35-nm Zigzag T-Gate $hbox{In}_{0.52}hbox{Al}_{0.48} hbox{As/In}_{0.53}hbox{Ga}_{0.47}hbox{As}$ Metamorphic GaAs HEMTs With an Ultrahigh $f_{max}$ of 520 GHz

Metamorphic GaAs high electron mobility transistors (mHEMTs) with the highest-f _max reported to date are presented here. The 35-nm zigzag T-gate In_0.52Al_0.48As/In_0.53Ga_0.47As metamorphic GaAs HEMTs show f _maxof 520 GHz, f _T of 440 GHz, and maximum transconductance (g _m) of 1100 mS/mm at a drain current of 333 mA/mm. The combinations of f _max and f _T are the highest data yet reported for mHEMTs. These devices are promising candidates for aggressively scaled sub-35-nm T-gate mHEMTs.     

A study of noise in surface and buried channel SiGe MOSFETs with gate oxide grown by low temperature plasma anodization

A study is made of noise in p- and n-channel transistors incorporating SiGe surface and buried channels, over the frequency range f=1 Hz–100 kHz. The gate oxide is grown by low temperature plasma oxidation. Surface n-channel devices are found to exhibit two noise components namely 1/f and generation–recombination (GR) noise. It is shown that the 1/f noise component is due to fluctuations of charge in slow oxide traps whilst bulk centers located in a thin layer of the semiconductor close to the channel, give rise to the GR noise component. The analysis of the noise data gives values for the density D_ot of the oxide traps in the SiGe and Si nMOSFETs of the order 1.8×10¹² and 2.5×10¹⁰ cm⁻² (eV)⁻¹, respectively. The density D_GR of the bulk GR centres is equal to 3×10¹⁰ cm⁻² in both the SiGe and Si devices. The electron and hole capture cross-sections for these centres as well as their energy level and their depth below the oxide/semiconductor interface are also the same in the devices of both types. This suggests that those GR centers are of the same nature in all devices studied. p-Channel devices show different behaviour with only a 1/f noise component apparent in the data over the same frequency range. Buried SiGe channel and Si control devices exhibit quite low and similar slow state densities of the order low to mid 10¹⁰ cm⁻² (eV)⁻¹ whereas surface p-channel devices show even higher slow state densities than n-channel counterparts. The Hooge noise characterized by the Hooge coefficient _H=2×10⁻⁵ is also detected in some buried p-channel SiGe devices.     

Two-dimensional analysis of the surface state effects in 4H-SiC MESFETs

Two-dimensional small-signal ac and transient analysis of surface trap effects in 4H-SiC MESFETs have been performed in this paper. The mechanism by which acceptor-type traps effect the transconductance and drain current changes has been discussed. The simulation results show that transconductance exhibits negative frequency dispersion behavior, which is caused by the charge exchange via the surface states existing between the gate-source and gate-drain terminals. The current degradation behavior is also observed due to acceptor-type traps, acting as electron traps, in MESFET devices. A detailed study involving the density, ionization and energy level of traps reveals conclusive results in the devices analyzed.     

Trap studies in GaInP/GaAs and AlGaAs/GaAs HEMT's by means oflow-frequency noise and transconductance dispersion characterizations

The presence of traps in GaInP/GaAs and AlGaAs/GaAs HEMT's was investigated by means of low frequency noise and frequency dispersion measurements. Low frequency noise measurements showed two deep traps (E _a1=0.58 eV, E_a2=0.27 eV) in AlGaAs/GaAs HEMT's. One of them (E_a2) is responsible for the channel current collapse at low temperature. A deep trap (E_a1'=0.52 eV) was observed in GaInP/GaAs HEMT's only at a much higher temperature (~350 K). These devices showed a transconductance dispersion of ~16% at 300 K which reduced to only ~2% at 200 K. The dispersion characteristics of AlGaAs/GaAs HEMT's were very similar at 300 K (~12%) but degraded at 200 K (~20%). The low frequency noise and the transconductance dispersion are enhanced at certain temperatures corresponding to trap level crossing by the Fermi-level. The transition frequency of 1/f noise is estimated at 180 MHz for GaInP/GaAs HEMT's and resembles that of AlGaAs/GaAs devices     

