A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. Th... more A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values extracted from the wireless measurements agree with those determined from capacitance-voltage measurements, providing strong evidence that the sensing arises from the variable quantum capacitance in graphene. These results represent a new sensor transduction mechanism and pave the way for graphene quantum capacitance sensors to be studied for a wide range of chemical and biological sensing applications.
2007 International Semiconductor Device Research Symposium, 2007
Abstract III-V nitride-based HEMT technology has made rapid progress over the last decade. Benefi... more Abstract III-V nitride-based HEMT technology has made rapid progress over the last decade. Benefiting from the extremely high polarization charge, in this work, we demonstrate record high DC current density (2.9 A/mm) and very high extrinsic transconductance (~ 430 mS/mm) AIN/GaN HEMTs.
In this work, we have developed a technique to circumvent the low-temperature limitations of conv... more In this work, we have developed a technique to circumvent the low-temperature limitations of conventional lift-off by using an inorganic bi-layer of silicon nitride (SiN) and Ge to replace conventional photoresist stacks. By using standard photolithography and anisotropic fluorine plasma etching, a lift-off profile was fabricated that has been exposed to temperatures as high as 670 °C with no failure in mechanical integrity. This paper documents the results from the initial proof-of-concept experiment and then shows the technique's application to patterning molecular-beam epitaxy (MBE) regrowth of n + GaN on GaN.
ABSTRACT A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–... more ABSTRACT A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.
Abstract Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations hi... more Abstract Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations higher than 1x1013 cm-2 can be produced at the AlN/GaN heterojunction with AlN barriers as thin as 2 nm. This ultra-shallow channel together with the wide bandgap of AlN (6.2 eV) makes AlN/GaN heterojunction field effect transistors (HFET) extremely attractive for high frequency (> 100 GHz) high power applications. At Notre Dame, these structures have been grown using molecular beam epitaxy and the record transport ...
ABSTRACT In this paper, we present the first demonstration of a HfO2-insulated gate N-polar GaN i... more ABSTRACT In this paper, we present the first demonstration of a HfO2-insulated gate N-polar GaN inverted high-electron-mobility transistor (iHEMT). HfO2-insulated gate devices showed an order of magnitude improvement in reverse-bias gate leakage current as compared to reference Schottky devices. With the reduced gate leakage current, the insulated gate iHEMTs were able to simultaneously demonstrate breakdown voltages in excess of 130 V and maximum current density of 0.87 A/mm. Pulsed I–V gate-lag measurements were performed to investigate the drain current transient behavior of these devices.
ABSTRACT We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterost... more ABSTRACT We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5 nm and 7.5 nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0 nm exhibit the highest Hall mobility, approximately 1700 cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5 nm, coincident with the onset of strain relaxation.
ABSTRACT AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular be... more ABSTRACT AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (LG) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. LG = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency fT of 165 GHz, a maximum frequency of oscillation fmax of 171 GHz, and intrinsic average electron velocity ve of 1.5 ×107 cm/s. The 40-GHz load-pull measurements of LG = 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 ve, and high RF performance in AlN/GaN transistors.
A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. Th... more A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values extracted from the wireless measurements agree with those determined from capacitance-voltage measurements, providing strong evidence that the sensing arises from the variable quantum capacitance in graphene. These results represent a new sensor transduction mechanism and pave the way for graphene quantum capacitance sensors to be studied for a wide range of chemical and biological sensing applications.
2007 International Semiconductor Device Research Symposium, 2007
Abstract III-V nitride-based HEMT technology has made rapid progress over the last decade. Benefi... more Abstract III-V nitride-based HEMT technology has made rapid progress over the last decade. Benefiting from the extremely high polarization charge, in this work, we demonstrate record high DC current density (2.9 A/mm) and very high extrinsic transconductance (~ 430 mS/mm) AIN/GaN HEMTs.
In this work, we have developed a technique to circumvent the low-temperature limitations of conv... more In this work, we have developed a technique to circumvent the low-temperature limitations of conventional lift-off by using an inorganic bi-layer of silicon nitride (SiN) and Ge to replace conventional photoresist stacks. By using standard photolithography and anisotropic fluorine plasma etching, a lift-off profile was fabricated that has been exposed to temperatures as high as 670 °C with no failure in mechanical integrity. This paper documents the results from the initial proof-of-concept experiment and then shows the technique's application to patterning molecular-beam epitaxy (MBE) regrowth of n + GaN on GaN.
ABSTRACT A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–... more ABSTRACT A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.
Abstract Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations hi... more Abstract Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations higher than 1x1013 cm-2 can be produced at the AlN/GaN heterojunction with AlN barriers as thin as 2 nm. This ultra-shallow channel together with the wide bandgap of AlN (6.2 eV) makes AlN/GaN heterojunction field effect transistors (HFET) extremely attractive for high frequency (> 100 GHz) high power applications. At Notre Dame, these structures have been grown using molecular beam epitaxy and the record transport ...
ABSTRACT In this paper, we present the first demonstration of a HfO2-insulated gate N-polar GaN i... more ABSTRACT In this paper, we present the first demonstration of a HfO2-insulated gate N-polar GaN inverted high-electron-mobility transistor (iHEMT). HfO2-insulated gate devices showed an order of magnitude improvement in reverse-bias gate leakage current as compared to reference Schottky devices. With the reduced gate leakage current, the insulated gate iHEMTs were able to simultaneously demonstrate breakdown voltages in excess of 130 V and maximum current density of 0.87 A/mm. Pulsed I–V gate-lag measurements were performed to investigate the drain current transient behavior of these devices.
ABSTRACT We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterost... more ABSTRACT We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5 nm and 7.5 nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0 nm exhibit the highest Hall mobility, approximately 1700 cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5 nm, coincident with the onset of strain relaxation.
ABSTRACT AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular be... more ABSTRACT AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (LG) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. LG = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency fT of 165 GHz, a maximum frequency of oscillation fmax of 171 GHz, and intrinsic average electron velocity ve of 1.5 ×107 cm/s. The 40-GHz load-pull measurements of LG = 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 ve, and high RF performance in AlN/GaN transistors.
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