Carrier Velocity Saturation Research Articles

Linearity improvement in graded channel AlGaN/GaN HEMTs for high-speed applications

AlGaN HEMTs are popular devices for high-frequency applications. At higher frequencies, nonlinearity effects are major concerns and need serious attention. In this work, we have attempted to improve the linearity performance of graded channel Al x Ga1−x N/GaN HEMTs in comparison with abrupt channel GaN HEMTs through Technology Computer Aided (TCAD) simulation. In graded channel HEMTs, linearly grading of Al composition in the AlGaN layer, reduces the local carrier densities and provides improvement in the carrier saturation velocity. This provides an avenue to improve the overall linearity performance. A complete set of figures of merits (FOMs) such as and for both devices are presented. The simulation results have been calibrated with reported experimental results available in the literature. The graded devices are observed to have better transconductance (g m ) and drain current for gate voltages greater than −2 V. The drop in g m is reduced by nearly 50% at the higher gate-to-source voltages. Moreover, the RF figure of merits such as the current gain cutoff frequency (f T) and a maximum frequency of oscillation (f max) have been calculated for both devices. Due to the graded channel f T increases by 20% and f max becomes twice which makes it a better choice for high-voltage and high-power applications. The impact of interface trap and leakage current performance on the graded channel device are also reported. The cutoff frequency reduces by 25% with an increase in trap charge concentration from 9e17 cm−2 to 1e19 cm−2. Also, it has been found that the graded channel device shows better linearity and lower intermodulation distortion in comparison to the abrupt device which has been calculated from the linearity parameters. The high gate leakage current in the case of the graded channel device can be reduced by adding a thin GaN cap layer on the AlGaN channel which can be a future prospect of improving the performance of the graded GaN device.

Robust Compact Model of High-Voltage MOSFET’s Drift Region

This brief presents a compact model to capture the major difference between high-voltage (HV) and low-voltage MOSFETs, i.e., the carrier velocity saturation effect in the drift region of HV MOSFETs. We discuss the numerical and behavioral issues that can arise in SPICE simulations with the existing current-dependent formulation in Berkeley-Short-Channel-IGFET model (BSIM) for HV transistors. We then demonstrate how a voltage-dependent formulation can mitigate them without losing simplicity and accuracy. We also validate the proposed model against experimental data of HV transistors.

Performance of neutron-irradiated 4H-silicon carbide diodes subjected to alpha radiation

The unique electrical and material properties of 4H-silicon-carbide (4H-SiC) make it a promising candidate material for high rate particle detectors. In contrast to the ubiquitously used silicon (Si), 4H-SiC offers a higher carrier saturation velocity and larger breakdown voltage, enabling a high intrinsic time resolution and mitigating pile-up effects. Additionally, as radiation hardness requirements grow more demanding in the context of future high luminosity high energy physics experiments, wide-bandgap materials such as 4H-SiC could offer better performance due to low dark currents and higher atomic displacement thresholds. In this work, the detector performance of 50 µm thick 4H-SiC p-in-n planar pad sensors was investigated at room temperature, using an 241Am alpha source at reverse biases of up to 1100 V. Samples subjected to neutron irradiation with fluences of up to 1 × 1016 neq/cm2 were included in the study in order to quantify the radiation hardness properties of 4H-SiC. A calibration of the absolute number of collected charges was performed using a GATE simulation. The obtained results are compared to previously performed UV transient current technique (TCT) studies. Samples exhibit a drop in charge collection efficiency (CCE) with increasing irradiation fluence, partially compensated at high reverse bias voltages far above full depletion voltage. At fluences of 5 × 1014 neq/cm2 and 1 × 1015 neq/cm2, CCEs of 64 % and 51 % are obtained, decreasing to 15 % at 5 × 1015 neq/cm2. A plateau of the collected charges is observed in accordance with the depletion of the volume the alpha particles penetrate for an unirradiated reference detector. For the neutron-irradiated samples, such a plateau only becomes apparent at higher reverse bias, roughly 600 V and 900 V for neutron fluences of 5 × 1014 neq/cm2 and 1 × 1015 neq/cm2. For the highest investigated fluence, CCE behaves almost linearly with increasing reverse bias. Compared to UV-TCT measurements, the reverse bias required to deplete a sensitive volume covering full energy deposition is lower, due to the small penetration depth of the alpha particles. At the highest reverse bias, the measured CCE values agree well with earlier UV-TCT studies, with discrepancies between 1% and 5%.