AlGaN/GaN MOS-HEMT With $hbox{HfO}_{2}$ Dielectric and $hbox{Al}_{2}hbox{O}_{3}$ Interfacial Passivation Layer Grown by Atomic Layer Deposition

We have developed a novel AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor using a stack gate HfO₂/Al₂O₃ structure grown by atomic layer deposition. The stack gate consists of a thin HfO₂ (30-A) gate dielectric and a thin Al₂O₃ (20- A) interfacial passivation layer (IPL). For the 50-A stack gate, no measurable C-V hysteresis and a smaller threshold voltage shift were observed, indicating that a high-quality interface can be achieved using a Al₂O₃ IPL on an AlGaN substrate. Good surface passivation effects of the Al₂O₃ IPL have also been confirmed by pulsed gate measurements. Devices with 1- mum gate lengths exhibit a cutoff frequency (fT) of 12 GHz and a maximum frequency of oscillation (f _MAX) of 34 GHz, as well as a maximum drain current of 800 mA/mm and a peak transconductance of 150 mS/mm, whereas the gate leakage current is at least six orders of magnitude lower than that of the reference high-electron mobility transistors at a positive gate bias.     

0.25 μm Self-Aligned AlGaN/GaN High Electron Mobility Transistors

Self-aligned AlGaN/GaN high electron mobility transistors grown on semiinsulating SiC substrates with a 0.25 mum gate-length were fabricated using a single-step ohmic process. Our recently developed Mo/Al/Mo/Au-based ohmic contact requiring annealing temperatures between 500degC and 600degC was utilized. Ohmic contact resistances between 0.35-0.6 Omega ldr mm were achieved. These 0.25 mum gate-length devices exhibited drain current density as high as 1.05 A/mm at a gate bias of 0 V and a drain bias of 10 V. A knee voltage of less than 2 V and a peak extrinsic transconductance (g_m ) of 321 mS/mm were measured. For their microwave characteristics, a unity gain cutoff frequency (f_T ) of 82 GHz and maximum frequency of oscillation (f _max) of 103 GHz were measured.     

Interpretation of transconductance dispersion in GaAs MESFET usingdeep level transient spectroscopy

The negative transconductance dispersion in a GaAs metal-semiconductor field-effect transistor (MESFET) was interpreted using both surface leakage current and capacitance deep level transient spectroscopy (DLTS) measurements. The transconductance of the device was reduced by 10% in the frequency range of 10 Hz ~1 kHz. The transition frequency shifted to higher frequency region with the increase of device temperature. The activation energy for the change of the transition frequency was determined to be 0.66±0.02 eV. It was found that the activation energy for the conductance of electrons on the surface of GaAs was 0.63±0.01 eV. In the DLTS spectra, two types of hole-like signals with activation energies, 0.65±0.07 eV (H1) and 0.88±0.04 eV (H2), were observed. The activation energy of H1 trap agrees well with those obtained from the transconductance dispersion and surface leakage current measurements. This demonstrates that surface state H1 causes the generation of surface leakage current, leading to the transconductance dispersion in the MESFET. Using the experimental results, a model for the evolution of hole-like signal by surface states in the capacitance DLTS is proposed     

Transport properties of AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors with Al2O3 of different thickness

The Al₂O₃ as a gate oxide and passivation was used to study the transport properties of AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOSHFETs). Performance of the devices with Al₂O₃ of different thickness between 4 and 14 nm prepared by metal–organic chemical vapor deposition (MOCVD) and with 4 nm thick Al₂O₃ prepared by Al sputtering and oxidation was investigated. All MOS-devices yielded higher transconductance than their HFET counterparts, i.e. the transconductance/capacitance expected proportionality assuming the same carrier velocity was not fulfilled. A different electric field near/below the gate contact due to a reduction of traps is responsible for the carrier velocity enhancement in the channel of the MOSHFET. The trap reduction depends on the oxide used, as follows from the capacitance vs frequency dispersion for devices investigated. It is qualitatively in a good agreement with the different velocity enhancement evaluated, and devices with thinner oxide show higher traps reduction as well as higher transconductance enhancement. It is also shown that obtained conclusions can be applied well on performance of SiO₂/AlGaN/GaN MOSHFETs.     

