Hydrogen sensing properties of a metamorphic high electron mobility transistor

Hydrogen sensing properties of a metamorphic high electron mobility

transistor

Tsung-Han Tsai,1Huey-Ing Chen,2Chung-Fu Chang,1Po-Shun Chiu,1Yi-Chun Liu,1 Li-Yang Chen,1Tzu-Pin Chen,1and Wen-Chau Liu1,a兲

1

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan 70101, Republic of China

2

Department of Chemical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan 70101, Republic of China

共Received 7 October 2008; accepted 24 November 2008; published online 6 January 2009兲 Hydrogen sensing properties of a metamorphic high electron mobility transistor 共MHEMT兲 are studied and presented. This MHEMT-based sensor exhibits good pinch-off characteristics upon exposing to hydrogen gases. Besides, the current variation and threshold voltage shift of the studied device reveal larger response under hydrogen-containing conditions. The studied device shows fast responses and exhibits a large current variation magnitude of the order of milliamperes and a relatively low sensitivity due to the high baseline current. Based on the Langmiur isotherm, experimental current responses are consistent with the simulated curve. This indicates that the surface reaction is the rate limited factor for this hydrogen adsorption reaction. © 2009 American Institute of Physics.关DOI:10.1063/1.3052698兴

Due to the increasing requirement of hydrogen, the de-velopment of the hydrogen sensor has become an important issue over the past years. Semiconductor-type hydrogen sen-sors have attracted considerable attention due to their com-patibility with conventional fabrication process. Previously, the Schottky diode-based hydrogen sensors were widely studied and reported.1–3With the rapid progress in epitaxial technology, more complicated device structures can be achieved. Electron devices, for instance, the light emitting diode and high electron mobility transistor共HEMT兲 largely require mature and high-quality epitaxy. Recently, much at-tention has been paid to HEMT-based hydrogen sensors.4,5 Yet, the report related metamorphic HEMT共MHEMT兲-based hydrogen sensors has not been found. Compared to the InP HEMT, the MHEMT provides the merits of low cost, less fragility, and large substrate size.6 The high indium content and large bandgap discontinuity at channel layer can largely increase the electron mobility and thus lead to good electron transport characteristics.7 In this work, a hydrogen sensor based on an InAlAs-MHEMT with a Pd metal gate is fabri-cated and demonstrated. This MHEMT-based sensor exhibits a larger current variation and voltage shift as compared with previous reports.8,9

The epitaxial structure was grown on a GaAs substrate by a metal organic chemical vapor deposition system. The epitaxial layers consisted of a 2 ␮m thick indium-graded InxAl1−xAs metamorphic buffer layer, a 3000 Å thick In_0.42Al_0.58As barrier layer, a 200 Å thick In_0.43Ga_0.57As channel layer, a 50 Å thick In0.42Al0.58As spacer layer, a single␦-doped sheet, a 200 Å thick In_0.42Al_0.58As Schottky contact layer, and a 50 Å thick n+_-In

0.43Ga0.57As cap layer. The metamorphic buffer layer can accommodate the lattice mismatch between the GaAs substrate and InGaAs channel layer. The large bandgap of InAlAs can supply good Schottky characteristics. Mesa isolation was made by wet

chemical etching 共H3PO4: H2O2: H2O兲. Ohmic contacts were formed on the cap layer by evaporating AuGe/Au metals, followed by a 603 K annealing for 15 s. Furthermore, a highly selective PH-adjusted solution 共succinic acid: H2O2: NH4OH兲 was employed to remove the cap layer for gate recess. Finally, the gate Schottky contact was achieved by evaporating the Pd catalytic on the 200 Å un-doped In0.42Al0.58As Schottky layer. The gate pattern was 1 ⫻100 ␮m2_{in dimension.}

Figure 1 illustrates the typical drain-source current-voltage 共I-V兲 characteristics of the studied Pd/InAlAs MHEMT hydrogen sensor at 303 K. All I-V curves reveal good pinch-off, saturation characteristics, and normally on field-effect transistor 共FET兲 characteristics. Clearly, the cur-rent response increases with increasing hydrogen concentra-tion. The large current variation can be attributed to the am-plification behavior of the transistor. A largest current variation value of about 2.2 mA is observed at VDS= 1.5 and VGS= 0 V upon exposing to a 1% H2/air gas. The current variation of the studied device is much larger than that of other HEMT-based hydrogen sensors.8,9 The current

varia-a兲_{Author to whom correspondence should be addressed. Electronic mail:}

wcliu@mail.ncku.edu.tw. FAX:⫹886-6-209-4786.

FIG. 1. Typical drain-source current-voltage 共I-V兲 characteristics of the studied Pd/InAlAs MEMT hydrogen sensor under different hydrogen gases at 303 K.

APPLIED PHYSICS LETTERS 94, 012102共2009兲

0003-6951/2009/94_{共1兲/012102/3/$23.00} 94, 012102-1 © 2009 American Institute of Physics

tion in response to hydrogen gas exposure can be interpreted as follows.10 The hydrogen molecules are effectively disso-ciated into hydrogen atoms by the catalytically active metal. The concentration gradients of hydrogen atoms between the Pd surface and Pd/InAlAs interface drive hydrogen atoms to diffuse through the Pd metal bulk to the Pd metal/ semiconductor interface. Then, the internal electric field po-larizes the hydrogen atoms and thus forms a dipole layer at the Pd/InAlAs interface. The density of the two-dimensional electron gas共2-DEG兲 at the channel layer is increased by the reduction in the depletion region due to dipole formation.10 The increased 2-DEG leads to the increase in current varia-tion when exposing to hydrogen gases. As compared to the FET sensor reported by Andrei et al.,11the proposed device exhibits a large current variation magnitude of the order of the order of milliamperes. However, the reported FET sensor has a relatively high sensitivity due to its low baseline current.

