-
[show abstract]
[hide abstract]
ABSTRACT: The interesting hydrogen sensing properties of a Pt-oxide-GaN metal-oxide-semiconductor-type Schottky diode are comprehensively studied and demonstrated. In the hydrogen-containing environment, the shift in current-voltage curves and decrease in turn-on voltage are found to be caused by the lowering of Schottky barrier height. Also, the corresponding series resistance is decreased from 191.8 (in air) to 155.3 Ω (for a 9970 ppm H <sub>2</sub>/ air gas) at 30 ° C . As the carrier gas is replaced by a nitrogen gas, a significant variation of 0.32 V and 19.56 Ω in the turn-on voltage V<sub>on</sub> and series resistance R<sub>s</sub> values, respectively, is obtained at 30 ° C , even at an extremely low hydrogen concentration of 4.3 ppm H <sub>2</sub>/ N <sub>2</sub> . Since the oxygen atoms will be dissolved on the Pt metal surface and react with hydrogen atoms by the formation of hydroxyl and water, the number of adsorbed hydrogen atoms on the Pt surface is reduced. Moreover, the shorter response time constant and the larger initial rate of current density variation are found even at room temperature.
Journal of Applied Physics 08/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this work, the characteristics of the
heterojunction bipolar transistors with different emitter-edge-thinning thickness were systematically investigated. A stronger
downward-band-bending phenomenon was observed at the edge of emitter-edge-thinning intersection with the exposed base surface.
This band bending induced the presence of a potential saddle point, which substantially increased the recombination rates
and electron densities. In addition, the decision of emitter-edge-thinning thickness plays a key role in reducing surface
recombination at the potential saddle point. As the emitter-edge-thinning thickness was selected between 100 and
, the lowest recombination rate and electron density and highest dc current gain could be obtained. Furthermore, good agreements
between the theoretical analyses and experimental results were found.
Journal of The Electrochemical Society. 03/2007; 154(4):H289-H292.
-
[show abstract]
[hide abstract]
ABSTRACT: Comprehensive and systematical comparisons of temperature-dependent characteristics of In0.42Al0.58As/In0.46Ga0.54As metamorphic heterostructure field-effect transistors (MHFETs) with various Schottky gate alloys are studied and demonstrated. The influence of the Schottky barrier height on the impact ionization effect and its associated device performance are also investigated. Better dc and microwave characteristics can be obtained by using the higher metal work function of gate alloys, e.g., Ti/Au, Ni/Au and Pt/Au. In particular, the device with a Pt/Au gate alloy shows the superior device performance in breakdown voltage, threshold voltage, maximum transconductance, output conductance, voltage gain and microwave properties at room temperature. Furthermore, the device with a Ti/Au gate alloy shows the thermally stable performance in threshold voltage, maximum transconductance, output conductance and voltage gain over a wide operating temperature range (from 300 to 510 K). Consequently, the studied devices with appropriate Schottky gate contacts provide the promise for high-speed and high-temperature electronic applications.