Charge collection efficiency study on neutron-irradiated planar silicon carbide diodes via UV-TCT

The material properties of silicon-carbide (SiC) make it a promising candidate for application as a particle detector at high beam rates. In comparison to silicon (Si), the increase in charge carrier saturation velocity and breakdown voltage allows for high intrinsic time resolution while mitigating pile-ups. A larger bandgap and higher atomic displacement threshold energy suppresses dark current and potentially improves radiation hardness, respectively. In addition to the lower susceptibility to temperature variations, it allows the operation of irradiated devices at room temperature and daylight illumination. Although already known for several decades, recent developments in industrial power electronics made SiC more accessible as a potential particle detector. Repeatability, large amplitudes and timing possibilities of signal pulses produced by the ultraviolet transient current technique (UV-TCT) allow for precise performance comparison of differing samples, while efficiently monitoring the influence of alternating external conditions. We present measurement results on the performance of neutron irradiated 4HSiC p-on-n planar diodes using such a setup. Dark current levels remain in the nA range for all fluences (5×1014neq/cm2−1×1016neq/cm2), while charge collection efficiency decreases with irradiation fluence, partially compensated when operating samples at reverse voltage conditions far above full depletion.

Design and Optimization of GeSn Waveguide Photodetectors for 2-µm Band Silicon Photonics.

Silicon photonics is emerging as a competitive platform for electronic–photonic integrated circuits (EPICs) in the 2 µm wavelength band where GeSn photodetectors (PDs) have proven to be efficient PDs. In this paper, we present a comprehensive theoretical study of GeSn vertical p–i–n homojunction waveguide photodetectors (WGPDs) that have a strain-free and defect-free GeSn active layer for 2 µm Si-based EPICs. The use of a narrow-gap GeSn alloy as the active layer can fully cover entire the 2 µm wavelength band. The waveguide structure allows for decoupling the photon-absorbing path and the carrier collection path, thereby allowing for the simultaneous achievement of high-responsivity and high-bandwidth (BW) operation at the 2 µm wavelength band. We present the theoretical models to calculate the carrier saturation velocities, optical absorption coefficient, responsivity, 3-dB bandwidth, zero-bias resistance, and detectivity, and optimize this device structure to achieve highest performance at the 2 µm wavelength band. The results indicate that the performance of the GeSn WGPD has a strong dependence on the Sn composition and geometric parameters. The optimally designed GeSn WGPD with a 10% Sn concentration can give responsivity of 1.55 A/W, detectivity of 6.12 × 1010 cmHz½W−1 at 2 µm wavelength, and ~97 GHz BW. Therefore, this optimally designed GeSn WGPD is a potential candidate for silicon photonic EPICs offering high-speed optical communications.

Graphene FET on Diamond for High-Frequency Electronics

Transistors operating at high frequencies are the basic building blocks of millimeter-wave communication and sensor systems. The high velocity and mobility of carriers in graphene can open way for ultra-fast group IV transistors with similar or even better performance than can be achieved with III-V based semiconductors. However, the progress of high-speed graphene transistors has been hampered due to fabrication issues, influence of adjacent materials, and self-heating effects. Here, we report a graphene field-effect transistor (FET) on a diamond substrate, with a $f_{max}$ up to 54 GHz for a gate length of 500 nm. The high thermal conductivity of diamond provides an efficient heat-sink, and its relatively high optical-phonon energy improves saturation velocity of carriers in the graphene channel. Moreover, we show that graphene FETs on diamond, with different gate lengths, exhibit excellent scaling behavior. These results indicate that graphene FETs on diamond technology can reach sub-terahertz frequency performance.

Assessing the Figures of Merit of Graphene-Based Radio Frequency Electronics: A Review of GFET in RF Technology

Graphene has been extensively investigated in the context of electronic components due to its attractive properties, such as high carrier mobility and saturation velocity. In the past decade, the graphene field-effect transistor (GFET) has been considered one of the potential devices to be used in future radio frequency (RF) applications and can help usher in the Internet of Things and the 5G communication network. This review presents recent developments of GFETs in RF applications with a focus on components such as amplifiers, frequency multipliers, phase shifters, mixers, and oscillators. Initially, the figures of merit (FoMs) for the GFET are briefly described to understand how they affect these RF components. Subsequently, the FoMs of GFET-based RF components are compared with other non-GFET-based RF components. It is found that, due to its zero-band gap and ambipolar characteristics, GFETs are more suitable for use in frequency multiplier and phase shifter applications, outperforming non-GFET-based RF components. Finally, future research on GFETs themselves as well as GFET-based RF components is recommended. This review provides valuable insights into such components that could give rise to innovative applications in industry.

