W. Kruppa

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 /spl mu/m gate length, an f/sub T/ of 30 GHz and an f/sub max/ of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

Fabrication and characterization of GaN FETs

Abstract The current status of GaN-based FET technology and performance is reviewed. The fabrication details and the dc and microwave characteristics of GaN MESFETs that utilize Si-doped channels on semi-insulating buffer layers are presented. MESFETs with a 0.8 μm gate have exhibited an f T and f max of 6 and 14 GHz, respectively. These devices have excellent pinchoff characteristics and a source-drain breakdown voltage of over 85 V. A high-field current-collapse phenomenon is observed in these MESFETs in the absence of light. The characteristics of this current collapse as a function of temperature, illuminating wavelength, and time are described. A model describing the current collapse in terms of hot electron injection into the buffer layer is presented.

AlSb/InAs HEMT's for low-voltage, high-speed applications

The design, fabrication, and characterization of 0.1 /spl mu/m AlSb/InAs HEMTs are reported. These devices have an In/sub 0.4/Al/sub 0.6/As/AlSb composite barrier above the InAs channel and a p/sup +/ GaSb layer within the AlSb buffer layer. The HEMTs exhibit a transconductance of 600 mS/mm and an f/sub T/ of 120 GHz at V/sub Ds/=0.6 V. An intrinsic f/sub T/ of 160 GHz is obtained after the gate bonding pad capacitance is removed from an equivalent circuit. The present HEMTs have a noise figure of 1 dB with 14 dB associated gain at 4 GHz and V/sub Ds/=0.4 V. Noise equivalent circuit simulation indicates that this noise figure is primarily limited by gate leakage current and that a noise figure of 0.3 dB at 4 GHz is achievable with expected technological improvements. HEMTs with a 0.5 /spl mu/m gate length on the same wafer exhibit a transconductance of 1 S/mm and an intrinsic f/sub T/L/sub g/, product of 50 GHz-/spl mu/m.

Scaling Properties of Gold Nanocluster Chemiresistor Sensors

The effect of geometric scaling on the performance of metal-insulator metal-ensemble (MIME) chemiresistors based on gold nanoclusters is investigated. The ultrasmall size of the nanoclusters is shown to enable extreme scaling of the sensors with reductions in area of at least a factor of 104 over conventional MIME devices. If the operating voltage is held constant, the absolute sensitivity of the devices is found to remain essentially unchanged by the geometric scaling. Interestingly, this occurs despite the fact that contact resistance appears to play a significant role in the smallest devices. The detection limit of the sensors is set by a signal-to-noise ratio, and because 1/f noise tends to dominate, reduction in sensor size raises the noise floor, leading to a degradation in the detection limit. Because of the importance of the 1/f noise, optimal performance will be obtained by operating the sensors under ac conditions with filtering. Despite the degradation in performance that results from scaling, nanocluster-based chemiresistors of reduced size can still be advantageous because of the possibility of achieving vapor-sensing systems of substantially reduced size, power, complexity, and cost, as well as new applications, e.g., for sensor arrays

Self-assembly of gold nanoclusters on micro- and nanoelectronic substrates

Layer-by-layer self-assembly of hexanethiol stabilized gold nanoclusters by a series of alkanedithiol coupling agents [HS(CH2)nSH, n = 6, 8, 9, 12] onto SiO2/Si substrates with micron- and nanometre-scale Au electrodes is investigated by electron transport and XPS measurements. The self-assembly process for each layer consists of a two-step cycle of alkanedithiol treatment and gold cluster deposition. For maximized electron transport, critical features to optimize are the alkanedithiol chain length and the extent of dithiol coupling agent displacement of the hexanethiol ligand. Substitution of a phenethyl for the hexyl structure in the cluster ligand shell significantly enhances conductivity while substitution of a phenylene structure in the dithiol coupling agent has little effect on electron transport. Current–voltage characteristics for self-assembled depositions on the micron-scale electrode are found to be ohmic whereas I–V characteristics for analogous self-assemblies on the nanometre-scale electrode are initially nonlinear but become increasingly ohmic after about 3 cycles of deposition. The nonlinear features observed at the nanoscale are believed to be associated with Coulomb blockade.

Low-Frequency Noise in AlSb/InAs and Related HEMTs

A comprehensive examination of the low-frequency noise characteristics of AlSb/InAs and related high-electron mobility transistors (HEMTs) in the 6.1-Aring-lattice-constant material system is reported. The effect of gate bias on the noise of devices in this technology is reported for the first time. The slope of the noise level in all the devices examined is nearly 1/f below 100 Hz, but some have significant generation-recombination Lorentzian components at higher frequencies, with an activation energy between 0.30 and 0.40 eV. The Hooge parameter alphaH for open-channel measurements is in the range between 5times10-4 and 5times10-3 based on measurements at low drain voltage. Comparisons are made to the noise performance of several earlier InAs-based HEMTs with considerably different layer structure and channel composition

Low-frequency noise in AlSb∕InAs high-electron-mobility transistor structure as a function of temperature and illumination

Measurements of the low-frequency noise in AlSb∕InAs high-electron-mobility transistor structures over the temperature range between 60 and 300K are reported. Without illumination, the slope of the noise level with frequency was found to be close to 1∕f with a Hooge parameter, αH, of 9×10−3 at room temperature. With broad-spectrum visible-light illumination at lower temperatures, however, the noise level increases greatly and displays a strong Lorentzian component with the characteristic 1∕f2 slope above the corner frequency. The associated sheet resistance also increases greatly, consistent with previously observed negative photoconductivity in AlSb∕InAs quantum wells.

Electrical noise in gold nanocluster sensors

Motivated by applications to chemiresistive vapor sensing, the electrical noise properties of large-area gold nanocluster films are investigated experimentally. Measurements show thermal noise to dominate at low voltages and high frequencies, with a 1∕f noise component becoming more important at a higher voltage and a lower frequency. The latter contribution obeys the Hooge formula in its frequency, voltage, and size dependences, and with a Hooge parameter whose relatively large size is attributed to constraints imposed by Coulomb blockade and disorder. Based on these results, a detection limit for nanocluster-based chemiresistors can be projected to approach one part per billion by volume.

Electron transport in nanocluster films with random voids

Nanocluster films are modeled as a network of tunnel junctions in which random voids have been introduced. The effects of network size and void distribution on electron transport are studied using Monte Carlo simulations of the Coulomb blockade mediated transport. The random void distributions of the model networks are systematically varied by randomly deleting junctions along each row and between the different rows. The nonlinearity and voltage threshold of the I-V curves are calculated for different void topologies and network sizes. Both the threshold voltage and nonlinearity are sensitive to lateral fluctuations of the conduction paths due to the random voids. The nonlinearity is found to be maximized for network aspect ratios of unity or larger and for particular network topologies. Both the nonlinearity and voltage threshold scale with network size. The behaviors seen in simulation are found to correspond well to Au nanocluster I-V measurements.

AlSb/InAs HEMTs with a TiW/Au gate metalization for improved stability

Abstract We report on the fabrication and characteristics of AlSb/InAs high electron mobility transistors (HEMTs) with a TiW/Au gate metalization. Using gate leakage and S -parameter measurements as a measure of stability, the HEMTs were found to be thermally stable up to 180 °C when heat treated in a H 2 /N 2 ambient.