1. Introduction
Gallium nitride (GaN) has been well developed as material for power electronics applications due to its wide band gap (3.4 eV) and two dimensional electron gas (2DEG) with large electron density of ~1013 cm−2 at the AlGaN/GaN heterostructure. AlGaN/GaN-based heterostructure field-effect transistors (HFETs) exhibit a normally-on operation due to the strong accumulation of high 2DEG density in the channel. To successfully achieve a normally-off mode, which plays an important role in applying power switching devices, a recessed-gate GaN metal-insulator-semiconductor (MIS) structure is required by adapting the removal of the AlGaN layer under the gate region [1,2,3,4,5]. The recessed-gate GaN MISFET with a normally-off operation has several advantages: (i) easy control of the threshold voltage (Vth) by varying the recess etch depth, (ii) obtaining a normally-off operation by accomplishing a simple dry etching technique and (iii) achieving low gate leakage by depositing the gate dielectric [1,2]. However, etching damages and protrusions happening during the recess etching process affect the deteriorated device performance. It is necessary to apply tetramethyl ammonium hydroxide (TMAH) wet solution in the damaged GaN surface under the recessed-gate region in order to prevent plasma damage and smooth the surface of etched GaN channel layer [2]. Many researchers [2,3,6,7,8,9] have reported enhancements of noise and device performance for the GaN-based devices by applying a TMAH treatment.
Noise source in a normally-off GaN MISFET stems from electron trapping/de-trapping into the interface of Al2O3/GaN and/or the buffer traps in the GaN buffer layer [10,11,12]. Low frequency noise (LFN) can analyze interface traps and also find buffer traps in GaN devices [10,11,12,13,14]. Fabricated GaN-based devices showed typical 1/f noise characteristics at the on-state (strong accumulation and subthreshold region), but Lorentzian or generation-recombination (g-r) noise at the off-state (deep-subthreshold region) [10,11]. Flicker noise, or 1/f noise, can be explained by two well-known models, one being the carrier number fluctuations (CNF) due to electron trapping/de-trapping from the channel into the gate insulator [15], and the being Hooge mobility fluctuations (HMF) due to fluctuations of electron mobility caused by phonon scattering [16]. On the other hand, g-r noise is originated by electron trapping/de-trapping into deep-level traps in the GaN buffer layer [10,11,12]. However, no detailed noise behaviors in a normally-off GaN MISFET have been investigated. We here report the 1/f noise characteristics at the on- and off-states to find the noise mechanism and investigate the traps in a normally-off GaN MISFET. Then, pulsed I-V measurements were conducted to observe current collapse behavior as well as to match with the noise results.
2. Materials and Methods
The AlGaN/GaN heterostructure was grown on a sapphire (0001) substrate by metal organic chemical vapor deposition (MOCVD) by the following steps: (1) 2 μm-thick highly-resistive undoped GaN buffer layer, (2) 50 nm-thick undoped GaN channel layer and (3) 16 nm-thick AlGaN barrier layer. In order to fabricate the recessed-gate GaN MISFET, the gate recess region was defined by photolithography and followed by inductively coupled plasma reactive ion etching (ICP-RIE). The gate region was then fully recessed by etching of a 16 nm-thick AlGaN barrier layer and the additional overetching of a 20 nm-thick GaN channel layer. Then, wet etching in TMAH solution (5% solution at 90 °C for 60 min) was applied to remove etching damages and protrusions [2]. To make the device isolation, the mesa region was defined and then a 17 nm-thick atomic layer deposited (ALD) Al2O3 as a gate insulator was deposited. After ohmic contact hole opening, ohmic metal with a Si/Ti/Al/Ni/Au structure (1/25/160/40/100 nm) was deposited by an electron-beam evaporator and sequentially annealed by rapid thermal annealing at 800 °C for 30 sec in N2 gas. Finally, the Ni/Al/Ni gate and pad metals were deposited. The schematic epitaxial and device structure with gate length (Lg) of 2 µm and width (Wg) of 50 µm are illustrated in Figure 1a. Cross-sectional transmission electron microscopy (TEM) (Thermo Fisher Scientific, Waltham, MA, USA) and energy-dispersive X-ray spectroscopy (EDX) (Thermo Fisher Scientific, Waltham, MA, USA) elemental mapping images clearly reveal that the fabricated device has the recessed GaN channel under the gate region, deposited with the ALD Al2O3 gate insulator and gate metal, as shown in Figure 1b,c.