Semiconductor Science and Technology 03/2007; 22(5):475. · 1.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The thermal stability performance of double -doped In 0.42 Al 0.58 As/In 0.46 Ga 0.54 As metamorphic heterostructure field-effect tran-sistors with Au and Ti/Au metal gates are comprehensively studied and demonstrated. By evaporating the Ti/Au metal gate, the thermal stability of device characteristics are significantly improved as compared with the device with conventional metal gate Au. Experimentally, the device with a Ti/Au metal gate simultaneously exhibits the considerably lower temperature degradation in turn-on voltage −2.19 mV/K, breakdown voltage −34 mV/K, logic swing −1.24 mV/K, transition region width 0.05 mV/K, on-off current ratio −3.55 /K, threshold voltage −0.25 mV/K, impact ionization-induced gate current 1.63 10 −3 A/mm K, output conductance 1.23 S/mm K, and voltage gain −0.33 /K as the temperature is increased from 300 to 510 K. Consequently, the studied device with a Ti/Au metal gate is a good candidate for high-speed and high-temperature digital and switching circuit applications. Recently, metamorphic heterostructure field-effect transistors MHFETs have received much attention for high-performance inte-grated electronic circuit applications. 1,2 As compared with InP-based transistors, 3,4 the metamorphic structure offers several advantages such as low cost, less fragility, large substrate size, high crystal quality, and mature back-side processing. In addition, the metamor-phic structures give a free choice of lattice constant and indium composition of In x Ga 1−x As channel. However, the high In mole frac-tion of InGaAs channel used in MHFET is observed to easily initiate the impact ionizations. 5 If the electric field is high enough to accel-erate carriers within the channel, the electron gains kinetic energy to collide with the lattice and generate electron-hole pairs. 6 Some holes injected across the Schottky layer and trapped close to the gate terminal could lead to a negative shift of the threshold voltage. Subsequently, through the degraded gate control ability as well as reduced gate depletion region, the kink effect and bell-shaped be-havior occur. 2,7 Therefore, the deposition of mechanically stable metal gates has become a critical issue. In particular, the require-ments of a thermally stable Schottky gate contact are that the metal gates can survive during high-temperature process and device opera-tion. Because the frequently used Pt metal is easy to chemically react with GaAs or AlGaAs layer and forms compounds at relatively low temperature, 8 it may be unsuitable for application in the gate electrode of metal semiconductor field-effect transistors. In this work, the double -doped In 0.42 Al 0.58 As/In 0.46 Ga 0.54 As MHFETs with different metal gates, e.g., Ti/Au and Au, are fabri-cated to comprehensively study their thermal stability performance over a wide temperature range 300–510 K. Also, to study the elec-trical properties in digital and switching circuit applications, the voltage transfer characteristics are included. The temperature degra-dation rates in device characteristics depend strongly on the specific metal gates. By the employment of Ti as a part of metal gate, the lower deviation and higher performance of Schottky gate contacts are obtained even at high ambient temperature. This also implies that the impact ionizations and related thermal runaway are sup-pressed. Therefore, the higher temperature operation capability and relatively thermal stability are simultaneously enhanced in device operations.
02/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: An interesting hydrogen sensor based on an Al<sub>0.24</sub>Ga<sub>0.76</sub>As Schottky barrier high-electron mobility transistor with a catalytic Pd metal/oxide/semiconductor is fabricated and demonstrated. In comparison with traditional Schottky diodes or capacitance-voltage type hydrogen sensors, the studied device exhibits larger current variation, lower hydrogen detection limit, and shorter transient hydrogen response time. Besides, good hydrogen-sensing properties, such as significant drain current change, threshold voltage shift, and transconductance change of transistor behaviors, are obtained. Therefore, the studied device provides the promise for high-performance solid-state hydrogen sensors, optoelectronic integrated circuits, and microelectromechanical system applications.
IEEE Sensors Journal 05/2006; · 1.52 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The temperature-dependent characteristics of an InGaP/InGaAs/GaAs heterostructure field-effect transistor (HFET), using the (NH<sub>4</sub>)<sub>2</sub>S<sub>x</sub> solution to form the InGaP surface passivation, are studied and demonstrated. The sulfur-passivated device shows significantly improved dc and RF performances over a wide temperature range (300-510 K). With a 1×100-μm<sup>2</sup> gate-dimension HFET by (NH<sub>4</sub>)<sub>2</sub>S<sub>x</sub> treatment, the considerably improved thermal stability over dc performances including lower temperature variation coefficients on the turn-on voltage (-1.23 mV/K), the gate-drain breakdown voltage (-0.05 mV/K), the gate leakage current (1.04 μA/mm·K), the threshold voltage (-1.139 mV/K), and the drain-saturation-current operating regimes (-3.11×10<sup>-4</sup>/K) are obtained as the temperature is increased from 300 to 510 K. In addition, for RF characteristics, the sulfur-passivated device also shows a low degradation rate on drain-saturation-current operating regimes (-3.29×10<sup>-4</sup>/K) as the temperature is increased from 300 to 400 K. These advantages provide the promise for high-speed high-frequency high-temperature electronics applications.
IEEE Transactions on Device and Materials Reliability 04/2006; · 1.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The influences of (NH<sub>4</sub>)<sub>2</sub>S<sub>x</sub> 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 1×100 μm<sup>2</sup> 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) μA/mm at V<sub>GD</sub>=-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</sub>), 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</sub> (maximum oscillation frequency f<sub>max</sub>) 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</sub>,f<sub>max</sub>) larger than 455 mA/mm. Moreover, the relatively lower variations of device performances over wide temperature range (300∼510 K) are observed.
