Gate-length High Electron Mobility Transistors Research Articles

Impact of the Channel Thickness on Electron Confinement in MOCVD‐Grown High Breakdown Buffer‐Free AlGaN/GaN Heterostructures

The 2D electron gas (2DEG) confinement on high electron mobility transistor (HEMT) heterostructures with a thin undoped GaN channel layer on the top of a grain‐boundary‐free AlN nucleation layer is studied. This is the first time demonstration of a buffer‐free epi‐structure grown with metal–organic chemical vapor deposition with thin GaN channel thicknesses, ranging from 250 to 150 nm, without any degradation of the structural quality and 2DEG properties. The HEMTs with a gate length of 70 nm exhibit good DC characteristics with peak transconductances of 500 mS mm−1and maximum saturated drain currents above 1 A mm−1. A thinner GaN channel layer improves 2DEG confinement because of the enhanced effectiveness of the AlN nucleation layer acting as a back‐barrier. An excellent drain‐induced barrier lowering of only 20 mV V−1at aVDSof 25 V and an outstanding critical electric field of 0.95 MV cm−1are demonstrated. Good large‐signal performance at 28 GHz with output power levels of 2.0 and 3.2 W mm−1and associated power‐added efficiencies of 56% and 40% are obtained at aVDSof 15 and 25 V, respectively. These results demonstrate the potential of sub‐100 nm gate length HEMTs on a buffer‐free GaN‐on‐SiC heterostructure.

Self-Heating and Equivalent Channel Temperature in Short Gate Length GaN HEMTs

In this paper, we study the self-heating mechanism and its impact on electrical performance of short gate length GaN high electron mobility transistors (HEMTs) based on electrothermal TCAD simulations. We propose an equivalent channel temperature to quantify the current degradation due to self-heating and also resolve the discrepancies between temperature measurements through electrical methods and thermal methods in the literature. We then explain the equivalent channel temperature’s behavior using the temperature- and field-dependent electron transport theory for short gate length HEMTs. The implications and guidelines to the various aspects of device design are also discussed.

Investigations of the Effect of Temperature-Dependent Access Resistances on Compact Model Parameters Extraction of AlGaN/GaN HEMTs

In this paper, the effect of temperature-dependent access resistances (TDARs) on intrinsic parameters extraction of compact model in AlGaN/GaN high electron–mobility transistors (HEMTs) is analyzed. Comparison of small-signal, large-signal, and dynamic load line results between the proposed method and conventional method are presented. It reveals that considering the TDARs effect can improve the accuracy of the large-signal model at high input power conditions. This is attributed to the improved accuracy of the extracted intrinsic parameters. Moreover, the nonlinearity of gate-drain series resistance (Rgd) is modeled for the first time. It shows that the consideration of nonlinear Rgd model can extend the applicable bias-voltage range of the large-signal model. A 0.15 μm gate-length GaN HEMT on SiC substrate is used for method validation. The results of the paper can be useful for millimeter-wave GaN circuits design.

Impact of barrier thickness on transistor performance in AlN/GaN high electron mobility transistors grown on free-standing GaN substrates

A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.

Engineering the (In, Al, Ga)N back-barrier to achieve high channel-conductivity for extremely scaled channel-thicknesses in N-polar GaN high-electron-mobility-transistors

Scaling down the channel-thickness (tch) in GaN/(In, Al, Ga)N high-electron-mobility-transistors (HEMTs) is essential to eliminating short-channel effects in sub 100 nm gate length HEMTs. However, this scaling can degrade both charge density (ns) and mobility (μ), thereby reducing channel-conductivity. In this study, the back-barrier design in N-polar GaN/(In, Al, Ga)N was engineered to achieve highly conductive-channels with tch < 5-nm using metal organic chemical vapor deposition. Compositional-grading was found to be the most effective approach in reducing channel-conductivity for structures with tch ∼ 3-nm. For a HEMT with 3-nm-thick-channel, a sheet-resistance of 329 Ω/◻ and a peak-transconductance of 718 mS/mm were demonstrated.