Figure 2 illustrates the maximum transconductance gm,max and threshold voltage Vth as a function of hydrogen concentration at 303 K. The magnitude of the threshold volt-age Vth increases by the increase in hydrogen concentration. The decrease in the Vthmagnitude with hydrogen concentra-tion can be ascribed to the increased density of 2-DEG at channel layer. Therefore, more negative gate bias is needed to deplete the channel layer. The studied device shows a significant threshold voltage shift⌬Vthof 260 mV upon ex-posing to a 1% H2/air gas. The maximum magnitude of ⌬Vth for the Pd MHEMT-based hydrogen sensor is larger than that of GaAs 共60 mV兲 and AlGaAs 共90.5 mV兲 HEMT-based ones.8,9Besides, the magnitude of V_thalso shows an interest-ing and relatively linear trend under the hydrogen concentra-tion from 50 to 1000 ppm H2/air. Thus, the studied Pd/ InAlAs MHEMT hydrogen sensor exhibits good linearity upon exposing to intermediate-concentration hydrogen gases. Figure 2also reveals that the maximum transconduc-tance gm,max is decreased with increasing hydrogen concen-tration. The gm,maxslightly decreasing with increasing hydro-gen concentration suggests that the gate control ability slightly decreases with the increased hydrogen concentration. The transient response curves of the studied device at 363 K are shown in Fig.3. Apparently, the current variation increases with the hydrogen concentration. Compared to the Pt-FET hydrogen sensor,12 the studied device has relatively short response times under the same testing conditions. For example, under exposing to a 50 ppm H2/air gas at 363 K,

the response time of the studied device is 30 s, which is remarkably shorter than that of the reported sensor共100 s兲.12 The overshooting phenomenon is observed while the hydro-gen concentration is larger than 50 ppm H₂/air. This phe-nomenon can be attributed to the hydrogen-oxygen interac-tion at high temperatures, which produces more water molecules and hydroxyl.13 The Pd surface is covered with oxygen molecules at air atmosphere. All hydrogen molecules impinging on the catalytic Pd metal react with oxygen mol-ecules to form the hydroxyl. Subsequently, the hydroxyl re-acts with the hydrogen atoms to form water molecules at higher temperatures.13 Therefore, the overshooting phenom-enon is observed due to the formation of water molecules. The abnormal current variation is not observed for the intro-duction of 50 ppm H2/air gas. This indicates that the pro-duced water is negligible under exposing to low-concentration hydrogen gases.

The reactions of hydrogen adsorption can be briefly ex-pressed as H2
d₁ c₁ 2Ha, 共1兲 O2+ 2Ha
d₂ c₂ 2OHa, 共2兲

where c1and c2are the forward reaction constants, d1and d2 are the constants for reverse reactions, Ha is the hydrogen atom, and OHa is the hydroxyl, respectively. The reverse reactions of Eqs. 共1兲 and 共2兲 are neglected to simplify the analyses. Thus, the relation between the steady-state current 共Ig,st兲 and hydrogen partial pressure PH₂ can be expressed as14 log

冉

Ig,st Io

冊

=

冉

␮ ␧

冊

冉

c1PH₂ 2c₂P_O 2

冊

1/2 1 +

冉

c1PH2 2c2PO₂

冊

1/2 N0 VT , 共3兲

where Iois the steady-state current in air ambiance, ␮is the dipole moment, ␧ is the permittivity of InAlAs, PO₂ is the oxygen partial pressure, Nois the total number of adsorption sites at the interface, and VTis the thermal voltage. Equation

共3兲 is a Langmiur-type equation.15 Figure 4 illustrates the logarithmic value of the ratio between Ig,stand Io共log Ig,st/Io兲 steady-state current共Ig,st兲 versus the square root of hydrogen FIG. 2. Maximum transconductance gm,maxand threshold voltage Vth as a

function of hydrogen concentration.

FIG. 3. Dynamic response curves of the studied Pd/InAlAs MHEMT hy-drogen sensor upon exposing to different H2/air gases.

012102-2 Tsai et al. Appl. Phys. Lett. 94, 012102共2009兲

partial pressure共

冑

PH₂兲 at 363 K. Although the overshooting phenomenon is revealed in Fig.3, the simulated curve is still consistent with experimental results. The consistence be-tween the experimental data and simulation indicates that the adsorption of hydrogen is dominated by the surface reaction i.e., the surface reaction is the rate-limited factor of this reaction.14

In conclusion, an interesting hydrogen sensor based on a MHEMT with a Pd/InAlAs structure is fabricated and dem-onstrated. The studied MHEMT sensor exhibits the larger signal response in terms of current variation and voltage shift as compared with previous reports. Based on the Langmiur isotherm, the experimental current responses are consistent with the simulated curve. This result indicates that the sur-face reaction is the rate limited factor for this hydrogen ad-sorption reaction. Consequently, based on the sensing

capa-bility and its well-known high-frequency properties, the studied Pd/InAlAs MHEMT hydrogen sensor provides the promise for the integration between high-performance hy-drogen sensors and microwave devices.

Part of this work was supported by the National Science Council of the Republic of China under Contract No. NSC-97–2221-E-006–237-MY3.

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FIG. 4. Logarithmic value of the ratio between Ig,stand Io共log Ig,st/Io兲 vs the

square root of hydrogen partial pressure共

冑

PH₂兲 at 363 K.

012102-3 Tsai et al. Appl. Phys. Lett. 94, 012102共2009兲