On the Explicit Saturation Drain Current in the Generalized EKV Compact MOSFET Model

We present and discuss explicit closed-form expressions for the saturation drain current of short-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) with gate oxide and interface-trapped charges, and including carrier velocity saturation, according to the generalized Enz–Krummenacher–Vittoz (EKV) MOSFET compact model. The normalized saturation drain current is derived as an explicit function of the normalized terminal voltages by solving the transcendental voltage versus charge equation using the Lambert ${W}$ function. Because this special function is analytically differentiable, other important quantities, such as the transconductance and the transconductance-to-current ratio, can be readily expressed as explicit functions of the terminal voltages.

Analysis of zero-temperature coefficient behavior on vertically stacked double nanosheet nMOS devices

This work presents an experimental study of the zero-temperature coefficient (ZTC) bias point of vertically stacked gate-all-around (GAA) double nanosheet nMOS devices (GAA-NS) for different dimensions, operating in linear and saturation regions. The experimental data is also compared to a simple analytical ZTC model in order to better understand which electrical parameters impact the ZTC behavior. The variation of the threshold voltage with the temperature (ΔVTH/ΔT) and temperature transconductance degradation factor (c) are the two important aspects that most impact the gate to source voltage at ZTC (VZTC). Although the ZTC behavior of the GAA-NS nMOS devices studied in this paper is well described by the simple analytical ZTC model in linear region, at high drain bias, factors such as series resistance and carrier saturation velocity play a significant influence in the ZTC performance of GAA-NS nMOS devices examined in this study.

Compact Modeling of Temperature Effects in FDSOI and FinFET Devices Down to Cryogenic Temperatures

We present compact models that capture published cryogenic temperature effects on silicon carrier mobility and velocity saturation, as well as fully depleted silicon on insulator (FDSOI) and fin field effect transistor (FinFET) devices characteristics within the industry-standard Berkeley short-channel IGFET model (BSIM) framework for cryogenic IC applications such as quantum computing. For the core model charge density/surface potential calculation, we introduce an effective temperature formulation to capture the effects of the band tail states. We also present a compact model that corrects the low-temperature threshold voltage for the band-tail states, Fermi–Dirac statistics, and interface traps. New temperature-dependent mobility and velocity saturation models are accurate down to cryogenic temperature. In addition, we propose that experimentally observed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{D}$ </tex-math></inline-formula> dependence of subthreshold swing (SS) at cryogenic temperatures is a consequence of the expectedly higher rate of Coulomb scattering of free carriers.

Design and modeling of a planar graphene structure as a terahertz cyclotron radiation source

With incredibly high carrier mobility and saturation velocity, graphene would be an ideal candidate for a miniaturized solid-state cyclotron radiation source. A planar semicircular graphene arc geometry was investigated for emission in the 0.5–1.5 THz range. Analytical studies, confirmed by finite element simulations, show that the emitted THz frequencies are inversely proportional to the arc radius given a fixed charge-carrier velocity. The simulations show that the desired frequency spectrum can be obtained with design radii ranging from 50 to 150 nm. Interestingly, the radiated spectrum is independent of the frequency of the stimulation of the graphene nano-arcs. The simulations also indicate that the total output power correlates well with the Larmor formulation. The device is expected to emit 1 nW/cm2, which confirms the findings of existing research in this field. Such a design could yield a scalable and cost-effective THz source.

Carbon Nanotube Based Radio Frequency Transistors for K-Band Amplifiers.