IEEE Transactions on Electron Devices 02/2006; · 2.32 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An interesting transistor-type hydrogen sensing detector based on a GaAs pseudomorphic high electron mobility transistor (PHEMT) with a Pd/Al0.24Ga0.76As metal–semiconductor Schottky gate structure is fabricated and investigated. Steady-state properties and transient responses under different temperatures and hydrogen concentrations are measured and studied. Significant modulations in electrical signals are observed, obviously due to the adsorption of hydrogen atoms at the Pd–semiconductor interface. Also, the studied device exhibits fast response and recovery properties. The corresponding adsorption and desorption time constants (τa and τb) are 2.5 and 6 s, respectively, under 9970 ppm H2/air gas at 160 °C. Furthermore, based on the Langmuir isotherm and the van't Hoff equation, a hydrogen adsorption heat of −37.02 kJ mole−1 is obtained at lower operating temperatures (≤72 °C). However, at a high temperature region (≥92 °C), the ΔH0 value is increased to −68.62 kJ mole−1. The hydrogen adsorption heat at lower and higher temperature regimes is demonstrated and studied. Consequentially, based on the experimental results, the studied device is promising for GaAs integrated circuit (IC) and micro electric and mechanic system (MEMS) applications.
Semiconductor Science and Technology 01/2006; 21(3):221. · 1.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The effect of (NH4)2Sx treatment on the device characteristics of an InGaP/InGaAs/GaAs heterostructure field-effect transistor are studied and demonstrated. Experimentally, it is found that the sulfur-passivated device shows significant dc characteristics including higher forward voltage, lower leakage current, lower output conductance, and higher voltage gain. The superior microwave performances with flat and wide operating region of drain saturation current are simultaneously obtained. Furthermore, the improved thermal stabilities over wide temperature range of sulfur-passivated devices are attained. Based on these advantages, the studied device with (NH4)2Sx treatment shows the promise for high-temperature and high-performance microwave applications.
Applied Physics Letters 08/2005; 87(8):083502-083502-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The interesting hydrogen sensing performances of a Pt-oxide-AlGaAs (MOS) high electron mobility transistor (HEMT) are studied and demonstrated. The effects of hydrogen adsorption on device performances such as the threshold voltage shift ΔV<sub> th </sub> , drain saturation current variation ΔI<sub> DS </sub> , and transient response are presented. ΔV<sub> th </sub> and ΔI<sub> DS </sub> decreased with increasing operating temperature. This suggests that, at higher temperature, less hydrogen atoms diffuse through the Pt bulk and reach the interface between the Pt metal and oxide layer resulting from the relatively faster formation rate of hydroxyl on the Pt surface. The response curves of the studied Pt-AlGaAs MOS HEMT show various profiles at different temperatures. The influences of hydrogen concentration and temperature on the interface sites occupied by adsorbed atoms are also studied.
Applied Physics Letters 04/2005; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this study, a triangular-barrier and a double-barrier structure were integrated to form a bi-directional switching device. In the structure center of the triangular barrier, a delta-doped (δ-doped) quantum well was inserted to enhance the carrier accumulation. Owing to the resonant tunneling through the double barrier and avalanche multiplication in the reverse-biased junction, N-shaped and S-shaped negative-differential-resistance (NDR) phenomena would occur in the current-voltage (I-V) characteristics under normal and reverse operation modes, respectively.
Vacuum Electron Sources Conference, 2004. Proceedings. IVESC 2004. The 5th International; 10/2004
-
[show abstract]
[hide abstract]
ABSTRACT: Pd- and Pt-Al0.3Ga0.7As metal–semiconductor (MS) Schottky diodes are fabricated and studied. The hydrogen-sensing characteristics including Schottky barrier height modulations and hydrogen detection sensitivity and transient responses are investigated and presented. Due to the formation of hydroxyl at higher temperature, different transient responses are found for the studied Pt- and Pd-MS Schottky diodes. According to the van't Hoff equation, the initial heat of adsorption for the Pd- and Pt-Al0.3Ga0.7As MS interfaces is calculated as −5.21 and −7.56 kJ mole−1, respectively.
