2DEG Sheet Carrier Concentration Research Articles

A Dielectrically Modulated GaN/AlN/AlGaN MOSHEMT with a Nanogap Embedded Cavity for Biosensing Applications

In this work, GaN/AlN/AlGaN MOS-HEMT with a cavity below the gate towards the drain side is studied for its sensitivity analysis and viability as a biosensor. The analysis is done by dielectric modulation of the cavity region to emulate the presence of different dielectric biomolecules and charged biomolecules by interface charge variation. MOSFET-based dielectrically modulated sensors have been a success experimentally and this work extends and demonstrates this concept with GaN HEMT. The device performance is evaluated through the shift in threshold voltage (V th) and drain current (I DS), which are used as performance metrics. The proposed device structure simulations were performed with ATLAS Silvaco device simulation tool which depicts the bio-immobilization in the cavity leads to the changes in electrostatic properties like conduction band offset, two-dimensional electron gas (2DEG) sheet carrier concentration and channel potential. The simulation analysis reveals V th and I DS shift up to 1.1 V and 153.7 mA/mm for the neutral biomolecules, whereas for deoxyribo nucleic acid, the shift is up to 0.30 mV and 65.2 mA/mm, respectively, implying a highly sensitive device. The AlGaN layer thickness and cavity fill height variations on device sensitivity are also reported.

Analytical modeling of 2DEG with 2DHG polarization charge density drain current and small‐signal model of quaternary AlInGaN HEMTs for microwave frequency applications

AbstractA physics‐based analytical model for quaternary AlInGaN high electron mobility transistors (HEMTs) is developed including two‐dimensional electron gas (2DEG) and two‐dimensional hole gas (2DHG) for microwave applications. The DC and RF performance characteristics are explored by considering the quasi‐triangular quantum well (QW). The derived charge carrier densities nsand psare considered for energy subbands Eo and E1 inside QW. The 2DEG sheet carrier concentration density remains constant as long as 2DHG exists. From the derived model, the drain current (Id), transconductance current gain (gm), and cutoff frequency (ft) for different gate length and width are verified with experimental data for upcoming nano‐scale devices.

InAlN/GaN high electron mobility micro-pressure sensors for high-temperature environments

A micro-scale pressure sensor leveraging a ring-shaped In0.17Al0.83N/GaN high electron mobility transistor (HEMT) sensing element was fabricated and characterized under applied pressure. The device design uses InAlN/GaN-on-Si as the material platform to enable monolithic integration with electronics and high-temperature operation. An analytical model of the pressure transduction was used to compare the change in the two-dimensional electron gas (2DEG) sheet carrier concentration for InAlN and AlGaN heterostructures upon applied pressure. The model confirms that the high aluminum content of InAlN and large piezoelectric constants of AlN results in a larger pressure response in InAlN/GaN heterostructures, in comparison to AlGaN/GaN heterostructures. To experimentally examine the InAlN/GaN pressure sensor architecture, a 500-μm-radius pressure sensor was electrically characterized under applied pressures from 0 to 28.5psig. The current sensitivity of the device increased as gate voltage approached the threshold voltage and as the drain to source bias increased, up to the saturation regime. The maximum change in current (or sensitivity) demonstrated was 0.64%/psig at VGS=−5V, VDS=2.2V. To observe the influence of temperature, the current–voltage (ID–VDS) response of released and unreleased ring-shaped devices was measured up to 300°C. At all temperatures, the current of the released ring-shaped devices decreased when compared to the solid state device, which is attributed to reduced thin film tensile-stress between the underlying GaN buffer layers and the silicon. This work demonstrates the feasibility of using InAlN/GaN sensor architectures for high temperature applications (space exploration, nuclear energy, downhole, and combustion).

The world’s first high voltage GaN-on-Diamond power semiconductor devices

This paper presents the detailed fabrication method and extensive electrical characterisation results of the first-ever demonstrated high voltage GaN power semiconductor devices on CVD Diamond substrate. Fabricated circular GaN-on-Diamond HEMTs with gate-to-drain drift length of 17μm and source field plate length of 3μm show an off-state breakdown voltage of ∼1100V. Temperature characterisation of capacitance-voltage characteristics and on-state characteristics provides insight on the temperature dependence of key parameters such as threshold voltage, 2DEG sheet carrier concentration, specific on-state resistance, and drain saturation current in the fabricated devices.

