Characteristics Of High Electron Mobility Transistors Research Articles

Influence of Growth Parameters and Thickness of AlN Spacer on Electrical Properties of AlGaN/AlN/GaN High-Electron-Mobility Transistors Grown on 4-Inch Si Substrate

Influence of thickness and growth condition of AlN spacer on electrical properties of AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) grown on 4-in. Si substrate was studied. Hall measurements show the mobility of AlGaN/AlN/GaN heterostructure varies with different thickness of AlN spacer and a narrow high-mobility window was obtained. The surface and structure studies indicate that both increasing growth temperature and lowering pressure benefit the quality of AlN and AlGaN, which leads to improvement of Hall mobility, except the temperature higher than that of GaN growth. Drain current–voltage (Ids–Vds) characteristics of such HEMTs exhibit the maximum current density (Imax) and tranconductance (gmax) have the similar trend with the mobility due to different interface roughness and piezoelectric field induced by part relaxation of tensile strain in AlGaN grown on AlN. The optimum properties of HEMT (Imax = 807 mA/mm, gmax = 221 mS/mm) are obtained when 1 nm AlN spacer grown at 1130 °C and 100 Torr.

A room temperature HEMT process for AlGaN/GaN heterostructure characterization

A simple, room temperature, AlGaN/GaN high electron mobility transistor (HEMT) process is presented. The process consists of only two steps which can be performed by hand. The first step is to deposit gallium (Ga) metal for ohmic contacts, which without thermal processing have a specific contact resistivity ρc = 0.10 Ω cm2. Silver (Ag)-based conductive paint is then deposited to form a Schottky contact. The simplicity of the process facilitates fast fabrication and characterization of HEMTs, without unwanted effects on the material. The process is useful for initial material characterization and screening. The process is also found to be a useful tool for process monitoring of the conventional HEMT micro-fabrication process by detecting material/process quality problems. In this work the process is used for initial material characterization and screening, where a leaky buffer can easily be detected. The process is also used to identify the ohmic contact annealing as a potentially damaging step in a conventional micro-fabrication process. A decrease in the electron sheet carrier concentration and an increase of leakage currents are measured after annealing.

Utilizing Diode Characteristics for GaN HEMT Channel Temperature Prediction

Measuring channel temperature in GaN high-electron mobility transistors (HEMTs) is challenging due to the submicrometer dimensions of the gate fingers. The HEMT characteristics are electrically and thermally dependent. The channel temperature is measured using the Schottky gate-diode forward characteristics and compared with results of simulation, theory, and experimental evidence. The pulsed gate-diode forward resistance and threshold voltage predict channel temperatures that agree well with other methods. The technique presented provides a fast, easily implementable methodology for estimating channel temperature.

Barrier-Layer Scaling of InAlN/GaN HEMTs

We discuss the characteristics of high-electron mobility transistors with barrier thicknesses between 33 and 3 nm, which are grown on sapphire substrates by metal-organic chemical vapor deposition. The maximum drain current (at VG = 2.0 V) decreased with decreasing barrier thickness due to the gate forward drive limitation and residual surface-depletion effect. Full pinchoff and low leakage are observed. Even with 3-nm ultrathin barrier, the heterostructure and contacts are thermally highly stable (up to 1000degC).

Improved temperature model of AlGaN/GaN HEMT and device characteristics at variant temperature

In this paper, an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. Research is being conducted for a high-performance building block for high frequency applications that combine lower costs with improved performance and manufacturability. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the transfer characteristics, output current-voltage characteristics and small-signal microwave parameters of HEMTs. The investigated temperature range is from 100–600 K. The small signal microwave parameters have been evaluated to determine the unity current gain cut-off frequency (f T ). High f T (10–70 GHz) values and high current levels (∼550 mA/mm) are achieved for a 1 μm AlGaN/GaN HEMTs. A custom DC measurement system is used to facilitate the DC characterization of the unpackaged GaN HEMT test device. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.

