Wen-Chau Liu

Hydrogen-sensitive characteristics of a novel Pd/InP MOS Schottky diode hydrogen sensor

Steady-state and transient hydrogen-sensing characteristics of a novel Pd/InP metal-oxide-semiconductor (MOS) Schottky diode under atmospheric conditions are presented and studied. In presence of oxide layer, the significant increase of barrier height improves the hydrogen sensitivity even at lower operating temperatures. Even at a very low hydrogen concentration environment, e.g., 15 ppm H/sub 2/ in air, a significant response is obtained. Two effects, i.e., the removal of Fermi-level pinning caused by the donor level in the oxide and the reduction of Pd metal work function dominate the hydrogen sensing mechanism. Furthermore, the reaction kinetics incorporating the water formation upon hydrogen adsorption is investigated. The initial heat of adsorption for the Pd/oxide interface is estimated to be 0.42 eV/hydrogen atom. The coverage dependent heat of adsorption plays an important role in hydrogen response under steady-state conditions. In accordance with the Temkin isotherm behavior, the theoretical prediction of interface coverage agrees well with the experimental results over more than three decades of hydrogen partial pressure.

Comparative Hydrogen-Sensing Study of Pd/GaAs and Pd/InP Metal-Oxide-Semiconductor Schottky Diodes

In this work, the hydrogen response characteristics and sensing properties of catalytic Pd metal-oxide-semiconductor (MOS) Schottky diodes based on both GaAs and InP substrates are comparatively investigated. The behaviors of interface Fermi-level pinning causing the apparent difference in the barrier height modulation and the sensitivity are discussed. Furthermore, the effects of temperature and hydrogen concentration in the initial rate of change in current and the response time are examined. In order to study the steady-state reaction kinetics, we also performed the Langmuir analysis to estimate the values of initial heat of adsorption for both devices. Based on the adsorption isotherm of Temkin type, the interface coverage dependent heat of adsorption is responsible for the wide hydrogen sensing range. From the theoretical prediction, both devices have a very low sensitivity limit under atmospheric conditions.

Characteristics of Pd/InGaP Schottky diodes hydrogen sensors

Pd/InGaP hydrogen sensors based on the metal-oxide-semiconductor (MOS) and metal-semiconductor Schottky diodes have been fabricated and systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, barrier height variation, hydrogen coverage, and heat of adsorption are investigated. The studied devices exhibit very wide hydrogen concentration detection regimes and remarkable hydrogen-sensing properties. Particularly, an extremely low hydrogen concentration of 15 ppm H/sub 2//air at room temperature can be detected. In addition, under the presence of oxide layers in the studied MOS device structure, the enhancements of barrier height and high-temperature operating capability are observed. The initial heat of adsorption for Pd/oxide and Pd/semiconductor interface are calculated as 355 and 65.9 meV/atom, respectively. Furthermore, the considerably short response times are found in studied devices.

An improved heterostructure-emitter bipolar transistor (HEBT)

An N-Al/sub 0.5/Ga/sub 0.5/As/n-GaAs heterostructure-emitter bipolar transistor (HEBT), grown by MBE, has been fabricated with improved performance. The Al/sub 0.5/Ga/sub 0.5/As layer with higher valence-band offset Delta E/sub v/ offers better confinement of minority carriers (holes). A small offset voltage of about 80 mV and a high common-emitter current gain of 180 were measured for large-area devices. On the other hand, the adequate design of an n-GaAs emitter layer also plays an important role in device performance. The strongly knee-shaped characteristics and reachthrough effect were observed in those devices fabricated with 12 a thin n-GaAs emitter layer. >

A Novel

On the basis of a Pt/In0.52Al0.48As metal-semiconductor structure, a novel hydrogen sensor is fabricated and demonstrated. The studied Pt/In0.52Al0.48As Schottky diode-type hydrogen sensor exhibits significant sensing performance including high relative sensitivity ratio of about 2600% (under the 1% H2/air gas and VR=-0.5 V at 30 degC), large current variation of 310 muA (under the 1% H2/air gas and VR=-5 V at 200 degC), widespread reverse-voltage regime (0~-5 V), stable hydrogen-sensing current-voltage (I-V) curves, and fast transient response time of 1.5 s. The calculated Schottky barrier-height change and series-resistance variation, from the thermionic-emission model and Norde method, are 87.0 meV and 288 Omega, respectively (under the 1% H2/air gas at 30 degC). The hydrogen concentrations and operating temperatures tested in this letter are in the range of 15 ppm-1% H2/air and 30 degC-250 degC, respectively. Based on the excellent integration compatibility with InP-based electronic devices, the studied device provides the potentiality in high-performance sensor-array applications

\hbox{Pt/In}_{0.52}\hbox{Al}_{0.48}\hbox{As}

The temperature-dependent characteristics of an n+-InGaAs/n-GaAs composite doped channel (CDC) heterostructure field-effect transistor (HFET) have been studied. Due to the reduction of leakage current and good carrier confinement in the n+-InGaAs/n-GaAs CDC structure, the degradation of device performances with increasing the temperature is insignificant. Experimentally, for a 1 x 100 μm2 device, the gate-drain breakdown voltage of 24.5 (22.0) V, turn-on voltage of 2.05 (1.70) V, off-state drain-source breakdown voltage of 24.4 (18.7) V, transconductance of 161 (138) mS/mm, output conductance of 0.60 (0.60) mS/mm, and voltage gain of 268 (230) are obtained at 300 (450) K, respectively. The shift of Vth from 300 to 450 K is only 13 mV. In addition, the studied device also shows good microwave performances with flat and wide operation regime.