Owing to the combination of high carrier mobility and saturation velocity, low intrinsic capacitance, and excellent stability, the carbon nanotube (CNT) has been considered as a perfect semiconductor to construct radio frequency (RF) field-effect transistors (FETs) and circuits with an ultrahigh frequency band. However, the reported CNT RF FETs usually exhibited poor real performance indicated by the as-measured maximum oscillation frequency (fmax), and then the amplifiers, which are the most important and fundamental RF circuits, suffered from a low power gain and a low frequency band. In this work, we build RF transistors on solution-derived randomly orientated CNT films with improved quality and uniformity. The randomly orientated CNT film FETs exhibit the record as-measured maximum fmax of 90 GHz, demonstrating the potential for over 28 GHz (at least one-third of 90 GHz) 5G mmWave (frequency range 2) applications. Benefiting from the large-scale uniformity of CNT films, FETs are designed and fabricated with a large channel width to present low internal resistance for the standard 50 Ω impedance matching guide line, which is critical to construct an RF amplifier. Furthermore, we first demonstrate amplifiers with a maximum power gain up to 11 dB and output third-order intercept point (OIP3) of 15 dBm, both at the K-band, which represents the record of a CNT amplifier and is even comparable with a commercial amplifier based on III-V RF transistors.

Multi-scale analysis of radio-frequency performance of 2D-material based field-effect transistors.

Two-dimensional materials (2DMs) are a promising alternative to complement and upgrade high-frequency electronics. However, in order to boost their adoption, the availability of numerical tools and physically-based models able to support the experimental activities and to provide them with useful guidelines becomes essential. In this context, we propose a theoretical approach that combines numerical simulations and small-signal modeling to analyze 2DM-based FETs for radio-frequency applications. This multi-scale scheme takes into account non-idealities, such as interface traps, carrier velocity saturation, or short channel effects, by means of self-consistent physics-based numerical calculations that later feed the circuit level via a small-signal model based on the dynamic intrinsic capacitances of the device. At the circuit stage, the possibilities range from the evaluation of the performance of a single device to the design of complex circuits combining multiple transistors. In this work, we validate our scheme against experimental results and exemplify its use and capability assessing the impact of the channel scaling on the performance of MoS2-based FETs targeting RF applications.

Enhanced High-Frequency Performance of Top-Gated Graphene FETs Due to Substrate- Induced Improvements in Charge Carrier Saturation Velocity

The high-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a large extent on the saturation velocity of the charge carriers, a velocity limited by inelastic scattering by surface optical phonons from the dielectrics surrounding the channel. In this work, we show that, by simply changing the graphene channel surrounding dielectric with a material having higher optical phonon energy, one could improve the transit frequency and maximum frequency of oscillation of GFETs. We fabricated GFETs on conventional SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates by adding a thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> interfacial buffer layer on top of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates, a material with about 30% higher optical phonon energy than that of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and compared performance with that of GFETs fabricated without adding the interfacial layer. From S-parameter measurements, a transit frequency and a maximum frequency of oscillation of 43 and 46 GHz, respectively, were obtained for GFETs on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with 0.5- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> gate length. These values are approximately 30% higher than those for state-of-the-art GFETs of the same gate length on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . For relating the improvement of GFET high-frequency performance to improvements in the charge carrier saturation velocity, we used standard methods to extract the charge carrier velocity from the channel transit time. A comparison between two sets of GFETs with and without the interfacial Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer showed that the charge carrier saturation velocity had increased from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\cdot 10^{{7}}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\cdot 10^{{7}}$ </tex-math></inline-formula> cm/s.

Carrier Velocity in Amorphous Metal–Oxide–Semiconductor Transistors

A model for carrier velocity and velocity saturation in amorphous metal oxide thin-film transistors (TFTs) is presented and discussed. The charge transport in such TFTs involves the interplay between the trapping and extended state transport when the mobility is large enough. The model considers multiple trapping and release and band transport between the trapping events. The dominant scattering mechanisms are polar optical phonon scattering and trapped carrier scattering. Our model predicts that the velocity will saturate due to emission of optical phonons at high electric fields (>105 V/cm) with a maximum value near 107 cm/s. Trapping reduces the velocity and also imparts an additional electric field dependence in which the velocity increases with field. The model is compared with the experimental data from several groups. Experimental data are strongly affected by contact resistance effects, which must be corrected for. Procedures to correct for the contact resistance are described. The corrected velocity-field characteristics of six amorphous indium gallium zinc oxide (IGZO) TFTs are presented.

Charge-Based Continuous Equations for the Transconductance and Output Conductance of Graded-Channel SOI MOSFET’s

This paper presents charge-based continuous equations for the transconductance and output conductance of submicrometer Graded-Channel (GC) Silicon-On-Insulator (SOI) nMOSFET. The effects of carrier velocity saturation, channel length modulation and drain-induced barrier lowering were taken into account in the proposed equations. Experimental results were used to test the validity of the equations by comparing not only the transconductance and the output conductance, but also the Early voltage and the open-loop voltage gain, showing a good agreement in a wide range of bias.