Semiconductor Science and Technology 04/2004; 19(6):778. · 1.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An interesting InGaP/InGaAs heterostructure field-effect transistor utilizing dual δ-doped quantum wells as double channels is studied and demonstrated. The employed dual δ-doped quantum wells and InGaP layer provide good carrier confinement and Schottky behavior, respectively. Good device performances including higher turn-on and breakdown voltages, high and linear transconductance and RF properties are obtained. For a 1 × 100 μm device, turn-on voltage of 1.74 V, maximum output current of 499 mA/mm, and maximum transconductance of 162 mS/mm with 303 mA/mm broad operation regime are obtained. The microwave properties of f<sub>T</sub> and f<sub>max</sub> are 16 and 32.3 GHz, respectively. Furthermore, even the device is operated at higher temperature regime (>400K), insignificant degradations of DC and RF performances are observed.
Junction Technology, 2004. IWJT '04. The Fourth International Workshop on; 04/2004
-
[show abstract]
[hide abstract]
ABSTRACT: The temperature-dependent characteristics of polysilicon and diffused resistors have been studied. By using the 0.18 μm CMOS technology, 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</sub>) and interface resistance (R<sub>interface</sub>) are obtained at different temperature. For diffused resistors, the R<sub>bulk</sub> and R<sub>interface</sub>, values are increased and decreased with the increase of temperature, respectively. Positive values of temperature coefficient of resistance (TCR) are observed. Furthermore, TCR values are decreased with the decrease of resistor size. For polysilicon resistors, the Rinterface values are decreased with the increase of temperature. In addition, negative and positive TCR values of RNA are found in n<sup>+</sup> and p<sup>+</sup> polysilicon resistors, respectively. In conclusion, by comparing the studied diffused and polysilicon resistors, the negative trends of TCR are observed when the resistor sizes are decreased.
Junction Technology, 2004. IWJT '04. The Fourth International Workshop on; 04/2004
-
[show abstract]
[hide abstract]
ABSTRACT: The hydrogen response characteristics and sensing properties or catalytic Pd/Al<sub>0.3</sub>Ga<sub>0.7</sub>As metal-oxide-semiconductor (MOS) and metal-semiconductor (MS) Schottky diodes are systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, sensitivity, barrier height variation, heat of adsorption, and transient response are investigated. The studied devices can be operated under very wide hydrogen concentration regimes with remarkable hydrogen-sensing properties. Particularly, at an extremely low hydrogen concentration of 15 ppm H<sub>2</sub>/air, both steady-state and transient responses at room temperature can be detected. In addition, under the presence of oxide layer in the studied MOS device, a larger change of barrier height and higher hydrogen response are observed. In addition, according to the van't Hoff equation, the initial values of heat adsorption for Pd/semiconductor and Pd/oxide interface are calculated as 7.29 and 49.6 KJ/mole, respectively.
IEEE Transactions on Electron Devices 01/2004; · 2.32 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Semiconductor Science and Technology 05/2003; 18(7):615. · 1.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The interesting hydrogen sensing characteristics of two transistors with an Al0.24Ga0.76As (device A) and In0.49Ga0.51P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H2/air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H2/air at room temperature. On the other hand, device B exhibits more stable hydrogen-sensing characteristics at high temperatures. Even at a low concentration of 14 ppm H2/air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al0.24Ga0.76As and In0.49Ga0.51P materials are lattice-matched to the GaAs substrate, the studied devices can be integrated as sensor arrays to obtain superior hydrogen sensing characteristics including higher sensing signals, lower detection limit, shorter response time, and widespread detection and temperature regimes.
Sensors and Actuators B: Chemical.
