Showing papers by "Xiaohua Ma published in 2015"

Improved Interface and Transport Properties of AlGaN/GaN MIS-HEMTs With PEALD-Grown AlN Gate Dielectric

Abstract: In this paper, we demonstrated AlGaN/GaN metal–insulator–semiconductor high-electron mobility tran- sistors (MIS-HEMTs) with AlN gate dielectric grown by plasma-enhanced atomic layer deposition (PEALD). The high–quality interface of AlN/AlGaN/GaN MIS-HEMTs resulted in a very small threshold voltage ( ${V}_{\mathrm {\mathbf {th}}}$ ) hysteresis and dispersion. Simultaneously, the MIS-HEMTs exhibited a high-peak transconductance of 289 mS/mm and a small ${V}_{\mathrm {\mathbf {th}}}$ shift of 0.8 V, while those for Al2O3/AlGaN/GaN metal–oxide–semiconductor HEMTs were 203 mS/mm and 5.2 V, respectively. Furthermore, analysis indicated that PEALD-grown AlN significantly reduced the interface charges at dielectric/III-N interface (from $1.2\,\times \,10^{\mathrm {\mathbf {13}}}$ to $8\times 10^{\mathrm {\mathbf {12}}}~\mathrm{cm}^{\mathrm {\mathbf {-2}}}~\mathrm{eV}^{\mathrm {\mathbf {-1}}}$ for interface traps, and from $1.01\times 10^{\mathrm {\mathbf {13}}}$ to $3.1\times 10^{\mathrm {\mathbf {11}}}~\mathrm{cm}^{\mathrm {\mathbf {-2}}}$ for fixed charges) and improved channel transport properties (the full-width at half-maximum of channel transconductance increased from 0.9 to 2.7 V), compared with ALD-grown Al2O3, which could explain the differences of device characteristics.

Proton Irradiation Effects on AlGaN/AlN/GaN Heterojunctions

Abstract: AlGaN/AlN/GaN heterojunctions were irradiated by 3 MeV protons with different fluences. Hall and C-V measurements showed that the density and mobility of 2DEG for heterojunctions decreased after proton irradiation. The crystal quality and optical properties of AlGaN/GaN heterojunctions were characterized by the variations of the micro-Raman scattering spectra, XRD and photo luminescence (PL) spectra with proton fluence. It has been obtained that the strain and dislocation of GaN and AlGaN were not affected by the proton injected. The proton irradiation caused the increase of structural defects in HT-AlN buffer layer, which lead to the red shift of ${{\rm A}_1}$ (LO) mode. The higher tails of XRD rocking curve indicated the introduction of point defects by irradiation. The full-width at half-maximum (FWHM) of ${\rm E}_2^{\rm high}$ phonon mode broadened, which was consistent with change of FWHM of PL near-band-edge emission (BE). The spectra of yellow band normalized to the intensity of BE demonstrated a great increase of Ga vacancies related defects, which may be the main reason for the degradation of optical properties.

Trap states in enhancement-mode double heterostructures AlGaN/GaN high electron mobility transistors with different GaN channel layer thicknesses

Abstract: This is the report on trap states in enhancement-mode AlGaN/GaN/AlGaN double heterostructures high electron mobility transistors by fluorine plasma treatment with different GaN channel layer thicknesses. Compared with the thick GaN channel layer sample, the thin one has smaller 2DEG concentration, lower electron mobility, lower saturation current, and lower peak transconductance, but it has a higher threshold voltage of 1.2 V. Deep level transient spectroscopy measurements are used to obtain the accurate capture cross section of trap states. By frequency dependent capacitance and conductance measurements, the trap state density of (1.98–2.56) × 1012 cm−2 eV−1 is located at ET in a range of (0.37–0.44) eV in the thin sample, while the trap state density of (2.3–2.92) × 1012 cm−2 eV−1 is located at ET in a range of (0.33–0.38) eV in the thick one. It indicates that the trap states in the thin sample are deeper than those in the thick one.

Enhancement‐mode AlGaN/GaN HEMTs with thin and high Al composition barrier layers using O2 plasma implantation

Abstract: We report normally off AlGaN/GaN high-electron mobility transistors realized by using the thin and high Al composition barrier design in conjunction with O2 plasma implantation. The AlGaN barrier layer under the gate is selectively treated by low-energy O2 plasma in order to positively shift the threshold voltage of devices. The obtained enhancement-mode HEMT exhibits a threshold voltage (VTH) of +1 V, a high peak transconductance of ∼230 mS mm−1, and a maximum drain current of 424 mA mm−1 at a gate bias of 3 V for a gate length of 0.6 µm. A suppressed gate reverse leakage current and a high breakdown voltage of 450 V are achieved. Moreover, gradually partial oxidation of AlGaN barrier layer is determined to be responsible for the plasma-induced VTH modulation effect through a systematic investigation. The results indicate O2 plasma implantation can be a promising method for fabrication of enhancement-mode GaN-based devices.