A study on influence of micropipes in SiC substrate on overgrown AlGaN/GaN HEMT DC characteristics

A deterioration mechanism of DC characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated around hollow cores that extend from micropipes in SiC substrates was studied. Various sizes of hollow cores were investigated to study the influence of size dependence. Prominent deterioration of the DC characteristics of HEMTs fabricated in the vicinity of hollow cores with a diameter of 5 μm was observed, while no major deterioration was observed for HEMTs fabricated around hollow cores whose diameters were 1.5 and 3 μm. Clear correlation between the size of hollow cores and free carrier densities at the peripheries of the cores was observed using micro-Raman imaging. The high density of free carriers around relatively large hollow cores causes deterioration of the DC characteristics of HEMTs fabricated in the vicinity of hollow cores. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Statistical study of the effect of interface charge fluctuations in HEMTs using a 3D simulator

The intrinsic parameter fluctuations associated with the discreteness of charge and matter become an important factor when the semiconductor devices are scaled to nanometre dimensions. The interface charge in the recess regions of high electron mobility transistors (HEMTs) has a considerable effect on the overall device performance. We have employed a 3D parallel drift-diffusion device simulator to study the impact of interface charge fluctuations on the I-V characteristics of nanometre HEMTs. For this purpose, two devices have been analysed, a 120 nm gate length pseudomorphic HEMT with an In0.2Ga0.8As channel and a 50 nm gate length InP HEMT with an In0.7Ga0.3As channel.

Correlation between micropipes on SiC substrate and dc characteristics of AlGaN∕GaN high-electron mobility transistors

We report the influence of the size of hollow cores that extend from micropipes in SiC substrates on the dc characteristics of AlGaN∕GaN high-electron mobility transistors (HEMTs) fabricated on the substrates. Significant deterioration of the dc characteristics of HEMTs fabricated in the vicinity of hollow cores with a diameter of 5μm was observed, while no major deterioration was observed for HEMTs fabricated around hollow cores whose diameters were 1.5 and 3μm. A clear correlation between the size of hollow cores and free carrier densities at the peripheries of the cores was observed using micro-Raman imaging. The high densities of free carriers around relatively large hollow cores were suggested to be the cause of deterioration of the dc characteristics of HEMTs fabricated in the vicinity of the hollow cores. The device deterioration length with respect to the hollow core size was empirically deduced, and the effective decrease in the wafer yield was estimated as well.

Effects of a thin Al layer insertion between AlGaN and Schottky gate on the AlGaN∕GaN high electron mobility transistor characteristics

To improve an AlGaN∕GaN high electron mobility transistor, an Al layer as thin as 3 nm was inserted between the AlGaN barrier layer and the gate contact. At our preceded experiments on Schottky diodes, we confirmed significant improvement in capacitance-gate voltage characteristics especially at a low frequency as well as drastic reduction in gate leakage current, which should be interpreted in terms of decrease in oxygen-related trap density at the AlGaN surface. As a result of the trap reduction, the transistor indicates marked improvement of current collapse with no degradation in transconductance.

A numerical study of dc characteristics of HEMT with p -type δ -doped barrier

The dc characteristics of an InGaP/InGaAs/GaAs pseudomorphic high-electron-mobility transistor (HEMT) with a p-type δ-doped InGaP barrier are numerically investigated with the ISE-TCAD simulation program. The simulation results indicate that a HEMT with such a structure has a higher gate turn-on voltage, better carrier confinement that results in a lower voltage-dependent transconductance, and a larger breakdown voltage when compared with the typical HEMT. The simulation results also suggest that this structure is beneficial for linear and large-signal application.

