R. Bass

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.

Effects of molecular properties on nanolithography in polymethyl methacrylate

High-resolution lithographic performance of polymethyl methacrylate (PMMA) of molecular weights (MWs) of 50, 100, 496, and 950 K is compared. A chain scission model is used to analyze the behavior of the four molecular weight resists. The chain scission model is combined with an empirical dissolution model to successfully describe the edge profile of a bar pattern. Isolated linewidth data for the 100 and 496 K resists both fit a Monte Carlo code generated linespread function that was convolved with a Gaussian of standard deviation 9 nm. The width was comparable to that in the 950 K resist, but a factor of 3 narrower than that found for the 50 K resist. The higher molecular weight, 496 and 950 K resists showed more developer induced swelling than the lower molecular weight resists. In fact, the developer induced swelling limited the ability to develop 40 nm gratings in the 496 and 950 K resists. Reduction in developer strength produced some improvement. Etching of the supporting resist structure in the gratings was also observed, particularly in the 50 and 100 K resists. The 50 K MW resist exhibited the worst grating contrast upon development. Grating enhanced etching relative to 10 μm bar areas exposed with comparable area dose was observed. A 40 nm period grating was defined in the 100 K resist.

Ohmic contacts in AlSb/InAs high electron mobility transistors for low-voltage operation

We report on the fabrication and characteristics of Pd/Pt/Au ohmic contacts that have been used in AlSb/InAs high electron mobility transistors (HEMTs) with low access resistance. The metalization exhibits minimal in-diffusion and a contact resistance of 0.08 Ω mm after a 175 °C hot-plate heat treatment. By comparison, AuGe/Ni/Pt/Au ohmic contact metalizations formed using a 300 °C rapid thermal anneal exhibit a contact resistance of 0.11 Ω mm, but with considerable Au in-diffusion. Using the Pd/Pt/Au contact, 0.6 μm gate-length AlSb/InAs HEMTs exhibit a low-field source-drain resistance of 0.47 Ω mm and a transconductance above 1 S/mm. After removal of the gate bonding pad capacitance from an equivalent circuit, an fTLg product of 38 GHz μm is obtained at VDS=0.4 V. HEMTs with a 60 nm gate length exhibit a low-field source-drain resistance of 0.35 Ω mm and a measured fT of 90 GHz at a drain voltage of only 100 mV. These fTLg and fT values are the highest reported for any field effect transistor at these ...

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

Photonic microharp chemical sensors

We describe a new class of micro-opto-mechanical chemical sensors: A photonic microharp chemical sensor is an array of closely spaced microbridges, each differing slightly in length and coated with a different sorbent polymer. They are optically interrogated using microcavity interferometry and photothermal actuation, and are coupled directly to an optical fiber. Simultaneous measurements of the fundamental flexural resonant frequency of each microbridge allow the real-time detection and discrimination of a variety of vapor-phase analytes, including DMMP at concentrations as low as 17 ppb.

Sb-Based n- and p-Channel Heterostructure FETs for High-Speed, Low-Power Applications

Heterostructure field-effect transistors (HFETs) composed of antimonide-based compound semiconductor (ABCS) materials have intrinsic performance advantages due to the attractive electron and hole transport properties, narrow bandgaps, low ohmic contact resistances, and unique band-lineup design flexibility within this material system. These advantages can be particularly exploited in applications where high-speed operation and low-power consumption are essential. In this paper, we report on recent advances in the design, material growth, device characteristics, oxidation stability, and MMIC performance of Sb-based HEMTs with an InAlSb upper barrier layer. The high electron mobility transistors (HEMTs) exhibit a transconductance of 1.3S/mm at VDS=0.2V and an fTLg product of 33GHz-μm for a 0.2μm gate length. The design, fabrication and improved performance of InAlSb/InGaSb p-channel HFETs are also presented. The HFETs exhibit a mobility of 1500cm2/V-sec, an fmax of 34GHz for a 0.2μm gate length, a threshold voltage of 90mV, and a subthreshold slope of 106mV/dec at VDS=-1.0V.

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

High radiation tolerance of InAs∕AlSb high-electron-mobility transistors

InAs∕AlSb-based high-electron-mobility transistors (HEMTs) were irradiated with 2MeV protons. Radiation damage caused the source-drain current Ids to decrease nearly linearly with fluence Φ at a rate of Δ[Ids(Φ)∕Ids(0)]∕ΔΦ≈7×10−16cm2. Radiation-induced decreases in Ids have been observed for other HEMT material systems, and have been attributed to high-efficiency defect-induced scattering of carriers out of the two-dimensional electron gas. However, in the InAs∕AlSb system the rate of decrease of Ids is about 140 times less than that for typical GaAs∕AlGaAs HEMTs. An explanation is presented in which the high radiation tolerance of InAs∕AlSb HEMTs is related to carrier reinjection and the unusually large energy offset between the AlSb barriers and the InAs quantum well.

Current-driven reversal in annular vertical giant magnetoresistive devices

Experiments are reported that demonstrate the asymmetric switching behavior reported earlier in vertical giant magnetoresistance devices arises from the solid disk geometry of the device that produces a magnetic singularity at the disk center. Annular devices having a 0.1 μm center hole and 0.5 μm outer diameter are shown to switch symmetrically with an Amperian field.

Atomic layer deposited Ta2O5 gate insulation for enhancing breakdown voltage of AlN/GaN high electron mobility transistors

AlN/GaN heterostructures with a 3.5 nm AlN cap have been grown by molecular beam epitaxy followed by a 6 nm thick atomic layer deposited Ta2O5 film. Transistors fabricated with 150 nm length gates showed drain current density of 1.37 A/mm, transconductance of 315 mS/mm, and sustained drain-source biases up to 96 V while in the off-state before destructive breakdown as a result of the Ta2O5 gate insulator. Terman’s method has been modified for the multijunction capacitor and allowed the measurement of interface state density (∼1013 cm−2 eV−1). Small-signal frequency performance of 75 and 115 GHz was obtained for ft and fmax, respectively.