Low ohmic-contact resistance in AlGaN/GaN high electron mobility transistors with holes etching in ohmic region

Abstract: Good Ti/Al/Ni/Au ohmic contacts were achieved in AlGaN/GaN high electron mobility transistors (HEMTs) by using holes etching in an ohmic region. The ohmic contact with a contact resistance of 0.1 Ωmm was obtained. Compared with other methods which could reduce the ohmic-contact resistance, this method has simple process steps, low cost and can obtain stable device characteristics. The output characteristics and transfer characteristics of devices were analysed. The HEMTs using this method show lower contact resistance, smaller knee voltage and larger saturation current than the conventional HEMTs.

Influence of the gate edge on the reverse leakage current of AlGaN/GaN HEMTs

Abstract: By comparing the Schottky diodes of different area and perimeter, reverse gate leakage current of AlGaN/GaN high mobility transistors (HEMT) at gate bias beyond threshold voltage is studied. It is revealed that reverse current consists of area-related and perimeter-related current. An analytical model of electric field calculation is proposed to obtain the average electric field around the gate edge at high revers bias and estimate the effective range of edge leakage current. When the reverse bias increases, the increment of electric field is around the gate edge of a distance of ΔL, and perimeter-related gate edge current keeps increasing. By using the calculated electric field and the temperature-dependent current-voltage measurements, the edge gate leakage current mechanism is found to be Fowler-Nordheim tunneling at gate bias bellows -15V caused by the lateral extended depletion region induced barrier thinning. Effective range of edge current of Schottky diodes is about hundred to several hundred nano-meters, and is different in different shapes of Schottky diodes.

Degradation mechanism of enhancement-mode AlGaN/GaN HEMTs using fluorine ion implantation under the on-state gate overdrive stress*

Abstract: The degradation mechanism of enhancement-mode AlGaN/GaN high electron mobility transistors (HEMTs) fabricated by fluorine plasma ion implantation technology is one major concern of HEMT’s reliability. It is observed that the threshold voltage shows a significant negative shift during the typical long-term on-state gate overdrive stress. The degradation does not originate from the presence of as-grown traps in the AlGaN barrier layer or the generated traps during fluorine ion implantation process. By comparing the relationships between the shift of threshold voltage and the cumulative injected electrons under different stress conditions, a good agreement is observed. It provides direct experimental evidence to support the impact ionization physical model, in which the degradation of E-mode HEMTs under gate overdrive stress can be explained by the ionization of fluorine ions in the AlGaN barrier layer by electrons injected from 2DEG channel. Furthermore, our results show that there are few new traps generated in the AlGaN barrier layer during the gate overdrive stress, and the ionized fluorine ions cannot recapture the electrons.

Through space charge-transfer emission in lambda (Λ)-shaped triarylboranes and the use in fluorescent sensing for fluoride and cyanide ions

Abstract: A new class of Λ-shaped triarylboranes 2-(4-(N,N-dimethylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (TBBN) and 2-(4-(N,N-diphenylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TBBN2), incorporating different electron-donating amino groups and an electron-accepting dimesitylboryl group through a rigid Λ-shaped Troger's base linker were designed and synthesized. The compounds display twisted structures and effective intramolecular change-transfer transitions. The twisted nonplanar arrangement of the chromophores on the one hand suppresses the fluorescence quenching in the aggregated states, and on the other hand produces a through-space donor–acceptor charge transfer. As a result, dual fluorescent pathways, namely through-space charge transfer from the amino group to the dimesitylboryl group, and the π*–π transitions located on the amino groups, are observed to coexist in each molecule. The dual emissions can be selectively switched on or off by addition of fluoride or cyanide ions. Thus the dyes can be used as “switch-on” probes. The complexation of TBBN and TBBN2 with fluoride or cyanide ions induces dramatic blue shifts (about 72–140 nm) and color changes in the fluorescence, making them potential visually colorimetric and ratiometric sensors for fluoride and cyanide ions.

Trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors*

Abstract: Frequency-dependent conductance measurements were carried out to investigate the trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors (HEMTs) quantitatively. For the non-recessed HEMT, the trap state density decreases from 2.48 × 1013 cm−2eV−1 at an energy of 0.29 eV to 2.79 × 1012 cm−2eV−1 at ET = 0.33 eV. In contrast, the trap state density of 2.38 × 1013–1.10 × 1014 cm−2eV−1 is located at ET in a range of 0.30–0.33 eV for the recessed HEMT. Thus, lots of trap states with shallow energy levels are induced by the gate recess etching. The induced shallow trap states can be changed into deep trap states by 350 °C annealing process. As a result, there are two different types of trap sates, fast and slow, in the annealed HEMT. The parameters of the annealed HEMT are ET = 0.29–0.31 eV and DT = 8.16 × 1012–5.58 × 1013 cm−2eV−1 for the fast trap states, and ET = 0.37–0.45 eV and DT = 1.84 × 1013 − 8.50 × 1013 cm−2eV−1 for the slow trap states. The gate leakage currents are changed by the etching and following annealing process, and this change can be explained by the analysis of the trap states.

Effect of alloying temperature on the capacitance–voltage and current–voltage characteristics of low‐pressure chemical vapor deposition SiNx/n‐GaN MIS structures

Abstract: The low-pressure chemical vapor deposition (LPCVD)-SiNx layer is deposited prior to the ohmic alloying as the passivation layer and gate dielectric layer in GaN device fabrication. The effect of alloying temperature on the LPCVD-SiNx/GaN interface and LPCVD-SiNx dielectric film is evaluated by the capacitance–voltage (C–V) and current–voltage (I–V) characteristics. The C–V analysis by frequency-dependent method and Terman method shows that the higher density of interface states occurs at the higher alloying temperature, while I–V analysis indicates that the higher alloying temperature can lead to smaller current density with slightly deeper traps for Poole–Frenkel conduction. The hydrogen-related bonds may be the cause and affects the interfacial properties of LPCVD-SiNx/GaN as well as the electrical properties of LPCVD-SiNx film.

Using in-process measurements of open-gate structures to evaluate threshold voltage of normally-off GaN-based high electron mobility transistors

Abstract: The parameters of open-gate structures treated with different etching time were monitored during the gate recess process, and their impacts on the threshold voltage (Vth) of final fabricated AlGaN/GaN high electron mobility transistors (HEMTs) based on open-gate structures were discussed in this paper. It is found that Vth can exceed 0 V when channel resistance in the recessed region (Ron-open) increases over ∼275 Ω mm, maximum current (IDmax) decreases below ∼29 mA/mm, or recessed barrier thickness (tRB) is below ∼7.5 nm. In addition, tRB obtained by atomic force microscopy measurements and C-V measurements are also compared. Finally, theoretical common criteria based on the experimental results of this work for tRB and Ron-open were established to evaluate the Vth of a regular normally-off AlGaN/GaN HEMTs. The results indicate that these parameters of open-gate structure can be utilized to achieve normally-off HEMTs with controllable Vth.

Contact Length Scaling in Staggered Organic Thin-Film Transistors

Abstract: In this letter, the contact length (source and drain electrodes length) scaling potential in staggered organic thin-film transistors (OTFTs) was studied. We performed the effect of gate–source voltage, channel length, semiconductor film thickness, mobility, material disorder, anisotropy of mobility, and Schottky barrier between organic semiconductor and source/drain electrodes on the contact length downscaling, and found that the contact resistance do not increase until the contact length is scaled down to sub-500 nm. Importantly, the cutoff frequency ( $f_{T}$ ) of OTFTs was described and it was found that $f_{T}$ would increase with the decrease of contact length and the highest $f_{T}$ could be obtained with contact length in the range of sub-100 nm with Ohmic contacts. This letter showed excellent contact length scaling potential of OTFTs and provided guidelines for the design of high frequency, low-cost, and printable OTFTs.

Improved performance of AlGaN/GaN HEMT by N 2 O plasma pre-treatment

Abstract: The influence of an N2O plasma pre-treatment technique on characteristics of AlGaN/GaN high electron mobility transistor (HEMT) prepared by using a plasma-enhanced chemical vapor deposition (PECVD) system is presented. After the plasma treatment, the peak transconductance (gm) increases from 209 mS/mm to 293 mS/mm. Moreover, it is observed that the reverse gate leakage current is lowered by one order of magnitude and the drain current dispersion is improved in the plasma-treated device. From the analysis of frequency-dependent conductance, it can be seen that the trap state density (DT) and time constant (τT) of the N2O-treated device are smaller than those of a non-treated device. The results indicate that the N2O plasma pre-pretreatment before the gate metal deposition could be a promising approach to enhancing the performance of the device.