Trap characterization in buried-gate n-channel 6H-SiC JFETs

We present a detailed characterization of deep traps present in buried gate, n-channel 6H-SiC JFETs, based on transconductance measurements as a function of frequency. Four different deep levels have been identified, which are characterized by activation energies of 0.16, 0.18, 0.28, and 0.54 eV. Furthermore, based on the transconductance frequency dispersion features (upward or downward dispersion), we have been able to infer that three deep levels (0.16, 0.18 and 0.54 eV) are hole traps localized in the p-gate layer and one (0.28 eV) is an electron trap localized in the n-channel     

30-nm InAs Pseudomorphic HEMTs on an InP Substrate With a Current-Gain Cutoff Frequency of 628 GHz

We report on InAs pseudomorphic high-electron mobility transistors (PHEMTs) on an InP substrate with record cutoff frequency characteristics. This result was achieved by paying attention to minimizing resistive and capacitive parasitics and improving short-channel effects, which play a key role in high-frequency response. Toward this, the device design features a very thin channel and is fabricated through a three-step recess process that yields a scaled-down barrier thickness. A 30-nm InAs PHEMT with t _ins = 4 nm and t _ch = 10 nm exhibits excellent g _m, _max of 1.62 S/mm, f _T of 628 GHz, and f _max of 331 GHz at V _DS = 0.6 V . To the knowledge of the authors, the obtained f _T is the highest ever reported in any FET on any material system. In addition, a 50-nm device shows the best combination of f _T= 557 GHz and f _max = 718 GHz of any transistor technology.     

Limitations of the MIS capacitance method resulting from semiconductor properties

The influence of doping profiles, traps and grain boundaries in the semiconductor on the C-V characteristics of ideal MIS diodes is calculated. In addition, it is shown that losses and frequency dispersion as well as hysteresis effects can result from traps in the semiconductor. If these semiconductor properties are ignored misinterpretations in terms of interface state density will arise which can be about 10¹² cm⁻². In the case of decreasing doping profiles and traps in a n-type semiconductor one will get an apparent positive surface charge while an apparent negative surface charge will result from increasing profiles and grain boundaries. A method for the experimental determination of the C-V characteristics of ideal MIS diodes is developed. This method is applicable to stable diodes with only shallow traps in the semiconductor.     

基于陷阱的4H-SiC MESFET频散效应分析

在建立正确模型的基础上,运用ISE软件的二维仿真,模拟了4H-SiC MESFET在交流小信号条件下,表面陷阱和体陷阱对跨导和漏电导随频率变化的影响。分析了产生频散效应的原因以及内部机理,同时考虑了不同环境温度对跨导的频散影响。结果表明,在低频条件下,陷阱会导致跨导和漏电导频散,漏电压越大,环境温度越高,频散越不明显。     

Low-frequency transconductance dispersion in InAlAs/InGaAs/InPHEMT's with single- and double-recessed gate structures

The effects of trapping mechanisms on the transconductance of single- and double-recessed InAlAs/InGaAs/InP HEMT's are examined. Measurements at room temperature indicate transconductance dispersion occurring primarily between 100 Hz and 1 MHz. A detailed examination of the dispersion yields two mechanisms with different activation energies which were determined by measuring the transition frequencies as functions of temperature. One mechanism, causing negative dispersion, has an activation energy of 0.17 eV and was found only in the double-recessed structure. The other mechanism, causing positive dispersion and common to both structures, has a dominant transition with an activation energy of 0.51 eV at low fields. The first mechanism appears to be associated with surface states, while the second is caused by electron traps in the InAlAs or its interface with the InGaAs channel. Transient response measurements were also used to examine the location of the traps and to study the field dependence of the characteristic times     

Numerical analysis of frequency dispersion of transconductance inGaAs MESFETs

A fully two-dimensional numerical model for the transconductance dispersion in GaAs MESFETs is presented. According to simulated results, the dominating surface traps belong to the hole trap type in order to obtain consistent results with reported measurements. The AC frequency-dependent modulation of negative surface charge can explain this anomalous phenomenon. The holes injecting from and emitting out of the gate metal electrode interact with the surface hole traps, and result in the change of the gate-to-source and the gate-to-drain resistances, which in turn cause the change in transconductance. The gate voltage and the gate length effects on the dispersion are also considered. Good agreement with reported results is obtained     