-
[show abstract]
[hide abstract]
ABSTRACT: The temperature-dependent characteristics of NH 4 2 S x -passivated AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors were studied and demonstrated. Due to the use of sulfur passivation, remarkable improvements in device performance, including higher forward turn-on voltage, higher reverse breakdown voltage, lower reverse leakage current, higher transconduc-tance, lower on-resistance, more linear operating regime, and superior microwave performance, were obtained. In addition, the sulfur-passivated devices also show good properties in the higher operating temperature regime and relatively thermally stable performance over the operation temperature range 300–510 K. Therefore, the studied device with NH 4 2 S x treatment provides promise for high-performance digital and microwave device applications. Due to progressive improvements in growth techniques, hetero-structure field-effect transistors HFETs have been studied exten-sively for high-speed microwave applications. 1-3 Nevertheless, the III-V HFETs may suffer from surface-related mechanisms. The sur-face of III-V semiconductor materials such as AlGaAs is known to be plagued by a high density of interface states. 4 Furthermore, the extrinsic surface, formed by the typical gate-recess etching pro-cesses, traps a significant portion of the electrons and causes the increase of the depletion layer between the source and the gate edge near the source. 5,6 This adversely influences the effect of applied gate bias on the active channel and results in reduced transconduc-tance and poor long-term device stability. Therefore, the lack of an adequate surface passivation for the III-V HFETs severely degrades device performance and restricts application of compound semicon-ductor circuits. In order to overcome this undesirable problem, the surface passivation becomes a crucial requirement for fabricating high-performance electronic and optoelectronic devices. 7-9 In par-ticular, sulfur passivation in a wet chemical solution of NH 4 2 S x was shown to be an effective way to substantially lower AlGaAs surface recombination centers. 10,11 In this work, the temperature-dependent characteristics of a sulfur-passivated AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor PHEMT were studied and demon-strated. Sulfur passivation was employed to a gate junction to avoid the difficulty in fabricating high-performance metal/AlGaAs Schottky contacts. The passivation method controls defective states originating from i air oxidation of the AlGaAs surface before Schottky metallization and ii interfacial reaction during metalliza-tion. In other words, the sulfur passivation effectively reduces the series resistances and interface states. This also indicates that the Fermi-level pinning effect produced by native surface oxides and nonradiative recombination centers can be eliminated. 11,12 There-fore, good device performance, high-temperature operation capabil-ity, and relatively thermally stable characteristics are simultaneously obtained.
-
[show abstract]
[hide abstract]
ABSTRACT: Comprehensive studies of temperature-dependent gate-metal-related impact ionization in metamorphic high electron mobility transistors MHEMTs are demonstrated. It is known that, from experimental results, the electric field and temperature dependences of impact ionization mechanisms in MHEMT's operation are dominated by the ionization threshold energy and hot electron population. The peak impact ionization-induced gate current could be substantially decreased by the presence of specific gate contact with a higher Schottky barrier height. Therefore, the suppressions of bell-shaped behavior and related impact ionization effect are observed by using the appropriate Schottky gate contacts. © 2006 American Institute of Physics. Recently, the metamorphic high electron mobility tran-sistors, MHEMTs InP based layers on GaAs substrate, have attracted considerable attention for high power and low noise microwave applications. 1,2 From the physical point of view, the increased gate leakage current as well as decreased breakdown voltage are mainly caused by the carrier genera-tion within the channel and carrier transport across the Schottky barrier layer. 3,4 Thus, the carrier generation through effective impact ionization in the channel layer is responsible for the breakdown characteristics. Since the impact ionization is inversely proportional to the band gap value, the narrow band gap of InGaAs channel makes it susceptible to induce the impact ionization effect. 3 This problem usually leads to the deteriorated device char-acteristics including the low breakdown voltage, high gate leakage current, and high output conductance. The extraction of impact ionization-induced gate current from the total mea-sured gate current becomes essential to study the impact ion-ization effect. It is noticeable that the impact ionization effect strongly depends on the Schottky barrier height of gate con-tacts. The adequate determination of gate metals can sup-press the impact ionizations and then improve the device characteristics. In this work, the temperature-dependent char-acteristics of gate-metal-related impact ionizations in MHEMTs are investigated. This temperature effect on gate-metal-related impact ionization has never been found in pre-vious reports. Different metals, including Pt/ Au, Ti/ Au, and Au, as the Schottky gate contacts are fabricated simulta-neously to study the impact ionization effects. In order to consider the impact ionization-induced gate current, the con-venient excess gate hole current model established by Web-ster et al. is employed. 4 Besides, two distinct mechanisms ionization threshold energy and hot electron population for impact ionization, which separately dominate in different field ranges, are also presented to interpret the anomalous electric field and temperature dependences. The MHEMT layer structure is similar to the previous work. 5 Drain-source Ohmic contacts were formed on the top of cap layer by rapid thermal annealing of AuGe/ Ni/ Au metallurgy. Gate recess etching of cap layer was done by highly selective pH-adjusted succinic acid solution. For comparison, gate Schottky contacts were achieved by evapo-rating Pt/ Au 20nm/ 130 nm, Ti/Au 20nm/ 130 nm, and Au 150 nm, three kinds of different metals, for devices A, B, and C, respectively, on the Schottky barrier layer by using the same process sequence. Finally, the active structure un-derneath the gate feeder was completely removed by wet chemical etching to build the air-bridge gate structure which included multiple piers under the gate-feeder metal. 6 The gate dimension was 1 100 m 2 . Figure 1 shows the schematic band diagrams and calcu-lated electron densities within channel layer regimes at ther-mal equilibrium. Experimentally, the measured Schottky bar-rier height B values of devices A, B, and C are 0.91 0.82, 0.78 0.71, and 0.71 0.63 eV at 300 480 K, respectively.