Effects of High-Field Velocity Saturation on the Performance of V-Doped 6H Silicon-Carbide Photoconductive Switches

Velocity saturation characteristics of a V-doped 6H silicon-carbide (SiC) photoconductive switch under high electric field are presented. A vertical-geometry switch device based on vanadium-compensated SiC is triggered by a 532-nm laser pulse with light peak power of several hundred kW and operational voltages from 2 to 20 kV. The maximum electrical peak power achieved by the device is up to 1 MW (~50-A, 20-kV, 1.1-ns pulsewidth). When the working voltage is increased to increase output power, the carrier-velocity saturation under high electric field unavoidably impedes the current growth and directly limits the peak power. The parameters of parallel high-field-dependent mobility are obtained with different fields and laser energies: low-field mobility μ = 227 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V s and fitting parameter β = 1.7. With working voltage increasing steadily, a crack-forming process results in device degradation as a result of large current and high electric field. The I - V characteristic curves of the simulations and tests show current saturation and possible damage under high electric field.

Roadmap for gain-bandwidth-product enhanced photodetectors: opinion

Photodetectors are key optoelectronic building blocks performing the essential optical-to-electrical signal conversion, and unlike solar cells, operate at a specific wavelength and at high signal or sensory speeds. Towards achieving high detector performance, device physics, however, places a fundamental limit of the achievable detector sensitivity, such as responsivity and gain, when simultaneously aimed to increasing the detector’s temporal response (speed) known as the gain-bandwidth product (GBP). While detector’s GBP has been increasing in recent years, the average GBP is still relatively modest (∼106-109 Hz-A/W). Here we discuss photoconductor-based detector performance limits and opportunities based on arguments from scaling length theory relating photocarrier channel length, mobility, electrical resistance with optical waveguide mode constrains. We show that short-channel detectors are synergistic with slot-waveguide approaches, and when combined, offer a high-degree of detector design synergy especially for the class of nanometer-thin materials. Indeed, we find that two-dimensional material-based detectors are neither limited by their low mobility nor by associated carrier velocity saturation limitations and can, in principle, allow for 100 GHz fast response rates, which is unlike traditional detector designs that are based on wide channel lengths. However, the contact resistance is still a challenge for such thin photo absorbing materials – a research topic that is still not addressed yet. An interim solution is to utilize heterojunction approaches for functionality separation. Nonetheless, atomistic and nanometer-thin materials used in such next-generation scaling length theory based detectors also demand high material quality and monolithic integration strategies into photonic circuits including foundry-near processes. As it stands, this letter aims to guide the community if achieving the next generation photodetectors aiming for a performance target of GBP ∼ 1012 Hz-A/W.

Effects of Self-Heating on ${f}_{\text{T}}$ and ${f}_{\text{max}}$ Performance of Graphene Field-Effect Transistors

It has been shown that there can be a significant temperature increase in graphene field-effect transistors (GFETs) operating under high drain bias, which is required for power gain. However, the possible effects of self-heating on the high-frequency performance of GFETs have been weakly addressed so far. In this article, we report on an experimental and theoretical study of the effects of self-heating on dc and high-frequency performance of GFETs by introducing a method that allows accurate evaluation of the effective channel temperature of GFETs with a submicrometer gate length. In the method, theoretical expressions for the transit frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) and the maximum frequency of oscillation (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) based on the small-signal equivalent circuit parameters are used in combination with the models of the field- and temperature-dependent charge carrier concentration, velocity, and saturation velocity of GFETs. The thermal resistances found by our method are in good agreement with those obtained by the solution of the Laplace equation and by the method of thermo-sensitive electrical parameters. Our experiments and modeling indicate that the self-heating can significantly degrade the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of GFETs at power densities above 1 mW/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , from approximately 25 to 20 GHz. This article provides valuable insights for further development of GFETs, taking into account the self-heating effects on the high-frequency performance.

Does carrier velocity saturation help to enhance f max in graphene field-effect transistors?

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (fmax). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced fmax. We have fabricated 500 nm GFETs with high extrinsic fmax (37 GHz), and later simulated with a drift–diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher the fmax are. However, the highest fmax can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highest fmax is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highest fmax depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highest fmax from ∼60 to ∼40 GHz.