Proposal of normally-off InN-channel high-electron mobility transistors

We propose N-polar normally-off InN-channel high-electron mobility transistors (HEMTs). A calculated energy band diagram shows full depletion of a quantum well channel provided by a negative polarization charge at an InN-channel/GaN-cap junction. For a 100 nm long gate-length HEMT with a 3 nm thick InN channel, 0.4 nm GaN cap and 4 nm thick HfO2/ZrO2 insulator we calculate a threshold voltage VT = 0.5 V and a transconductance of 3 S mm−1. Selective plasma etching of the GaN cap over the InN channel may facilitate monolithic integration of normally-off and -on InN channel HEMTs in a new generation of extremely fast digital circuits.

AlGaN/GaN HEMTs on Silicon Substrate With 206-GHz $F_{ \rm MAX}$

This letter reports on AlGaN/GaN high-electron-mobility transistors (HEMTs) on high-resistive silicon substrate with a record maximum oscillation cutoff frequency FMAX. Double-T-shaped gates are associated with an optimized technology to enable high-efficiency 2-D electron gas control while mitigating the parasitic resistances. Good results ogate-length HEMTf FMAX = 206 GHz and FT = 100 GHz are obtained for a 90-nm gate-length HEMT with 0.25-μm source-to-gate spacing. The associated peak extrinsic transconductance value is as high as 440 mS·mm-1. To the authors' knowledge, the obtained FMAX and Gmext are the highest reported values for GaN HEMTs technology on silicon substrate. The accuracy of the cutoff frequency values is checked by small-signal modeling based on extracted S-parameters.

RF High-Power Operation of AlGaN/GaN HEMTs Epitaxially Grown on Diamond

We epitaxially grow AlGaN/GaN high-electron-mobility transistors (HEMTs) on IIa-type single-crystal diamond (111) substrates. A 0.4-μm gate-length HEMT showed a dc drain-current density IDS of 770 mA/mm and a breakdown voltage of 165 V. In the RF large-signal measurements at 1 GHz, an RF output-power density POUT of 2.13 W/mm was obtained. This is the first report of RF power operation of AlGaN/GaN HEMTs epitaxially grown on diamond. The AlGaN/GaN HEMTs epitaxially grown on diamond showed a low thermal resistance of 1.5 K·mm/W.

Fabrication of 150-nm T-Gate Metamorphic AlInAs/GaInAs HEMTs on GaAs Substrates by MOCVD

Metamorphic AlInAs/GaInAs high-electron-mobility transistors (HEMTs) of 150-nm gate length with very good device performance have been grown by metal-organic chemical vapor deposition, with the introduction of an effective multistage buffering scheme. By using a combined optical and e-beam photolithography technology, submicrometer mHEMT devices have been achieved. The devices exhibit good dc and RF performance. The maximum transconductance was 1074 mS/mm. The nonalloyed ohmic contact resistance <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rc</i> was as low as 0.02 Ω·mm. The unity current gain cutoff frequency ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fT</i> ) and the maximum oscillation frequency ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) were 279 and 231 GHz, respectively. This device has the highest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fT</i> yet reported for 150-nm gate-length HEMTs. Also, an input capacitance to gate-drain feedback capacitance ratio <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gd</sub> of 3.2 is obtained in the device.

Improved modeling method to account for the kink effect of gaas phemts

Short gate-length High Electron Mobility Transistors (HEMTs) have been observed to exhibit kinks in their drain current-voltage (I–V) characteristics. To model this nonlinear effect, we present an effective approach that is easily incorporated into most existing empirical HEMT I–V models. This has been done by modifying the channel length modulation parameter (λ) to account for the kink effect. Moreover, the definitions of the left parameters in the original model will not be influenced, and the improved HEMT I–V model enhances its bias range of operation for which accuracy is maintained. The proposed modeling method is validated through DC/ Pulsed I–V as well as large-signal power measurements.

AlGaN/GaN High Electron Mobility Transistor degradation under on- and off-state stress

AlGaN/GaN High Electron Mobility Transistors (HEMTs) with various gate lengths have been step-stressed under both on- and off-state conditions. On-state, high power stress tests were performed on 0.17 μm gate length HEMTs and a single 5 μm spaced TLM pattern. Significant degradation of the submicron HEMTs as compared to the excellent stability of the TLM patterns under the same stress conditions reveal that the Schottky contact is the source of degradation. Off-state stress showed a linear relationship between the critical degradation voltage and gate length, though two dimensional ATLAS/Blaze simulations show that the maximum electric field is similar for all gate lengths. Additionally, as the drain bias was increased, the critical voltage decreased. However, the cumulative bias between the gate and drain remained constant, further indicating that the electric field is the main mechanism for degradation.