3. Results and Discussion
Figure 2 shows the transfer and output curves of the fabricated GaN MISFET. The device successfully demonstrates a normally-off operation with a large threshold voltage (Vth) of around 3.5 V, which is preferred for power devices, as in the linear region (Vd = 0.1 V) of Figure 2a. The fully recessed gate region with etching depth of 36 nm is attributed to the enhanced Vth and the degraded on-current. The GaN buffer layer with relatively low resistance also deteriorates the off-state leakage current of the device, which leads to the reduced ION/IOFF ratio. The ION/IOFF ratio can be further improved by controlling the gate recess and/or increasing buffer resistance by doping deep-level impurities [17,18,19]. The gate voltage can sweep up to 10 V without degradation of the drain current, thanks to the high quality of the Al2O3 gate insulator. The output curves in Figure 2b present good cut-off and pinch-off properties with a small knee voltage of ~3 V.
LFN measurements were performed using a NOSISYS7 fully automatic noise analyzer (Synergie Concept) [20]. The drain current noise power spectral densities (SId) are plotted in the frequency ranges from 4 Hz to 104 Hz at Vd = 0.1 V and two representative gate biases: (i) Vg = 2 V (deep-subthreshold region) and (ii) Vg = 10 V (strong accumulation region) are shown in Figure 3a. At the on-state, LFNs clearly exhibit a 1/fγ shape with γ ≈ 1 from the subthreshold region (Vg = 2.6 V) to the strong accumulation region (Vg = 10 V). Similar 1/f noise curves were obtained for the LFN measured at Vg bias conditions of 2.6 V ≤ Vg ≤ 10 V (not presented in Figure 3a). The measured SId values increased at increased Vg, which was attributed to the increased drain current.
The normalized SId (SId/Id2) values are displayed as a function of Id at f = 10 Hz and Vd = 0.1 V, as shown in Figure 3b. If LFN follows the correlated mobility fluctuations (CMF) model, which is explained by the CNF noise model plus additional mobility fluctuations, SId/Id2 can be explained as follows [21,22],
SIdId2=(gmId)2 SVfb (1+ΩIdgm)2
with SVfb=q2kTλNtWLCox2f
where SVfb is the flat-band voltage spectral density, Ω = αsc × μeff × Cox, is the correlated mobility fluctuation term, which includes that αsc is the Coulomb scattering coefficient, μeff is the effective carrier mobility, Cox is the gate dielectric capacitance per unit area, q is the electron charge, kT is the thermal energy, λ is the tunnel attenuation distance, Nt is the gate dielectric trap density, WL is the channel area and f is frequency. Considering values of SVfb = 7 × 10−10 V−2/Hz and Ω = 0.65 V−1, SId/Id2 is perfectly matched with (gm/Id)2 at the drain current level. It is obvious that the dominant LFN mechanism in the fabricated GaN MISFET is the CMF noise model. According to the Equation (2) with λ = 0.11 nm [12], the trap density of Nt is acquired as 2.1 × 1019 cm−3·eV−1, which is a value one or two orders lower than those of the GaN iunctionless FET (JLFET) [12] and the GaN nanowire gate-all-around (GAA) FET [23]. The reason for the decreased Nt in the fabricated GaN MISFET is that the improved quality of the GaN channel layer compared to the n-type doped GaN in GaN JLFET and the smart-cut GaN in GaN GAA FET [12,23].
To further clearly observe the CMF noise model, the gate voltage noise power spectral density, (SVg)1/2, is plotted using Equation (3) [24],
SVg=SIdgm2=SVfb(1+ΩIdgm)
Figure 3c shows a good linear relationship, which means that the CMF model fits very well with the experimental noise data. From the curves of (SVg)1/2 measured at Vd = 0.1 V and f = 10 Hz, the SVfb and Ω can be determined from the intercept with the Y-axis and the slope between (SVg)1/2 and (Id/gm), respectively. The corresponding SVfb and Ω values are extracted to be 7.3 × 10−10 V−2/Hz and 0.75 V−1, respectively, which are almost consistent with those values obtained in the curves of SI/Id2 versus Id of Figure 3b.