Low-frequency noise in Si0.7Ge0.3 surface channel pMOSFETs with ALD HfO2/Al2O3 gate dielectrics

Low-frequency noise was characterized in Si_0.7Ge_0.3 surface channel pMOSFETs with ALD Al₂O₃/HfO₂/Al₂O₃ stacks as gate dielectrics. The influences of surface treatment prior to ALD processing and thickness of the Al₂O₃ layer at the channel interface were investigated. The noise was of the 1/f type and could be modeled as a sum of a Hooge mobility fluctuation noise component and a number fluctuation noise component. Mobility fluctuation noise dominated the 1/f noise in strong inversion, but the number fluctuation noise component, mainly originating from traps in HfO₂, also contributed closer to threshold and in weak inversion. The number fluctuation noise component was negligibly small in a device with a 2 nm thick Al₂O₃ layer at the SiGe channel interface, which reduced the average 1/f noise by a factor of two and decreased the device-to-device variations.     

Static and dynamic transconductance of MOSFETs

A model for the dynamic transconductance of small-channel-length MOSFETs operating in the linear region is presented. The model includes the effects of interface traps and their frequency-dependent admittance, Coulombic scattering due to all charges near the Si-SiO₂ interface, and surface roughness scattering. This model is used to explain the behavior of measured devices that have been subjected to ionizing radiation that introduces charges in the insulator and at the insulator-semiconductor interface. In particular, it is shown that the transconductance peak increases with the frequency of operation. The size of this effect is related mainly to the density of interface traps but is also controlled by the number of trapped charges near the Si-SiO ₂ interface, which increases during irradiation. Static and high-frequency measurements of the transconductance of n-channel MOSFETs are compared with simulated results using the proposed model     

Low-frequency dispersion of transconductance in GaAs JFETs andMESFETs with an ion-implanted channel layer

An analytical model of low-frequency dispersion of transconductanced in GaAs FETs which have nonuniform profiles of carrier concentration and mobility is reported. The frequency dependence of surface charge density is incorporated into the model as a variation in the source resistance of the FETs. The model explains the low-frequency dispersion of transconductance in GaAs p-n junction FETs (JFETs) and metal-semiconductor FETs (MESFETs), both of which have a channel layer formed by ion implantation. It is suggested that the low-frequency dispersion of transconductance can be attributed to the charge exchange which occurs with the surface states in GaAs FETs     

Surface influence on the conductance DLTS spectra of GaAs MESFET's

The observation of hole traps in small-signal GaAs MESFET's has been extensively reported in the literature. Previously these have been attributed to trapping at the active layer-substrate interface. Evidence is presented here, based on conductance DLTS and low-field low-frequency transconductance dispersion measurements on MESFET's of various geometries, to suggest that the main contribution to the "hole trap-like" spectrum in conductance DLTS is not bulk hole traps. Instead we believe that this phenomenon arises from changes in the population of surface states in the ungated access regions of the device, resulting in modulation of the surface depletion layer in series with the gate depletion region.     

A transconductance spectroscopy approach to device level surfacestate characterization

A device level transconductance spectroscopy approach is developed for characterizing surface states in metal-semiconductor field-effect transistors. A comparison of the theoretical results and the available experimental observations shows that the model can successfully explain both the surface leakage current dependence of transconductance dispersion magnitude reported by M. Ozeki et al. (1982) and the temperature dependence of transconductance dispersion observed by S.R. Blight et al. (1986)     

Determination of interface and bulk traps in the subthreshold region of polycrystalline silicon thin-film transistors

A simple method to determine the Interface and bulk density of states in polycrystalline silicon thin-film transistors is presented. The energy distribution of the interface trap density has been extracted from analysis of the transfer characteristics in the subthreshold region of operation. Using the obtained interface state distribution, the energy distribution of the bulk traps has been determined by fitting the surface potential at each gate voltage with an analytical theoretical model. Both interface and bulk traps were found to consist of deep states with constant density near the mid-gap and band-tails with density increasing exponentially with the energy when the trap energy approaches the conduction band-edge.