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation

Abstract: The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor (HEMT) becomes one of the most important reliability issues with the downscaling of feature size. In this paper, the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K. Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current. By comparing the experimental data with the numerical transport models, it is found that neither Fowler–Nordheim tunneling nor Frenkel–Poole emission can describe the transport of reverse surface leakage current. However, good agreement is found between the experimental data and the two-dimensional variable range hopping (2D-VRH) model. Therefore, it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer. Moreover, the activation energy of surface leakage current is extracted, which is around 0.083 eV. Finally, the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied. It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV, which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.

Improved Interface and Transport Properties of AlGaN/GaN MIS-HEMTs With PEALD-Grown

Abstract: In this paper, we demonstrated AlGaN/GaN metal-insulator-semiconductor high-electron mobility tran- sistors (MIS-HEMTs) with AlN gate dielectric grown by plasma- enhanced atomic layer deposition (PEALD). The high-quality interface of AlN/AlGaN/GaN MIS-HEMTs resulted in a very small threshold voltage (Vth) hysteresis and dispersion. Simultaneously, the MIS-HEMTs exhibited a high-peak transcon- ductance of 289 mS/mm and a small Vth shift of 0.8 V, while those for Al2O3/AlGaN/GaN metal-oxide-semiconductor HEMTs were 203 mS/mm and 5.2 V, respectively. Furthermore, analysis indicated that PEALD-grown AlN significantly reduced the interface charges at dielectric/III-N interface (from 1.2 × 10 13 to 8× 10 12 cm �2 eV �1 for interface traps, and from 1.01× 10 13 to 3.1 × 10 11 cm �2 for fixed charges) and improved channel transport properties (the full-width at half-maximum of channel transconductance increased from 0.9 to 2.7 V), compared with ALD-grown Al2O3, which could explain the differences of device characteristics.

Germanium-Tin P-channel tunneling field-effect transistors: Impacts of biaxial tensile strain and surface orientation

Abstract: This work investigates the impacts of biaxial tensile strain and surface orientation on performance of GeSn pTFET. Multi-bands k • p method is used to calculate the band structure of biaxially tensile strained GeSn on various orientations. The electrical characteristics of tensile strained GeSn line- and point-pTFETs are computed implementing the dynamic nonlocal BTBT algorithm. Our simulation demonstrates that 1) tensile strained GeSn pTFETs achieve significantly improved |I on | over relaxed devices; 2) With the same tensile strain, GeSn pTFETs on (011) and (111) orientations demonstrate higher |I On | compared to (001)-oriented device.

Theoretical analysis of proton irradiation effects on AlGaN/GaN high-electron-mobility transistors

Abstract: To study radiation damage, the authors irradiated AlGaN/GaN high-electron-mobility transistors with 3 MeV protons at various fluences. This irradiation caused displacement damage, which decreased the saturated drain current, maximum transconductance, cutoff frequency, and maximum frequency of oscillation. The authors extracted the damage factors of the threshold voltage, two-dimensional electron gas (2DEG) surface density, and mobility, which are usually used to simulate and estimate device performance in radiation environments. Calculations based on the charge control model show that the acceptor defects induced in the GaN layer play a leading role, while defects induced in the AlGaN barrier layer rarely matter. The removal rate of carriers from the 2DEG is unrelated to the thickness of undoped AlGaN layer, the conduction band discontinuity, and the doping concentration of AlGaN barrier layer; it only depends on the concentration of acceptor defects induced.

A 5–8 GHz wideband 100 W internally matched GaN power amplifier

Abstract: A 5–8GHz internally matched Gallium Nitride (GaN) power amplifier (PA) with 100Woutput power was realized in this letter. The theory of load line match was used and extended. Power contour was depicted and revised by the output capacitance of GaN High Electron Mobility Transistor (HEMT). Impedance was matched into the −1 dB power contour in a wide frequency band due to the ladder transmission line matching network and broadband power combiner. With the package size of 14.5 ∗ 14.8mm, the proposed power amplifier has the maximum output power of 102W with 45.8% associate power added efficiency (PAE) at the frequency of 6.5GHz, and output power over 85W and PAE over 42.8% at the frequency band of 5–8GHz.

Analysis of the third harmonic for class-F power amplifiers with an I–V knee effect

Abstract: The appearance of third-generation semiconductors represented by gallium nitride (GaN) material greatly improves the output power of a power amplifier (PA), but the efficiency of the PA needs to be further improved. The Class-F PA reduces the overlap of drain voltage and current by tuning harmonic impedance so that high efficiency is achieved. This paper begins with the principle of class-F PA, regards the third harmonic voltage as an independent variable, analyzes the influence of the third harmonic on fundamental, and points out how drain efficiency and output power vary with the third harmonic voltage with an I?V knee effect. Finally, the best third harmonic impedance is found mathematically. We compare our results with the Loadpull technique in advanced design system environment and conclude that an optimized third harmonic impedance is open in an ideal case, while it is not at an open point with the I?V knee effect, and the drain efficiency with optimized third harmonic impedance is 4% higher than that with the third harmonic open.