At the off-state (deep subthreshold region), the spectral deformation is clearly acquired, which is totally different from the noise characteristics at the on-state (Figure 3a). The noise spectra at the off-state are observed as 1/f noise at low frequency, but suddenly decrease with 1/f2 at higher frequency. Generally, the noise spectra consist of two noise sources: one is 1/f noise and the other is g-r noise, as in Equation (4) [24],
SI=Kff+∑i=0NAi1+(ffoi)2 with τi=12πfoi
where Kf is the coefficient of the 1/f noise component, Ai is the plateau value of the g-r component, foi is the cut-off frequency and τi is the trap time constant. To clearly attain the cutoff frequency, the product SI × f is displayed according to the increased Vd from 0.1 V to 5 V in Figure 4a. The obtained g-r noise levels are increased at enhanced Vd, which is caused by the increased off-state leakage current. The fabricated GaN MISFET shows a foi of 700 Hz, which corresponds to a τi of 23 msec. Similar results were also obtained from the GaN JLFET with a partially covered-gate structure (τi = 50 ms) [12]. The reason for this small trap time constant is that the proposed device exhibits much less trapping effects than that of the reference device [12]. According to the increased Vd up to 5 V, the estimated foi is attained at 500 Hz (the corresponding τi is 32 ms). This confirms that the trapping/detrapping process in the GaN buffer layer becomes deeper according to the increased drain voltage. This tendency is coincident with the drain lag phenomenon, which is related to current collapse induced by bulk traps in the GaN buffer layer at high drain voltage.
To check the drain lag of the proposed device, pulsed I-V measurements were conducted as in Figure 4b. The drain voltage is swept from 0 V to 10 V, varying Vg = 0 ~ 8 V with step of 1 V. The pulse conditions with pulse width of 1 ms are set at (1) Vg,Q = Vd,Q = 0 V, (2) Vg,Q = −2 V, Vd,Q = 0 V, and (3) Vg,Q = −2 V, Vd,Q = 10 V. To check the gate lag and drain lag, these pulsed I-V characteristics are compared. From the difference between curves (1) and (2), the fabricated device presents almost negligible gate lag due to the effective surface passivation of the high quality Al2O3 gate insulator. However, the severe drain lag from the difference between curves (2) and (3) is demonstrated in the fabricated device (Figure 4b). This is reflected by deep-trapping/de-trapping in the GaN buffer layer, which is well matched with the g-r noise implemented from the noise results at the off-state, as discussed earlier.
4. Conclusions A normally-off GaN MISFET was investigated through DC, LFN, and pulsed I-V measurements. Normally-off operation with Vth of 3.5 V was successfully obtained in the fabricated device using a recessed-gate MIS structure. LFN clearly indicated 1/f noise behavior at the on-state, but Lorentzian characteristics at the deep-subthreshold region (off-state). The dominant channel mechanism in the proposed GaN MISFET is a CMF noise model, which was confirmed by both SId/Id2 versus Id and (SVg)1/2 versus (Id/gm) curves. The τi calculated from foi measured at the off-state was 23 ms and increased to 32 ms at increased Vd, which clearly explains to the deep-trapping/de-trapping process in the GaN buffer layer. The drain lag observed from the pulsed I-V measurements was also well matched with that of the noise performances at the off-state.
Figure 1.(a) Schematic illustration of a normally-off GaN metal-insulator-semiconductor field-effect transistor (MISFET) including epitaxial and device information. (b) Cross-sectional transmission electron microscopy (TEM) and (c) energy-dispersive X-ray spectroscopy (EDX) images of the recessed-gate GaN MIS structure. Elemental mapping of Al (green), GaN (red), N (purple), and O (blue) in the device structure obtained using the EDX machine.
Figure 2.(a) Linear (filled black) and logarithmic (empty blue) scale of Id versus Vg at Vd = 0.1 V and (b) Id-Vd curves sweeping Vd = 0 ~ 10 V in the fabricated GaN MISFET.
Figure 3.(a) Noise spectral density (SId) versus frequency at Vg = 2 V and 10 V. (b) Normalized SId (black square) versus Id matching with the correlated mobility fluctuations (CMF) noise model and (c) variation of the gate voltage noise spectral density (SVg)1/2 as a function of (Id/gm) in the fabricated GaN metal-insulator-semiconductor field-effect transistor (MISFET), measured at Vd = 0.1 V and f = 10 Hz. The dashed line in (b) indicates the fitting line.
Figure 4.(a) Product SId × frequency as a function of frequency in the fabricated GaN MISFET varied Vd = 0.1 ~ 5 V. (b) Pulsed Id-Vd curves for the device sweeping Vd = 0 ~ 10 V.