-
[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent characteristics of high electron mobility transistors HEMTs with sulfur and SiN x passivations are com-prehensively studied and demonstrated. Experimentally, for the studied device with formal passivations, better dc and microwave characteristics are obtained over a wide operating temperature range. In particular, as compared with the device only with sulfur passivation, the slight degradations of device performance are caused by the temperature stress during the deposition of SiN x layer and presence of surface traps at the SiN x /AlGaAs interface. However, under an accelerated stress test, this formal-passivated device shows improved reliability performance. Based on these good results, the formal-passivated HEMT is expected to exhibit relatively better long-term operation stability and reliable device characteristics. Recently, high electron mobility transistors HEMTs have been applied widely in high-performance microwave and power circuits. 1-3 It is known that, practically, the manufacturing cost, yield, device reliability, and longevity are mandatory factors for sys-tem applications. Because metal-semiconductor MS-based struc-tures are widely employed in III-V HEMTs, the surface effect in-duced by high density of interface traps at MS contacts has attracted much attention. The surface effects crucially influence device per-formance because the electron transport within channel layer is strongly affected by the air-exposed surface region. 4 Hence, the lack of an adequate surface passivation usually results in the formation of native oxide and degradation of electrical properties. To optimize device behaviors and achieve thermal stability, the sulfur passivation has been developing as a promising technique. 5-7 Previously, the sulfur passivation effects on the temperature-dependent characteristics of HEMTs with Pt/Au gate Schottky con-tact were demonstrated in our report. 7 However, the Pt metal is easy to chemically react with GaAs or AlGaAs layer at the relatively low temperature. 8 Thus, it is difficult to control the accuracy and preci-sion of sulfur passivation on the device with Pt/Au gate alloy. Also, the sulfur passivation effect could be degraded quickly when the device was exposed to ambient atmosphere. 9 This certainly leads to reliability problems for device performance. Therefore, deposition of an oxygen-free dielectric layer after sulfur passivation is neces-sary to protect the sulfide layer and achieve long-term stable passi-vation. Above all, silicon nitride SiN x is one of the dominant ma-terials for further stabilizing the sulfur passivation effect. 10,11 In this work, a formal passivation method that combines the sulfur and SiN x passivations is employed to achieve improved op-eration stability and reliable device performance in HEMTs with Ti/Pt/Au gate Schottky contact. To meet the desired accuracy, the Ti metal as a part of gate Schottky contact is employed. 12 This im-proved experimental condition would be useful to study the inherent effect of sulfur and SiN x passivations without any unpredictable effects. Moreover, the proposed passivation technique is substan-tially different from previous work. 7 For the formal passivation, an additional SiN x passivation layer is deposited on the mesa sidewall and the exposed sulfur-passivated barrier surface between drain-source and gate edge. The effects of sulfur and SiN x passivations in the electric parameters are analyzed and comprehensively studied. The comparisons between the formal-passivated device and nonpas-sivated device are made to show the effect of formal passivation on device performance. Furthermore, in order to demonstrate the supe-riority of the proposed formal passivation over standard sulfur pas-sivation, the sulfur-passivated device is also included in the reliabil-ity test.