Characterization of gate recessed GaN/AlGaN/GaN high electron mobility transistors fabricated using a SiCl4/SF6 dry etch recipe

Incorporating GaN capping layers in conjunction with recessing has been identified as a means to maximize the high frequency performance of AlGaN/GaN high electron mobility transistors (HEMTs). Doping the cap heavily n-type is required in order to ensure minimal loss of carriers from the channel. Using a SiCl4/SF6 dry etch plasma recipe, 250 nm gate length HEMTs with recess lengths varying from 300 nm to 5 μm are fabricated. Heavily doped n+GaN caps enabled contact resistances of 0.3 Ω mm to be achieved. Recessing using a SiCl4/SF6 recipe does not introduce significant numbers of bulk traps. Gate recessing in conjunction with Si3N4 passivation reduces rf dispersion to negligible levels.

T-gate geometric (solution for submicrometer gate length) HEMT: Physical analysis, modeling and implementation as parasitic elements and its usage as dual gate for variable gain amplifiers

In order to achieve superior RF performance, short gate length is required for the compound semiconductor field effect transistors, but the limitation in lithography for submicrometer gate lengths leads to the formation of various metal–insulator geometries like T-gate [Sandeep R. Bahl, Jesus A. del Alamo, Physics of breakdown in InAlAs/ n +-InGaAs heterostructure field-effect transistors, IEEE Trans. Electron Devices 41 (12) (1994) 2268–2275]. These geometries are the combination of various Metal-Semiconductor (MS)/Metal–Air-Semiconductor (MAS) contacts. Moreover, field plates [S. Karmalkar, M.S. Shur, G. Simin, M. Asif Khan, Field-plate engineering for HFETs, IEEE Trans. Electron Devices 52 (2005) 2534–2540] are also being fabricated these days, mainly at the drain end ( Γ -gate) having Metal–Insulator-Semiconductor (MIS) instead of MAS contact with the intention of increasing the breakdown voltage of the device. To realize the effect of upper gate electrode in the T-gate structure and field plates, an analytical model has been proposed in the present article by dividing the whole structure into MS/MIS contact regions, applying current continuity among them and solving iteratively. The model proposed for Metal–Insulator Semiconductor High Electron Mobility Transistor (MISHEMT) [R. Gupta, S.K. Aggarwal, M. Gupta, R.S. Gupta, Analytical model for metal insulator semiconductor high electron mobility transistor (MISHEMT) for its high frequency and high power applications, J. Semicond. Technol. Sci. 6 (3) (2006) 189–198], is equally applicable to High Electron Mobility Transistors (HEMT) and has been used to formulate this model. In this paper, various structures and geometries have been compared to anticipate the need of T-gate modeling. The effect of MIS contacts has been implemented as parasitic resistance and capacitance and has also been studied to control the middle conventional gate as in dual gate technology by applying separate voltages across it. The results obtained using the proposed analytical scheme has been compared with simulated and experimental results, to prove the validity of our model.

Influence of high-temperature AlN interlayer on the electrical properties of AlGaN/GaN heterostructure and HEMTs

GaN buffer with insertion of a 40-nm-thick high-temperature （HT） AlN interlayer is studied. The HT-AlN interlayer enhances the in-plane compressive strain of GaN film and thereby improves the electrical properties of AlGaN/GaN high electron mobility transistors （HEMTs）. Base on the precise measurement of Bragg angle, the strain states of GaN are calculated. It is found that the increased compressive stress enhances the piezoelectric polarization field in GaN, which consequently causes accumulation of more electrons at the AlGaN/GaN interface. On the other hand, the influence of AlGaN layer induced by the enhanced compressive stress on the two-dimensional electron gas （2DEG） sheet carrier density, both positive and negative, are proved to counteract each other. Meanwhile, the employment of the HT-AlN interlayer reduces the lattice mismatch between the GaN and AlGaN and smoothes the AlGaN/GaN interface, thus increases the 2DEG mobility by weakening the interface roughness scattering. The 1-μm gate-length HEMTs by using the GaN buffer layer with the HT-AlN interlayer are fabricated. The measurements show that the maximum drain current and transconductance are increased by 42% and 20% respectively compared with the conventional HEMTs without HT-AlN interlayer.