Temperature‐dependent microwave noise performances of AlGaN/GaN HEMTs with post‐gate annealing

In this paper, we report the temperature dependence of microwave noise performances of AlGaN/GaN high electron mobility transistors (HEMTs) up to 573 K and the influences of post-gate-annealing (PGA) technique on temperature dependant characteristics of HEMTs. From room temperature (R.T.) to 573 K, the maximum current of devices with PGA decreased from 998 mA/mm at R.T. to 478 mA/mm while maximum current of devices without PGA dropped from 848 mA/mm at R.T to 641 mA/mm at 393 K. The gate leakage current of devices with PGA is four orders smaller than that of devices without PGA. For RF performance, the cut-off frequency (fT) and the maximum frequency of oscillation (fmax) of devices decreased significantly with increasing temperature. In noise measurements, annealed devices exhibited significant improvements in minimum noise figure (NFmin) and associated gains (Ga). Specifically, NFmin and Ga at 10 GHz are measured at different temperatures. At R.T., the devices after PGA showed an NFmin of 1.16 dB that was 0.7 dB lower than the un-annealed devices. At 573 K, the devices with PGA still exhibit excellent characteristics with an NFmin of 3.94 dB and a Ga of 7.02 dB while devices without PGA can only work up to 393 K. To our knowledge, this is the first report of microwave noise performances of AlGaN/GaN HEMTs for substrate temperatures higher than 473 K. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A compact analytical I– V model of AlGaAs/InGaAs/GaAs p-HEMTs based on non-linear charge control model

A simple non-linear charge control model proposed by DasGupta et al. for the AlGaAs/GaAs based high electron mobility transistors (HEMTs) is applied for AlGaAs/InGaAs/GaAs based pseudomorphic HEMTs. This non-linear charge control model is used to calculate the I– V characteristics of pHEMTs. As electron velocity is one of the important parameters which determine the I– V characteristics of devices, a best analytical expression is used for modeling the I– V characteristics of HEMTs. The I– V characteristics obtained from the present model are compared with the experimental I– V data of the 0.25 μm gate length Al 0.25Ga 0.75As/In 0.2Ga 0.8As/GaAs pHEMTs fabricated in our laboratory, and show good agreement. The present model, being analytical and simple, can be implemented in circuit simulator for design and analysis of circuits based on AlGaAs/InGaAs/GaAs based pHEMTs.

Bias and frequency dependence of FET characteristics

A novel measurement of the dynamics of high electron-mobility transistor (HEMT) and MESFET behavior permits classification of rate-dependence mechanisms and identification of operating regions that they affect. This reveals a simple structure to the otherwise complicated behavior that has concerned circuit designers. Heating, impact ionization, and trapping contribute to transient behavior through rate-dependence mechanisms. These are illustrated by a simple description. Each has an effect on specific regions of bias and operating frequency. With this insight, it is possible to determine true isodyamic characteristics of HEMTs and MESFETs and to predict operating conditions that will or will not be affected by rate dependence. It is interesting to note that, for some devices, rate dependence can be seen to exist at microwave frequencies and may, therefore, contribute to intermodulation distortion.

InAs-inserted-channel InAlAs/InGaAs inverted HEMTs with direct ohmic contacts

We have investigated the device characteristics of InAs-inserted-channel In 0.52Al 0.48As/ In 0.53Ga 0.47As inverted high electron mobility transistors (HEMTs) with a novel ohmic structure. The ohmic contact between the ohmic electrodes and the two-dimensional electron gas (2DEG) formed in the InAs layer is obtained by direct contact with the ohmic electrodes-InAs, instead of an alloyed normal-metal and semiconductor as in a conventional inverted HEMT. The contact resistance of 0.11 Ωmm between the ohmic electrodes and the channel is smaller by a factor of 4, than that obtained using a conventional AuGeNi alloyed ohmic contact. For a 0.5 μm-gate device, a maximum extrinsic transconductance of 1.2 S/mm was obtained at 4.2 K, even at a very low drain voltage of 0.2 V. These results show that this ohmic contact formation allows us to obtain the improved HEMT characteristics.