Schottky Diode-Type Hydrogen Sensor

The temperature-dependent characteristics of polysilicon and diffused resistors have been studied. By using 0.18-/spl mu/m CMOS technology, a cobalt salicide process is employed and silicide is formed at the ends of resistors. Based on a simple and useful model, some important parameters of resistors including bulk sheet resistance (R/sub bulk/) and interface resistance (R/sub interface/) are obtained at different temperature. For diffused resistors, the R/sub bulk/ and R/sub interface/ values increase and decrease with increase of temperature, respectively. Positive values of temperature coefficient of resistance (TCR) are observed. Furthermore, TCR values decrease with decreasing resistor size. For polysilicon resistors, the R/sub interface/ values decrease with increase of temperature. In addition, negative and positive TCR values of R/sub bulk/ are found in n/sup +/ and p/sup +/ polysilicon resistors, respectively. In conclusion, by comparing the studied diffused and polysilicon resistors, negative trends of TCR are observed when the resistor size decreases.

Investigation of temperature-dependent characteristics of an n/sup +/-InGaAs/n-GaAs composite doped channel HFET

A new and interesting Pd-oxide-Al/sub 0.3/Ga/sub 0.7/As MOS hydrogen sensor has been fabricated and studied. The steady-state and transient responses with different hydrogen concentrations has been measured at various temperatures. Based on the large Schottky barrier height and presence of oxide layer, the studied device exhibits a high hydrogen detection sensitivity and wide temperature operating regime. The studied device exhibits the low-leakage current and obvious current changes when exposed to hydrogen-contained gas. Even at room temperature, a very high hydrogen detection sensitivity of 155.9 is obtained when a 9090 ppm H/sub 2//air gas is introduced. Furthermore, when exposed to hydrogen-contained gas at 95/spl deg/C, both the forward and reverse currents are substantially increased with increased hydrogen concentration. In other words, the studied device can be used as a hydrogen sensor under the applied bidirectional bias. Under the applied voltage of 0.35 V and 9090 ppm H/sub 2//air hydrogen ambient, a fast adsorption response time about 10 s is found. The transient and steady-state characteristics of hydrogen adsorption are also investigated.

Temperature-dependent characteristics of polysilicon and diffused resistors

The influences of (NH/sub 4/)/sub 2/S/sub x/ treatment on an AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) are studied and demonstrated. Upon the sulfur passivation, the studied device exhibits better temperature-dependent dc and microwave characteristics. Experimentally, for a 1/spl times/100 /spl mu/m/sup 2/ gate/dimension PHEMT with sulfur passivation, the higher gate/drain breakdown voltage of 36.4 (21.5) V, higher turn-on voltage of 0.994 (0.69) V, lower gate leakage current of 0.6 (571) /spl mu/A/mm at V/sub GD/=-22 V, improved threshold voltage of -1.62 (-1.71) V, higher maximum transconductance of 240 (211) mS/mm with 348 (242) mA/mm broad operating regime (>0.9g/sub m,max/), and lower output conductance of 0.51 (0.53) mS/mm are obtained, respectively, at 300 (510) K. The corresponding unity current gain cutoff frequency f/sub T/ (maximum oscillation frequency f/sub max/) are 22.2 (87.9) and 19.5 (59.3) GHz at 250 and 400 K, respectively, with considerably broad operating regimes (>0.8f/sub T/,f/sub max/) larger than 455 mA/mm. Moreover, the relatively lower variations of device performances over wide temperature range (300/spl sim/510 K) are observed.

A new Pd-oxide-Al/sub 0.3/Ga/sub 0.7/As MOS hydrogen sensor

A new and interesting Pt/oxide/In/sub 0.49/Ga/sub 0.51/P metal-oxide-semiconductor (MOS) Schottky diode hydrogen sensor has been fabricated and studied. The steady-state and transient responses with different hydrogen concentrations and at different temperatures are measured. The presence of dipoles at the oxide layer leads to an extra electrical field and the variation of Schottky barrier height. Even at room temperature, a very high hydrogen detection sensitivity of 561% is obtained when a 9090 ppm H/sub 2//air gas is introduced. In addition, an absorption response time less than 1 s under the applied voltage of 0.7 V and 9090 ppm H/sub 2//air hydrogen ambient is found. The roles of hydrogen adsorption and desorption for the transient response at different temperatures are also investigated.