Shao-Yen Chiu

High-Sensitivity Metal–Semiconductor–Metal Hydrogen Sensors With a Mixture of Pd and

New metal-semiconductor-metal hydrogen sensors are fabricated to take advantages of symmetrically bidirectional detection. Unlike commonly used single catalytic metal layers, a mixture of Pd and SiO2 inserted between Pd and GaN was employed as sensing media. There are three sensing regions (i.e., 2-D dipole, transient, and 3-D dipole regions) observed in static response. Room-temperature sensitivity larger than 107 was obtained in 1080-ppm H2/N2 ambient. The barrier-height variation is as high as 422 mV. To our best knowledge, these are the highest values ever reported. According to transient response, a short response time of 70 s is obtained at room temperature. Thus, a newly developed concept of forming 3-D dipoles is introduced to possibly explain experimental results.

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A metal–semiconductor–metal Pd/GaN hydrogen sensor with a porous-like mixture of Pd and SiO2 is investigated. Besides symmetrically bidirectional sensing characteristics with a widespread voltage regime (at least −5–5 V), a high sensing response of 8 × 105 (7.7 × 106) corresponding to a Schottky barrier height variation of 352 (411) meV is obtained in a 4890 ppm H2/N2 ambience at a voltage of −1.5 (−5) V. A highly efficient dissociation of hydrogen molecules due to an enhanced catalytic activity of the mixture explains the improved performance. Furthermore, dynamic responses by alternately switching voltage polarity and introducing and removing hydrogen-containing gases are also included to evaluate the proposed device as a high sensing response and low power sensor.

Forming Three-Dimensional Dipoles

New hydrogen-sensing amplifiers are fabricated by integrating a GaAs Schottky-type hydrogen sensor and an InGaP-GaAs heterojunction bipolar transistor. Sensing collector currents (ICN and ICH) reflecting to N2 and hydrogen-containing gases are employed as output signals in common-emitter characteristics. Gummel-plot sensing characteristics with testing gases as inputs show a high sensing-collector-current gain (ICH/ICN) of > 3000. When operating in standby mode for in situ long-term detection, power consumption is smaller than 0.4 ¿W. Furthermore, the room-temperature response time is 85 s for the integrated hydrogen-sensing amplifier fabricated with a bipolar-type structure.

High sensing response Pd/GaN hydrogen sensors with a porous-like mixture of Pd and SiO2

Depositing gate metal across a step undercut between the Schottky barrier layer and the insulator-like layer is employed to obtain a reduced gate length of 0.4 mum with an additional 0.6-mum field plate from a 1-mum gate window. Most dc and ac characteristics including current density (IDSS=451mA/mm), transconductance (gm,max=225mS/mm), breakdown voltages (VBD(DS)/V BD(GD)=22/-25.5V), gate-voltage swing (GVS=2.24V), cutoff, and maximum oscillation frequencies (ft/fmax=17.2/32GHz) are improved as compared to those of a 1-mum gate device without field plate. At a VDS of 4.0 V, a maximum power added efficiency of 36% with an output power of 13.9 dBm and a power gain of 8.7 dB are obtained at a frequency of 1.8 GHz. The saturated output power and the linear power gain are 316 mW/mm and 13 dB, respectively

Integrated Hydrogen-Sensing Amplifier With GaAs Schottky-Type Diode and InGaP – GaAs Heterojunction Bipolar Transistor

Temperature-dependent dark and optical characteristics of the InGaP/GaAs heterojunction phototransistors (HPTs) with and without sulfur treatment are studied. As compared to the HPT without (NH 4 ) 2 S treatment (HPT A), treatment at 50°C for 20 min leads to a reduced p-i-n dark current (/dark) and a reduced collector dark current (IC dark ) for the HPT (HPT D) in the emitter-floated and base-floated configurations, respectively. Moreover, the effective reduction of the surface defects also induces an enhanced p-i-n photocurrent (I ph ). The enhanced I ph combined with the promoted dc current gain results in an enhanced optical gain (G) and signal-to-noise ratio (SNR). For HPT A (D) under P in = 107.6 nW at 298 K, the G is 1.42 (20.3) while the SNR is 42 (94) dB. Experimental results indicate that the treated HPTs, compared to the untreated one, are more sensitive to low-power illumination.

InGaP/InGaAs Pseudomorphic Heterodoped-Channel FETs With a Field Plate and a Reduced Gate Length by Splitting Gate Metal

In this article, a novel InGaP/GaAs pnp δ-doped heterojunction bipolar transistor is first demonstrated. Though the valence band discontinuity at InGaP/GaAs heterojunction is relatively large, the addition of a δ-doped sheet between two spacer layers at the emitter-base (E-B) junction effectively eliminates the potential spike and increases the confined barrier for electrons, simultaneously. Experimentally, a high current gain of 25 and a relatively low E-B offset voltage of 60 mV are achieved. The offset voltage is much smaller than the conventional InGaP/GaAs pnp HBT. The proposed device could be used for linear amplifiers and low-power complementary integrated circuit applications.