Novel Analytical model for current-voltage characteristics and transconductance of undoped AlGaN/GaN MISHFETs and performance comparison with different high-k dielectrics

In this paper an improved charge controlled model of undoped, lattice mismatched AlGaN / GaN MISHFET is presented .This model gives an accurate estimation of the 2-DEG sheet carrier concentration at the interface contributed entirely by spontaneous and piezoelectric polarization .The dependence of this sheet carrier density on Al composition and the insulator thickness is investigated in detail. Based on these developments an analytical current – voltage model incorporating Source/Drain parasitic resistance is presented. Also, the non-linear behavior of saturated Drain current is properly formulated and computation of transconductance is carried out for the same. Even without any intentional doping the proposed model exhibits fairly encouraging values of output current and conductance.

2-D Theoretical Model for Current–Voltage Characteristics in AlGaN/GaN HEMT’s

A threshold-voltage-based 2-D theoretical model for the Current–Voltage characteristics of the AlGaN/GaN high electron mobility transistors (HEMT’s) is developed. The present work proposes an improved charge-control model by employing the Robin boundary condition when introduced the solution of the 2-D Poisson’s equation in the density of charge depleted in the AlGaN layer. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model in order to take into account the impact of gate lengths. The relation between the kink effect and existing deep centers has also been confirmed by using an electrical approach, which can allow to adjust some of electron transport parameters in order to optimize the output current.

2DEG properties in InGaN/InN/InGaN‐based double channel HEMTs

AbstractThis paper describes the 2DEG properties of InGaN/InN/InGaN double channel high electron mobility transistor (DHEMT). These include an in‐depth theoretical investigation of 2DEG sheet carrier concentration and mobility. The 2DEG mobility is calculated using ensemble Monte Carlo taking into account all relevant scatterings. The calculated sheet carrier concentration reaches as high as 1.77×1014 cm‐2 for DHEMT. While for single channel high electron mobility transistor (SHEMT) the calculated sheet carrier concentration was found to be 7.06×1013 cm‐2. The maximum total 2DEG mobility is found to be around 12×104 and 8×104 cm2V‐1s‐1, respectively, for DHEMT and SHEMT for a sheet carrier concentration of ns = 5×1012 cm‐2 at 77 K. Thus, the 2DEG sheet carrier concentration and mobility for dual channel is higher than conventional single channel HEMTs. The high values of 2DEG sheet carrier concentration and mobility strongly suggest that the InGaN/InN/InGaN based double channel HEMT is very promising for the fabrication of high performance high speed device (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Impact of gate dielectric thickness on the electrical properties of AlGaN/GaN MISHFETs on Si(111) substrate

AbstractWe report on the fabrication and characterization of AlGaN/GaN metal–insulator–semiconductor heterostructure field effect transistors (MISHFETs) using HfO2 and Al2O3 as gate dielectric, deposited by atomic layer deposition (ALD). Improved device performance has been observed for all MISHFET devices as compared to the reference HFET. The additional dielectric layer underneath the gate metallization leads to a significant decrease of gate leakage currents and thus prevents device degradation and losses. Furthermore, the 2DEG sheet carrier concentration increases depending on the k‐value of the oxide as well as the oxide thickness. The thin (3–9 nm) dielectric layer is serving simultaneously as surface passivation, mitigating dc‐to‐rf dispersion. However, less has been reported about the impact of gate insulator thickness on the electrical performance, in particular in terms of dc‐to‐rf dispersion. In this paper, full recovery of dc drain current is demonstrated with a MISHFET with 9 nm Al2O3 oxide thickness.

