Advanced Semiconductor Devices

PREFACE.

CONTENTS.

In this paper, we discuss requirements of power devices for automotive applications, especially hybrid vehicles and the development of GaN power devices at Toyota. We fabricated AlGaN/GaN HEMTs and measured their characteristics. The maximum breakdown voltage was over 600V. The drain current with a gate width of 31mm was over 8A. A thermograph image of the HEMT under high current operation shows the AlGaN/GaN HEMT operated at more than 300°C. And we confirmed the operation of a vertical GaN device. All the results of the GaN HEMTs are really promising to realize high performance and small size inverters for future automobiles.

This paper presents Freescale's baseline GaN device technology for wireless infrastructure applications. At 48 V drain bias and 2.1 GHz operating frequency 10-11 W/mm, 62-67% power-added efficiency (PAE) is realized on 0.3 mm devices and 74 W (5.9 W/mm), 55% PAE is demonstrated for 12.6 mm devices. A simple thermal model shows that a more than twofold increase in channel temperature is responsible for limiting the CW power density on the 12.6 mm compared to 0.3 mm devices. The addition of through wafer source vias to improve gain and tuning the device in a fixture optimized for efficiency yield an output power of 57W (4.7 W/mm), PAE of 66%, and a calculated channel temperature of approximately 137°C at a 28 V bias.

Additional friction due to Pauli constraint, channel self-heating, alloy scattering, and hot phonons is reconsidered.

We report on two-dimensional isothermal simulations of recessed gate and field-plated AlGaN-GaN HFETs with submicron gates. The optimization of the recessed gate shape allows us to reduce the electric field at the drain-side gate edge by approximately 30%. Our simulations reveal a dramatic increase of the effective gate length with increasing drain-to-source bias with a commensurate decrease of the cutoff frequency (up to 40% decrease for 50V). To improve the cutoff frequency for the high drain-to-source bias, we suggest using the second field plate connected to the drain with a small gap between the two field plates. In this design, the electric field in the gap between the gate and the drain field plate is higher leading to a significant reduction of the effective gate length and, as a consequence, to an increase in the cutoff frequency at high drain-to-source biases (compared to the conventional design).

Electroluminescence of GaInN/GaN multiple-quantum-well (QW) light-emitting diodes emitting in the green spectral region is analyzed at variable low temperature. Spectra are dominated by QW emission at RT throughout 7.7 K. Below 150 K, donor-acceptor pair recombination appears that must be assigned to residual impurities in either the barriers or the p-layers. The current-voltage behavior reveals shunt paths that carry up to several mA at low bias voltages. Below 20 K those paths are frozen out, but the device still emits predominantly from the QW. The peak energy exhibits a blue shift from RT to 158 K followed by a red shift from 158 K to 7.7 K. The deeper binding energy at low temperatures can hardly be affect by the injection current indicating that saturation of low-density states cannot be responsible for the transition between red and blue shifts.

We analyze GaInN based light emitting diodes emitting in the range of 400-550 nm using a new intensity-quantitative spectroscopic cathodoluminescence mapping method. Spectroscopic information of arbitrary sample locations is generated from sequences of quantitative image scans. From the temperature dependence of the intensity, we derive thermal activation energies of the dominant loss processes. Those compare well with the hole binding energies in the piezoelectric and quantized quantum well structures.

In this work we investigate a pseudo-random surface texturing technique of sapphire by means of inductively coupled plasma reacting ion etching in chlorine chemistry, for which no sophisticated lithographic process is required. Such a surface texturing technique, which we believe offers indicative promise for enhanced light extraction in deep ultraviolet light-emitting diodes has allowed us to texture sapphire samples having a surface larger than 1 cm² with controlled structures. Fabrication parameters have been characterized, and textured Al₂O₃ surfaces having submicron features, and nano-scale periodicity have been obtained. Performance, and characterization of our textured Al₂O₃ surfaces is the hinge of addition work in progress.

A simplified model of electron transport by tunneling within a GaAs/AlGaAs/GaAs heterojunction is developed. The model is applied specifically to tunneling through a triangular barrier formed by the compositional grading of the AlGaAs region, but can in principle be extended to a range of barrier geometries encountered at heterojunction or metal/semiconductor interfaces. The experimental data for the current-voltage characteristics obtained for a range of temperatures from 77 K to 273 K are used to test the functional dependence obtained from calculations. Good agreement has been obtained between theory and experiment, thus confirming the usefulness of the simple model for device evaluation.