Optical and electrical characteristics of GaAs/InGaAs quantum-well device

A GaAs/InGaAs quantum-well structure was grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The quantum well was graded from 25% to 15% indium (from the bottom to the top of the channel). Hall measurements were made to characterize the concentration and mobility of the two-dimensional electron gas (2DEG). The temperature-dependent photoluminescence (PL) and photoreflectance (PR) spectra of the structure of interest were obtained. Various intersuband features were observed in the PR spectra. Furthermore, a 1.5 μm gate-length high-electron mobility transistor (HEMT), fabricated on these layers, had an extrinsic transconductance of 127 mS/mm. The optical and electrical characteristics were determined simultaneously.

AlGaN/AlN/GaN/SiC HEMT structure with high mobility GaN thin layer as channel grown by MOCVD

Enhancement of the electrical properties in an AlGaN/GaN high electron mobility transistor (HEMT) structures was demonstrated by employing the combination of a high mobility GaN channel layer and an AlN interlayer. The structures were grown on 50 mm semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD). The room temperature (RT) two-dimensional electron gas (2DEG) mobility was as high as 2215 cm 2/V s, with a 2DEG concentration of 1.044×10 13 cm −2. The 50 mm HEMT wafer exhibited a low average sheet resistance of 251.0 Ω/square, with a resistance uniformity of 2.02%. The 0.35 μm gate length HEMT devices based on this material structure, exhibited a maximum drain current density of 1300 mA/mm, a maximum extrinsic transconductance of 314 mS/mm, a current gain cut-off frequency of 28 GHz and a maximum oscillation frequency of 60 GHz. The maximum output power density of 4.10 W/mm was achieved at 8 GHz, with a power gain of 6.13 dB and a power added efficiency (PAE) of 33.6%.

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.

Microwave noise performances of AlGaN/GaN HEMTs on semi-insulating 6H-SiC substrates

High-performance, low-noise AlGaN/GaN high electron mobility transistors (HEMTs) with 0.25 µm gate-length have been fabricated on semi-insulating 6H‐SiC substrates. The devices exhibited a unity current gain cutoff frequency (fT) of 52.3 GHz, and maximum frequency of oscillation (fMAX) of 112 GHz. At 10 GHz, a minimum noise figure (NFmin) of 0.75 dB and an associated gain (Ga) of 10.84 dB was obtained when biased at VDS=10 V and IDS=40.6 mA/mm. The corresponding values were 1.15 and 7.49 dB at 18 GHz. These results are the first reported microwave noise characteristics obtained from 0.25 µm gate-length GaN HEMTs on 6H‐SiC substrates. The use of 6H‐SiC substrates provides an alternative solution to the full commercialisation of GaN-based technologies for low-noise and high-power electronics.

High performance 0.25 μm gate-length AlGaN/GaN HEMTs on sapphire with transconductance of over 400 mS/mm

Metal organic chemical vapour deposition-grown AlGaN/GaN high electron mobility transistors (HEMTs) having record high transconductance of over 400 mS/mm have been fabricated on sapphire substrates. These 0.25 µm gate-length devices exhibited maximum drain current density as high as 1.4 A/mm, unity gain cutoff frequency ( fT) of 85 GHz, and maximum frequency of oscillation ( fmax) of 151 GHz. The fT of 85 GHz and fmax of 151 GHz are the highest ever reported values for 0.25 µm gate-length GaN-based HEMTs.

Ultrafast monolithic receiver OEIC composed of multimode waveguide p-i-n photodiode and HEMT distributed amplifier

A multimode waveguide p-i-n photodiode (WGPD) and a distributed baseband amplifier consisting of high-electron mobility transistors (HEMTs) were monolithically integrated on InP substrate using a stacked layer structure for both components. The multimode WGPD has a 3-dB bandwidth of 49 GHz. The distributed baseband amplifier has a 3-dB bandwidth of 47 GHz, though its 0.5-/spl mu/m gate-length HEMTs have modest cutoff frequencies f/sub T//f/sub max/ of 47/100 GHz. The receiver optoelectronic integrated circuit has a bandwidth of 46.5 GHz. It was packaged into a fiber-pig-tailed module, and the WGPD in the module has a high responsivity of 0.62 A/W for 1.55-/spl mu/m wavelength. The module achieves a sensitivity of -22.7 dBm at 40 Gb/s and exhibits a clear eye-opening at 50 Gb/s.