Comparison of two-dimensional electron gas of etched and unetched InAlAs/InGaAs/InAlAs metamorphic high electron mobility transistor structures

We present the photoluminescence (PL) and Hall studies on the two-dimensional electron gas (2DEG) of etched and unetched In 0.5Al 0.5As/In 0.5Ga 0.5As metamorphic high electron mobility transistor (mHEMT) structures. The PL technique is shown to be capable of extracting the 2DEG sheet carrier concentration in a complete mHEMT structure directly without making contacts or processing, while the Hall measurement gives a substantially higher concentration due to the parallel conduction of the heavily doped cap layer. We also report a new frequency-dependent photo-Hall technique to obtain the absorption coefficient of the In 0.5Ga 0.5As quantum-well channel layer.

Comparison of measuring methods of sheet carrier density in AlGaN/GaN heterostructures

Hall measurement with Van der Pauw method and the Capacitance-Voltage(C-V) characteristics method are performed on AlGaN/GaN heterostructures with different Al contents grown on sapphire substrates by metalorganic chemical vapor deposition. It is found that the value of sheet carrier density obtained from Hall measurement is larger than that deduced from C-V carrier density profile, and both values, as well as the difference between them increase with increasing Al content. This result is ascribed to two reasons. On the one hand, Ni/Au Schottky contact deposited on AlGaN/GaN heterostructure changes the surface states of the AlGaN barrier layer. Some electrons in the two-dimensional electron gas (2DEG) are extracted to the void surface donor states, and consequently the 2DEG sheet carrier concentration is reduced. And with the Al content increasing, the more the surface states of the AlGaN layer, the more the electrons are extracted to the void surface donor states. On the other hand, the precision of C-V measurement is influenced by the series resistance, which causes underestimation of the magnitude of the depletion-layer capacitance and hence the carrier concentration.

Influence of Ni Schottky contact area on two-dimensional electron-gas sheet carrier concentration of strained AlGaN∕GaN heterostructures

The influence of Ni Schottky contact area on two-dimensional electron-gas (2DEG) sheet carrier concentration of strained AlGaN∕GaN heterostructures has been investigated by depositing different areas of Ni Schottky contact on the structure. With the measured capacitance-voltage (C-V) curves, the 2DEG sheet carrier concentrations with different Ni Schottky contact areas were calculated. It is shown that with the increased area of Ni Schottky contact on strained AlGaN∕GaN heterostructures, the 2DEG sheet carrier concentration is decreased, and when the area of Ni Schottky contact is increased to some extent, the decreased 2DEG sheet carrier concentration tends to be constant. A hypothetic model of the interaction among the electrons of Ni Schottky metal, the surface donor states of AlGaN barrier layer, and 2DEG electrons in strained AlGaN∕GaN heterostructures is proposed to explain the above results. The current-voltage (I-V) characteristics for the different areas of Ni Schottky contact confirm the results obtained by C-V characteristics.

Influence of annealed ohmic contact metals on polarisation of AlGaN barrier layer

The influence of annealed ohmic contact metals on the polarisation of the AlGaN barrier layer has been investigated by the Schottky contacts on the AlGaN/GaN HFET structure. The analysed result shows that annealed ohmic contact metals weaken the polarisation of the AlGaN barrier layer. When ohmic contact metals are close to Schottky contact metals, the weakened polarisation decreases the 2DEG sheet carrier concentration in the channel.

An accurate charge control model for spontaneous and piezoelectric polarization dependent two-dimensional electron gas sheet charge density of lattice-mismatched AlGaN/GaN HEMTs

The present paper proposes an improved charge control model of lattice-mismatched AlGaN/GaN HEMTs, valid over the entire operating region. The model for estimation of two-dimensional electron gas (2-DEG) sheet carrier concentration accounts for the strongly dominant spontaneous and piezoelectric polarization at the AlGaN/GaN heterointerface. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model include the effect of field dependent mobility, velocity saturation and parasitic source/drain resistances. Close proximity with experimental data confirms the validity of the proposed model.