The key material and device parameters governing the electrical performance of high voltage 4H-SiC PiN diodes have been investigated using experimental results and numerical simulations. Reverse recovery characteristics show an increase in both carrier lifetime and anode injection efficiency at elevated temperature. Open circuit voltage decay measurements are used to estimate carrier lifetimes (τ ≈ 0.6μs at T=25°C increasing to τ≈2μs at T=225°C) that are comparable to values measured on starting material prior to fabrication using micro-wave photoconductivity decay techniques.

In this paper, we have fabricated and compared the performance of lateral enhancement-mode GaN MOSFETs with linear and circular geometries. Circular MOSFETs show 2 to 4 orders of magnitude lower leakage current than that of linear MOSFETs. We also studied short channel behaviors and found that they are similar to those previously reported Si MOSFET.

This work presents a vertical RESURF structure, which utilizes 2-dimensional depletion in semiconductor to achieve high blocking capability. This approach is then implemented into both 4H-SiC Schottky rectifiers and MOSFETs. Device characteristics are analyzed with numerical simulations and compared with both conventional Schottky rectifiers and Superjunction structure. Design-of-Experiment (DOE) is also used to optimize the trade-off between several design parameters.

The implementation of challenging novel materials and process techniques has led to remarkable device improvements in state-of-the-art high-performance SiGe HBTs, rivaling their III-V compound semiconductor counterparts. Vertical scaling, lateral scaling, and device structure innovations required to improve SiGe HBTs performance have benefited from advanced materials and process techniques developed for next generation CMOS technology. In this work, we present a review of recent process and materials development enabling operational speeds of SiGe HBTs approaching 400 GHz. In addition, we present device simulation results that show the extendibility of SiGe HBT technology performance towards half-terahertz and beyond with further scaling and device structure improvements.

Optical properties of green emission Ga_0.80ln_0.20N/GaN multi-quantum well and light emitting diode have been investigated by using photoluminescence, cathodoluminescence, electroluminescence, and photoconductivity. The temperature dependent photoluminescence and cathodoluminescence studies show three emission bands including GaInN/GaN quantum well emission centered at 2.38 eV (∼ 520 nm). The activation energy of the non-radiative recombination centers was found to be ∼ 60 meV. The comparison of photoconductivity with luminescence spectroscopy revealed that optical properties of quantum well layers are strongly affected by the quantum-confined Stark effect.

A significant improvement of contact transfer resistance on undoped GaN/AlGaN/AlN (10 Å)/GaN high electron mobility transistor (HEMT) structure was demonstrated using a Ta/Ti/Al/Mo/Au metallization scheme compared to a Ti/Al/Mo/Au metallization scheme. A contact resistance as low as 0.16 ± 0.03 ohm-mm was achieved by rapid thermal annealing of evaporated Ta (125 Å)/Ti (150 Å)/Al (900 Å)/Mo(400 Å)/Au(500 Å) metal contact at 700 °C for 1 min followed by 800 °C for 30 sec in a N₂ ambient. An excellent edge acuity was also demonstrated for the annealed Ta/Ti/Al/Mo/Au ohmic contacts.

We report on the investigation of an InAlN/GaN HEMT structure, delivering higher sheet carrier density than the commonly used AlGaN/GaN system. We achieved in a reproducible way more than 2 A/mm maximum drain current density for a gate length of 0.25 μm with unpassivated undoped devices realized on sapphire substrates. Small signal measurements yield a F_T = 31 GHz and F_MAX = 52 GHz, which illustrates the capability of these structures to operate at high frequencies. Moreover, the pulsed analysis indicates a more stable surface in the case of AlInN than that of AlGaN, attributed to the lattice matched growth of this barrier with 17 % In content on GaN, avoiding strain piezo polarization in the material.

