermi-Dirac energy distributions were fitted to the measured energy distributions, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations. This is the first experimental demonstration of this kind of transport in AlN.

Band Structure Effects on the Transient Electron Velocity Overshoot in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurtzite GaN. The velocity overshoot increases with electric field up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. However, theoretical Monte Carlo calculations incorporating a GaN full-zone band structure show that the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (> 325 kV/cm). Insight into this behavior can be gleaned from the band nonparabolicity deduced from the constant energy surfaces in the Γ valley, which shows that the effective mass in the c-direction can be viewed as becoming larger at high k values.

Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN

A femtosecond time-resolved electroabsorption technique employing an AlGaN/GaN heterojunction p–i–n diode with a p-type AlGaN window layer and a semitransparent p contact has been used to measure the transient electron velocity overshoot in GaN. A peak transient electron velocity of 7.25×107 cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a negative differential resistivity region of the steady-state velocity-field characteristic in this high field range.

Comparison between wurtzite phase and zincblende phase GaN MESFETs using a full band Monte Carlo simulation

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. The effect of interface states on the device performance is modeled by including uniformly depleted regions at the device surface under the passivation layers. It is found that the drain current increases gradually with increasing drain-source voltage at the onset of breakdown for both phases. The calculated breakdown voltage for the wurtzite device is considerably higher than the breakdown voltage calculated for the zincblende device. On the other hand, the zincblende device is calculated to have 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 device is apparently due to the greater electron velocity overshoot for this phase compared to that for the wurtzite phase. The maximum cutoff frequency and transconductance of a 0.1 /spl mu/m gate-length zincblende GaN MESFET are calculated to be 220 GHz and 210 mS/mm, respectively. The corresponding quantities for the wurtzite GaN device are calculated to be 160 GHz and 158 mS/mm.

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

The 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 temperature behavior of subpicosecond electron transport transient in 3C- and 6H-SiC

A study of the subpicosecond high-field electron transport transient in 3C- and 6H-SiC polytypes at high lattice temperatures is performed. Electric field intensities up to 1000 kV/cm and lattice temperatures of 300, 673, and 1073 K are considered. It is shown that the transient regime behavior depends on the electric field intensity as well as on the lattice temperature, but it is always shorter or of the order of 0.3 ps. For the lowest lattice temperature, an electron velocity overshoot can always occur when the electric field intensity is higher than 300 kV/cm, but it decreases and can even disappear when the lattice temperature is raised.

High-temperature effects on the velocity overshoot of hot electrons in 6H- and 3C-SiC

Lattice temperature effects on the high-field transport transient of electrons in 6H- and 3C-SiC during the subpicosecond regime are studied. The calculations are performed considering a nonparabolic band structure, and the results are obtained through the numerical solution of Boltzmann-like transport equations for the electron drift velocity v and energy within the momentum and relaxation time approximations, p and , respectively. It is shown that an increase of the lattice temperature reduces both the electron drift velocity and energy, being even able to preclude a drift velocity overshoot in 6H- and 3C-SiC. For a given electric field, the growth rate of the electron energy decreases strongly when the 6H- and 3C-SiC lattice temperatures are raised.

Exploration of velocity overshoot in a high-performance deep sub-0.1-μm SOI MOSFET with asymmetric channel profile

The electron velocity overshoot phenomenon in the inversion layer is experimentally investigated using a novel thin-film silicon-on-insulator (SOI) test structure with channel lengths down to 0.08 μm. The uniformity of the carrier density and tangential field is realized by employing a lateral asymmetric channel (LAC) profile. The electron drift velocity observed in this work is 9.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm/s for a device with L/sub eff/=0.08 μm at 300 K. The upward trend in electron velocity can be clearly noticed for decreasing channel lengths.