Investigation of trap states in Al 2 O 3 InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors

Abstract: In this paper the trapping effects in Al2O3/In0.17Al0.83N/GaN MOS-HEMT (here, HEMT stands for high electron mobility transistor) are investigated by frequency-dependent capacitance and conductance analysis. The trap states are found at both the Al2O3/InAlN and InAlN/GaN interface. Trap states in InAlN/GaN heterostructure are determined to have mixed de-trapping mechanisms, emission, and tunneling. Part of the electrons captured in the trap states are likely to tunnel into the two-dimensional electron gas (2DEG) channel under serious band bending and stronger electric field peak caused by high Al content in the InAlN barrier, which explains the opposite voltage dependence of time constant and relation between the time constant and energy of the trap states.

Recovery of PMOSFET NBTI under different conditions

Abstract: Negative bias temperature instability (NBTI) has become a serious reliability issue, and the interface traps and oxide charges play an important role in the degradation process. In this paper, we study the recovery of NBTI systemically under different conditions in the P-type metal–oxide–semiconductor field effect transistor (PMOSFET), explain the various recovery phenomena, and find the possible processes of the recovery.

Preparation method of high performance reduced graphene for producing electrode of super capacitor

Abstract: The invention belongs to the technical field of material preparation, and specifically relates to a preparation method of high performance reduced graphene for producing the electrode of a super capacitor. The preparation method comprises the following steps: taking commercial oxidized graphene as the initial raw material, dehydrating the commercial oxidized graphene in advance; reacting the preprocessed oxidized graphene with titanium borohydride in a solvent to reduce the oxidized graphene; carrying out separation, and drying the obtained solids so as to obtain the reduced graphene. The prepared reduced graphene is a super capacitor electrode material with an excellent performance. The electric capacity of a super capacitor electrode made of the provided reduced graphene can reach 290 F/g.

Contact-Size-Dependent Cutoff Frequency of Bottom-Contact Organic Thin Film Transistors*

Abstract: The contact-size-dependent characteristic of cutoff frequency fT in bottom-contact organic thin film transistors (OTFTs) is studied. The effects of electrode thickness, field-effect mobility, channel length and gate-source voltage on the contact length (source and drain electrodes' length) related contact resistance of bottom-contact OTFTs are performed with a modified transmission line model. It is found that the contact resistance increases dramatically when the contact length is scaled down to 200 nm. With the help of the contact length related contact resistance, contact-size-dependent fT of bottom-contact OTFTs is studied and it is found that fT increases with the decrease of the contact length in bottom-contact OTFTs.

On-wafer de-embedding techniques from 0.1 to 110 GHz*

Abstract: On-wafer S-parameter de-embedding techniques from 0.1 to 110 GHz are researched. The solving results of thru-reflect-line (TRL) and line-reflect-match (LRM) de-embedding algorithms, when the input and output ports are asymmetric, are given. The de-embedding standards of TRL and LRM are designed on an InP substrate. The validity of the de-embedding results is demonstrated through two passive components, and the accuracy of TRL and LRM de-embedding techniques is compared from 0.1 to 110 GHz. By utilizing an LRM technique in 0.1–40 GHz and a TRL technique in 75–110 GHz, the intrinsic S-parameters of active device HBT in two frequency bands are obtained, and comparisons of the extracted small-signal current gain and the unilateral power gain before and after de-embedding are presented. The whole S-parameters of actual DUT from 0.1 to 110 GHz can be obtained by interpolation.

Relaxed Ge 0.97 Sn 0.03 P-channel tunneling FETs with high drive current fabricated on Si and further improvement enabled by uniaxial tensile strain

Abstract: We fabricated relaxed Ge 097 Sn 003 pTFETs on Si(001) The devices show much higher I ON than SiGe, Ge, and compressively strained GeSn planer pTFETs in literatures For the first time, I ON enhancement in GeSn pTFET utilizing uniaxial strain is reported By applying 014% uniaxial tensile strain along channel direction, Ge 097 Sn 003 [110] pTFETs achieve ∼ 10% I ON improvement, over relaxed devices at |V GS − V TH | = |V DS | = 10 V Calculation demonstrates that the reduction of direct E G by tensile strain results in an enhanced G BTBT in GeSn, leading to improvement of I ON in uniaxially tensile strained pTFET