During InGaN Molecular Beam Epitaxy (MBE) growth, the material surface is exposed to a small diameter pulse laser beam that is controlled by scanning mirrors. Local heating effects are observed at the points of exposure. The materials are characterized by Wavelength Dispersive Spectroscopy (WDS), Scanning Electron Microscopy (SEM), and Photoluminescence (PL). Indium mole fraction of materials is reduced where laser exposure takes place. The effect of local thermal heating appears to enhance surface diffusion while not causing ablation or evaporation under the conditions studied. PL efficiency is significantly increased by focused thermal beam exposure. Laser written regions have 7 times higher PL intensity compared to non-written areas, which might be due to surface texturing that causes higher extraction efficiency.

The temperature dependence of heterostructure backward diodes based on the InAs/AlGaSb/GaSb material system for millimeter-wave detection has been investigated experimentally. Measured dc curvatures of 36 V⁻¹ at 298 K and 74 V⁻¹ at 4.2 K have been obtained. Variable-temperature on-wafer s-parameters to 110 GHz reveal that the junction capacitance of a typical 2×2 μm² area device decreases from 18 fF at 298 K to 11 fF at 77K, while the junction resistance decreases from 13.9 kΩ to 10.2 kΩ. Directly measured voltage sensitivities at 50 GHz of 3650 V/W and 7190 V/W were obtained at 298 K and 4.2 K, respectively, consistent with the expected value from measured dc curvature. A 1 dB compression point of 18.5 μW and 7.2 μW at 298 K and 77 K, respectively, was measured. A physical model based on self-consistent Poisson-Schrödinger equation solutions was obtained to explain the experimental observations, and suggests the ways to further improve the device performance.

The range of mm-wave radio communications is severely constrained by high losses arising from the short wavelength and from atmospheric attenuation. Large phased arrays can overcome these limitations, but it is very difficult to realize them using present monolithic beamsteering IC architectures. We propose an alternative architecture for large monolithic phased arrays. The beam is steered in altitude and in azimuth by separately imposing vertical and horizontal phase gradients. This choice reduces IC complexity, making large arrays feasible. Since extensive digital processing provides robust amplitude control and reduces die area, the LOs are processed as digital signals. Being very sensitive to compression, the IF signals are processed as analog signals and distributed by means of synthetic transmission-line buses. With careful frequency planning, this mixed-signal approach can allow large phased arrays to operate at frequencies much higher than those achievable with pure analog design.

In this paper, we report on current pumped THz emitting devices based on intersubband transitions in SiGe quantum wells. The spectral lines occurred in a range from 5 to 12 THz depending on the quantum well width, Ge concentration in the well, and device temperature. A time-averaged power of 15 nW was extracted from a 16 period SiGe/Si superlattice with quantum wells 22 Å thick, at a device temperature of 30 K and a drive current of 550 mA. A net quantum efficiency of approximately 3 × 10⁻⁴ was calculated from the power and drive current, 30 times higher than reported for comparable quantum cascades utilizing heavy-hole to heavy-hole transitions and, taking into account the number of quantum well periods, approximately four times larger than for electroluminescence reported previously from a device utilizing light-hole to heavy-hole transitions.

Biomolecules such as DNA and proteins exhibit a wealth of modes in the Terahertz (THz) range from the rotational, vibrational and stretching modes of biomolecules. Many materials such as drywall that are opaque to human eyes are transparent to THz. Therefore, it can be used as a powerful tool for biomolecular sensing, biomedical analysis and through-the-wall imaging. Experiments were carried out to study the absorption of various materials including DNA and see-through imaging of drywall using FTIR spectrometer and Time Domain Spectroscopy (TDS) system.

This work studies the effects of negative bias temperature instability (NBTI) on p-channel MOSFETS with TiN/HfSi_xO_y (20% SiO₂) based high-κ gate stacks under different gate bias and elevated temperature conditions. For low bias conditions, threshold voltage shift (ΔV_T) is most probably due to the mixed degradation within the bulk high-κ. For moderately high bias conditions, H-species dissociation in the presence of holes and subsequent diffusion may be initially responsible for interface state and positively charged bulk trap generation. Initial time, temperature and oxide electric field dependence of ΔV_T in our devices shows an excellent match with that of SiO₂ based devices, which is explained by reaction-diffusion (R-D) model of NBTI. Under high bias condition at elevated temperatures, due to higher Si-H bond-annealing/bond-breaking ratio, the experimentally observed absence of the impact ionization induced hot holes at the interfacial layer (IL)/Si interface probably limits the interface state generation and ΔV_T as they quickly reach saturation.