Promoted Potential of Heterojunction Phototransistor for Low-Power Photodetection by Surface Sulfur Treatment

The roles of micro-metal–oxide (MO) interfaces inside a sensing metal formed by coevaporating Pd and SiO2 in metal–semiconductor–metal GaN sensors are investigated. The porous property of the Pd and SiO2 mixture together with the presence of micro-MO interfaces gives rise to a highly efficient dissociation of hydrogen molecules and hence an enhanced barrier height variation (ΔB) of a reverse-biased Schottky diode. The measured ΔB increases from 294 to 392 mV at a concentration coefficient of 25 mV/decade as the hydrogen concentration increases from 2.13 to 10100 ppm H2/N2. Therefore, when the sensor is subjected to 0.02 ppm H2/N2, ΔB as high as 245 mV is still expected. The sensor in a 2.13 ppm H2/N2 ambience has a sensing response of 8.7×104. Excellent dynamic responses are demonstrated by switching voltage polarity or continuously changing hydrogen concentration, showing that the proposed structure is a promising hydrogen sensor.

High-performance InGaP/GaAs pnp δ-doped heterojunction bipolar transistor

Depletion-mode Al0.24Ga0.76As/In0.22Ga0.78As double-heterojunction high electron mobility transistors (DH-HEMTs) were fabricated with an as deposited gate to compare with those with a buried gate by annealing. Instead of a recessed gate, a buried gate used to control the distance between the gate and channel (and hence the aspect ratio) improves the series resistance. Measured transconductance of 150 mS mm−1 and an open-drain voltage gain of 136 for the DH-HEMT with an as deposited gate are enhanced to 175 mS mm−1 and 160 for the DH-HEMT with a 330 °C annealed gate. Good device linearity is also obtained with a low second harmonic to fundamental ratio of 3.55%. The measured maximums fts (fmaxs) are 13.5, 13.5 and 14.5 (35, 37, and 37.5) GHz for DH-HEMTs with an as deposited gate, and with 280 °C and 330 °C annealed gates, respectively. At a measured frequency of 2.4 GHz, the DH-HEMT with a 330 °C annealed gate exhibits the highest PAE = 44.8% at VDS = 3 V and VGS = −1.0 V and the lowest Fmin = 1.89 dB at VDS = 3 V and ID = 200 mA mm−1.

GaN Sensors with Metal–Oxide Mixture for Sensing Hydrogen-Containing Gases of Ultralow Concentration

In this article, high-performance InGaP∕InGaAs∕GaAs δ-doped pseudomorphic modulation-doped field effect transistors (pMODFETs) employing n+-GaAs∕p+-InGaP∕n-InGaP camel-gate structure are characterized. Because of the depleted p-n junction in the gate region and the presence of the large conduction-band discontinuity of the InGaP∕InGaAs heterostructure, a large gate turn-on voltage is obtained. The dc and microwave characteristics of single- and double-δ-doped pMODFETs are demonstrated. The double-δ-doped pMODFET exhibits a higher drain saturation current, a larger transconductance, a broader gate voltage swing, and better high-frequency responses than the single-δ-doped device. The excellent performance of the studied devices is promising for linear and large signal amplifiers and high-frequency circuit applications.

Performance of Al0.24Ga0.76As/In0.22Ga0.78As double-heterojunction HEMTs with an as deposited and a buried gate

Depletion-mode δ-doped In0.5Ga0.5As/In0.5Al0.5As mHEMTs have been metamorphically grown on a GaAs substrate and successfully fabricated with different kinds of gate-metal formation. The gate-metal formations include a combination of mesa- or air-type gate feeder with or without a buried gate (before or after annealing). Only the air-type mHEMT with a buried gate shows no clear kink-effect behaviour. For a 1 µm gate mesa-type (air-type) mHEMT, a maximum extrinsic transconductance of 412 (414) and 535 (472) mS mm−1 is obtained before annealing and after annealing. There is a 207 mV (205 mV) shift in VTH after the mesa-type (air-type) mHEMT is annealed. Experimental results indicate that both the gate-feeder metal and the annealing process have a significant effect on output conductance, gate leakage current, breakdown voltage, high frequency and noise performances. The peak gate leakage current density of 12 (120) µA mm−1 for the air-type (mesa-type) mHEMT before annealing is improved to 8 (55) µA mm−1 after annealing. At 2.4 (5.4) GHz, gain = 23 (20) dB can be obtained at Fmin = 1.27 (1.76) dB for the air-type mHEMT after annealing, while gain = 22 (18.5) dB is obtained at Fmin = 1.36 (2.0) dB before annealing. For the mesa-type mHEMT, these values are gain = 20 (16.5) dB at Fmin = 1.47 (2.25) dB and gain = 19.5 (14) dB at Fmin = 1.68 (3.11) dB.