Influence of Growth Temperature and Thickness of AlGaN Caps on Electron Transport in AlGaN/GaN Heterostructures Grown by Plasma-assisted Molecular Beam Epitaxy

The influence of Al0.05Ga0.95N cap thickness and growth temperature on the electrical properties of the Al0.05Ga0.95N/GaN two-dimensional electron gas (2DEG) was investigated. Varying the thickness of the Al0.05Ga0.95N barrier led to regions in which the dominant 2DEG scattering mechanisms were either Coulombic or interface roughness/alloy disorder scattering. The 2DEG sheet carrier concentration was found to increase with Al0.05Ga0.95N cap thickness and saturated at ∼ 25 nm. By increasing the growth temperature from 650°C to 750°C, the measured low temperature 2DEG sheet carrier concentration was found to decrease and the measured low temperature mobility was found to increase while the Al composition remained constant. Temperature dependent Hall measurements revealed that by increasing the growth temperature, the incorporation of impurities is decreased. A maximum 77 K mobility of ∼ 19,000 cm2/Vs was observed for films grown at 750°C with 20 nm thick Al0.05Ga0.95N caps.

Influence of Ga flux on the growth and electron transport properties of AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The growth and electrical properties of the Al 0.15Ga 0.85N/GaN two-dimensional electron gas (2DEG) were studied for a range of III/V ratios during growth. All films were grown by RF-plasma assisted molecular beam epitaxy. Al 0.15Ga 0.85N growth in the Ga-rich regime has a surface structure that is controlled by the Ga flux, ranging from morphologies indicative of statistical roughening effects to step-flow growth. The 2DEG sheet carrier concentration was insensitive to Ga flux—this is because the saturated 2DEG charge is approximately equal to the fixed polarization charge at the AlGaN/GaN interface. By increasing the Ga flux during growth, the measured low temperature 2DEG mobility was found to increase, reaching a maximum just before Ga droplets began to form on the surface. In the Ga droplet growth regime, the mobility decreased and was insensitive to further increase in Ga flux. The highest quality surface structure and maximum mobility were realized for structures grown with Ga fluxes just below those necessary to form droplets.

Characterization of GaAs/AlxGa1−xAs selective reactive ion etching in SiCl4/SiF4 plasmas

Selective reactive ion etching of GaAs over AlxGa1−xAs using SiF4/SiCl4 gas mixtures is studied. Etch rates of GaAs and AlxGa1−xAs are measured versus pressure, etch gas ratio, and plasma self-bias voltage. A selectivity ratio of 500:1 has been obtained for GaAs over Al0.35Ga0.65As at a gas composition ratio of SiCl4/SiF4=0.25, a pressure of 60 mTorr, and a self-bias voltage of −60 V. The etch stop mechanism is studied using Auger electron spectroscopy. Auger surface scan spectra indicate a high concentration of fluorine at the surface of etched AlGaAs samples suggesting the formation of an AlF3 etch stop layer.1 It is also shown that the AlF3 layer is easily removed with a very short dip in HCl/H2O or NH4OH/H2O. Hall effect measurements at 300 and 77 K are made on etched GaAs/Al0.3Ga0.7As modulation doped heterostructures to determine the effect of low-power etching on two-dimensional electron gas (2-DEG) sheet concentrations and carrier mobility. Slight reduction in both 2-DEG sheet carrier concentration and mobility are observed after the RIE process. This may be an indication of ion-beam induced damage to the AlGaAs/GaAs heterostructure. Finally, the effects of extended etching on the dc and microwave device characteristics of short gate length GaAs/AlGaAs/InGaAs MODFETs are reported.

On the charge control of the two-dimensional electron gas for analytic modeling of HEMTs

A simple charge control model is developed for the two-dimensional electron gas (2-DEG) of high-electron-mobility transistors (HEMTs). This model explicitly takes into account the effective distance of the 2-DEG from the heterointerface and has been developed for use in analytic I-V and C-V modeling. In this model, the Fermi energy level versus the 2-DEG sheet carrier-concentration is represented by a simplified expression derived from the triangular potential well approximation and is shown to be dominated by terms with different functional forms in two distinct operation regions: a moderate carrier-concentration region and a subthreshold region. The validity of the analytic charge control model is supported by the calculated results of a self-consistent quantum mechanical model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>