The works examines the power adaptive control of back-gated planar transistor employing a novel super-self-alignment design. The availability of such back-gate provides additional freedom for the improvement of the transistor performance of higher switching speed and lower power consumption. Both of these two capabilities are investigated through the studies of transistor's output and transfer characteristics.

We report the operational characteristics of ultra-small-scaled SONOS (below 50 nm gate width and length) and SiO₂/SiO₂ structural devices with 0.5 um gate width and length where trapping occurs in a very narrow region. The experimental work summarizes the memory characteristics of retention time, endurance cycles, and speed in SONOS and SiO₂/SiO₂ structures. Silicon nitride has many defects to hold electrons as charge storage media in SONOS memory. Defects are also incorporated during growth and deposition in device processing. Our experiments show that the interface between two oxides, one grown and one deposited, provides a remarkable media for electron storage with a smaller gate stack and thus lower operating voltage. The exponential dependence of the time on the voltage is reflected in the characteristic energy. It is ∼0.44 eV for the write process and ∼0.47 eV for the erase process in SiO₂/SiO₂ structural device which is somewhat more efficient than those of SONOS structure memory.

Numerical simulations are performed for SiGeC/Si electrooptic modulators and photodetectors operating at near-IR wavelengths. The addition of carbon provides the ability to lattice match layers with high germanium composition to silicon, which is shown to allow structures with a substantial increase in the optical confinement factor. In addition, SiGeC/Si heterostructures provide strong confinement of large electron and hole concentrations. The large optical confinement factor and strong carrier confinement enable broadband electrooptic modulators with sub-100 μm lengths and switching times below 0.5 ns with 25 mA current as well as photodetectors with quantum efficiencies as high as 90% for 300 μm length.

Quantum dot infrared photodetectors (QDIPs) using quantum dots (QDs) grown by strained-layer epitaxy have demonstrated low dark current, multi-spectral response, high operating temperature, and infrared (IR) imaging. However, achieving near room-temperature, multi-spectral operation is a challenge due to randomness in QD properties. The ability to control dopant incorporation is important since charge carrier occupation influences dark current and IR spectral response. In this work, dopant incorporation is investigated in two classes of QDs; epitaxial InAs/GaAs QDs and CdSe colloidal QDs (CQDs) embedded in MEH-PPV conducting polymers. The long-term goal of this work is to combine these hybrid nanomaterials in a single device heterostructure to enable multi-spectral IR photodetection. Two important results towards this goal are discussed. First, by temperature-dependent dark current-voltage and polarization-dependent Fourier transform IR spectroscopy measurements in InAs/GaAs QDIPs featuring different doping schemes, we have provided experimental evidence for the important contribution of thermally-activated, defect-assisted, sequential resonant tunneling. Second, the enhanced quantum confinement and electron localization in the conduction band of CdSe/MEH-PPV nanocomposites enable intraband transitions in the mid-IR at room temperature. Further, by controlling the semiconductor substrate material, doping type, and doping level on which these nanocomposites are deposited, the intraband IR response can be tuned.

We present the experimental development and characterization of GaN ballistic diodes for THz operation. Fabricated devices have been described and gathered experimental data is discussed. The major problem addressed is the domination of the parasitic resistances which significantly reduce the accelerating electric field across the ballistic region (intrinsic layer).

Time-resolved photoluminescence studies of nitride semiconductors and ultraviolet light emitters comprised of these materials are performed as a function of pump intensity as a means of understanding and evaluating device performance. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Similar behavior is observed in optically pumped UV lasers. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

The results of calculations of the low-frequency and the high-frequency barrier capacitance of selectively doped AlGaAs/GaAs heterostructures containing deep traps in the AlGaAs layer are presented. The calculations are done for the samples in the dark and under extrinsic illumination. It is shown that the high-frequency photocapacitance of these structures exhibits a positive peak, and the low-frequency photocapacitance has a positive peak followed by a negative valley. The underlying physical mechanisms are discussed.

AUTHOR INDEX.