InAs-inserted-channel InAlAs/InGaAs inverted HEMTs with superconducting electrodes

We investigate the device characteristics of InAs-inserted-channel In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As inverted high electron mobility transistors (HEMTs) with superconducting Nb electrodes. In these transistors, the ohmic contact between Nb and the two-dimensional electron gas formed in the InAs layer is obtained by contact with the Nb-InAs direct, instead of with an alloyed normal metal and semiconductor as in a conventional HEMT. The contact resistance of 0.15 /spl Omega/mm between the Nb ohmic electrodes and the channel decreased by a factor of 3 compared with that when a conventional AuGe/Ni alloyed ohmic contact is used. For a 0.5-/spl mu/m-gate device, the maximum extrinsic transconductance at 4.2 K was 1 S/mm, even at a very low drain voltage of 0.2 V. These results indicate that this ohmic contact formation will allow us to utilize in this device both the improved HEMT characteristics and the superior performance of superconducting electrodes.

An analytical model for the photodetection mechanisms in high-electron mobility transistors

The use of microwave high-electron mobility transistors (HEMTs) as photodetectors or optically controlled circuit elements have attracted interest. A model of the optical characteristics of HEMTs, which takes into account carrier transport as well as the quantum mechanical nature of the two-dimensional (2-D) electron gas channel, is presented. It is shown that the effect of illumination is equivalent to a shift in the gate to source bias voltage, referred to as the internal photovoltaic effect. The theoretical model is supported by experimental results that demonstrate that the HEMT photoresponse is a nonlinear function of light intensity with very high responsivity at low optical power levels.

A new fabrication technology for AlGaAs/GaAs HEMT LSIs using InGaAs nonalloyed ohmic contacts

The authors studied the nonalloyed ohmic characteristics of HEMTs (high electron mobility transistors). At high integration levels, nonalloyed ohmic contacts were found to have two advantages: an extremely short ohmic length with low parasitic source series resistance and direct connection between the source/drain and gate with the same metal. The propagation delay in a ring oscillator with a single-metal source/drain and gate formed simultaneously was 37 ps/gate (L/sub g/=0.9 mu m). The very short ohmic metal contacts and just three contact holes made it possible to reduce the memory cell area greatly. The cell is 21.5*21.5 mu m/sup 2/, one of the smallest ever reported for a GaAs-based static RAM. Using smaller load HEMTs or resistor loads in the memory cell, combined with nonalloyed ohmic technology with quarter- or subquarter-micrometer-gate HEMTs it is possible to fabricate a very-high-speed LSI such as a 64-kb static RAM with a reasonable chip size. >

Influence of surface defects on the characteristics of high electron mobility transistors grown by molecular-beam epitaxy

Surface defects on layers grown by molecular-beam epitaxy were classified, and their effect on high electron mobility transistor characteristics was studied. The defects originating from the Ga source consist of cores and hillocks. These defects had a strong influence only when their cores were located in the gate region. They decreased the transconductance and K value significantly and shifted the threshold voltage to the negative direction. This indicates the cores are highly conductive. The other defects, originating from substrate surface contamination, slightly decreased the K value and transconductance while increasing the deviation in the threshold voltage.

The influence of device physical parameters of HEMT large-signal characteristics

The small- and large-signal high-frequency characteristics of submicrometer HEMTs (high-electron-mobility transistors) are analyzed by taking into account parasitic effects such as parallel conduction, fringing capacitances, and substrate leakage. The dependence of large-signal properties on device physical parameters is reported. This includes device gate length, donor layer thickness and doping, and spacer thickness. Satisfactory agreement is shown to exist between theoretically and experimentally obtained device characteristics. >

Modelling DC characteristics of HEMTs

A simple empirical equation is presented to relate the drain current to drain-source and gate-source voltages of the high electron mobility transistors (HEMTs). This equation covers the entire ID/VDS characteristics. The accuracy of this equation is better than the theoretically developed separate equations.