Huey-Ing Chen

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.

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}

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.

Schottky Diode-Type 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.

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

In this paper, we study and compare the hydrogen sensing performances of Pd/InP Schottky diode sensors fabricated by electroless plating and thermal evaporation. Experimental results show that the used electroless plated Schottky diode exhibits superior detection sensitivity on hydrogen with a wider detection range and lower detection limit. The Schottky barrier height change ΔΦBn and the ideality factor n are increased with the increase of hydrogen concentration. Under a hydrogen concentration about 5000 ppm H2/air, ΔΦBn and n reach their saturation values. Compared with the thermal evaporated diode, the performances of larger maximum barrier height lowering and nearer unity ideality factor are observed in the Schottky diode fabricated by electroless plating. It is comprehensible that more charge states are created at the electroless plated Pd/InP interface, which allows the adsorption of more hydrogen atoms and results in superior sensing performances.

A new Pt/oxide/In/sub 0.49/Ga/sub 0.51/P MOS Schottky diode hydrogen sensor

A new Pt/oxide/InGaP metal-oxide semiconductor (MOS) Schottky diode has been fabricated and studied. Upon exposure to hydrogen, the steady-state and transient responses under different hydrogen concentrations and temperatures are measured. Due to the inherent property of InGaP material, e.g. the wide energy gap, a wide hydrogen-sensing range as large as 300 K (from room temperature to 600 K) is obtained. Even at room temperature, a very high sensitivity over 500% for 9090 ppm hydrogen in air is acquired. Furthermore, the measured absorption response time is less than 1 s at the applied voltage of 0.7 V and 9090 ppm hydrogen concentration atmosphere condition. Simultaneously, based on the analysis of the variation of barrier height and hydrogen coverage, the characteristics of the studied Pt/oxide/InGaP MOS Schottky diode is in good agreement with the Lundstrom isotherm.

A comparative study of hydrogen sensing performances between electroless plated and thermal evaporated Pd/InP Schottky diodes

In general, two factors, i.e. the Schottky barrier height (ΦBn) and the ideality factor (n) based on thermionic emission current–voltage (I–V) analysis, are used to evaluate the interfacial quality of Schottky diodes. In this paper, we propose an alternative method, different from the traditional I–V analysis, to evaluate the perfection of the Pd–InP Schottky interface. By introducing hydrogen adsorbates into the Pd–InP interface, it is found that the interfacial hydrogen energy state can reflect the perfection of the Pd–InP Schottky interface. The Pd/InP Schottky diode with a more perfect interface infers that the interfacial hydrogen adsorbates possess higher energy states. Compared with the thermal evaporated Pd–InP Schottky interface, the electroless plated diode exhibits superior I–V characteristics. Also, it demonstrates that hydrogen adsorbates at the electroless plated Pd–InP interface truly possess higher energy states, e.g., Gibbs energy (G) and enthalpy (H). Therefore, this proposed method can be used for evaluating the perfection of the Pd–InP Schottky interface. Also, the result is consistent with that obtained by traditional I–V analysis.

Characteristics of a new Pt/oxide/In0.49Ga0.51P hydrogen-sensing Schottky diode

An interesting hydrogen sensing Pd/InGaP metal-semiconductor (MS) Schottky diode has been fabricated and studied. Both the steady state and the transient condition of the hydrogen adsorption process are investigated. Even at room temperature, an extremely low hydrogen concentration of 15 ppm H2/air can be detected. In addition, the wide operating temperature range of 250 K of the studied Pd/InGaP hydrogen sensor is found. From experimental results, it is shown that the variation of Schottky barrier height increases with the increase of the operating temperature and hydrogen concentration. As the operation temperature is elevated, the water formation effect is also studied in the quasi-equilibrium region under the transient condition.