Author Contributions
Writing-review and editing, M.S.P.R., Y.H., Y.J.C., S.J.A., J.-S.R. and K.-S.I.; investigation, K.-S.I.; fabrication, Y.J.C., K.-S.I.; data collection of DC, LFN, and pulse I-V, Y.J.C., K.-S.I.; All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. NRF-2018R1A6A1A03025761, NRF-2019R1I1A1A01064011).
Conflicts of Interest
The authors declare no conflict of interest.
1. Im, K.-S.; Ha, J.-B.; Kim, K.-W.; Lee, J.-S.; Kim, D.-S.; Hahm, S.-H.; Lee, J.-H. Normally Off GaN MOSFET Based on AlGaN/GaN Heterostructure with Extremely High 2DEG Density Grown on Silicon Substrate. IEEE Electron Device Lett. 2010, 31, 192-194.
2. Kim, K.-W.; Jung, S.-D.; Kim, D.-S.; Kang, H.-S.; Im, K.-S.; Oh, J.-J.; Ha, J.-B.; Shin, J.-K.; Lee, J.-H. Effects of TMAH Treatment on Device Performance of Normally Off Al2O3/GaN MOSFET. IEEE Electron Device Lett. 2011, 32, 1376-1378.
3. Sakong, S.; Lee, S.-H.; Rim, T.; Jo, Y.-W.; Lee, J.-H.; Jeong, Y.-H. 1/f Noise Characteristics of Surface-Treated Normally-Off Al2O3/GaN MOSFETs. IEEE Electron Device Lett. 2015, 36, 229-231.
4. Yin, R.; Li, Y.; Sun, Y.; Wen, C.P.; Hao, Y.; Wang, M. Correlation between border traps and exposed surface properties in gate recessed normally-off Al2O3/GaN MOSFET. Appl. Phys. Lett. 2018, 112, 233505.
5. Choi, H.-S. Mobility Degradation Effect to Hooge's Constant in Recessed-Gate Al2O3/AlGaN/GaN MIS Power Transistors. IEEE Electron Device Lett. 2014, 35, 624-626.
6. Sindhuri, V.; Son, D.-H.; Lee, D.-G.; Sakong, S.; Jeong, Y.-H.; Cho, I.-T.; Lee, J.-H.; Kim, Y.-T.; Cristoloveanu, S.; Bae, Y.; et al. 1/f Noise Characteristics of AlGaN/GaN FinFETs with and without TMAH surface treatment. Microelectron. Eng. 2015, 147, 134-136.
7. Reddy, M.S.P.; Lee, J.-H.; Jang, J.-S. Electrical Characteristics of TMAH-Surface Treated Ni/Au/Al2O3/GaN MIS Schottky Structures. Electron. Mater. Lett. 2014, 10, 411-416.
8. Reddy, M.S.P.; Park, H.; Kim, S.-M.; Jang, S.-H.; Jang, J.-S. High-performance light-emitting diodes using hierarchically mplane GaN nano-prism light extractors. J. Mater. Chem. C 2015, 3, 8873-8880.
9. Reddy, M.S.P.; Son, D.-H.; Lee, J.-H.; Jang, J.-S.; Reddy, V.R. Influence of tetramethylammonium hydroxide treatment on the electrical characteristics of Ni/Au/GaN Schottky barrier diode. Mater. Chem. Phys. 2014, 143, 801-805.
10. Im, K.-S.; Choi, J.S.; Hwang, Y.M.; An, S.J.; Roh, J.-S.; Kang, S.-H.; Lee, J.-H.; Lee, J.-H. 1/f Noise Characteristics of AlGaN/GaN HEMTs with Periodically Carbon-doped GaN Buffer Layer. Microelectron. Eng. 2019, 215, 110985.
11. Reddy, M.S.P.; Caulmilone, R.; Cristoloveanu, S.; Im, K.-S.; Lee, J.-H. Trap and 1/f-noise effects at the surface and core of GaN nanowire gate-all-around FET structure. Nano Res. 2019, 12, 809-814.
12. Im, K.-S.; An, S.J.; Theodorou, C.G.; Ghibaudo, G.; Cristoloveanu, S.; Lee, J.-H. Effect of Gate Structure on the Trapping Behavior of GaN Junctionless FinFETs. IEEE Electron Device Lett. 2020, 41, 832-835.
13. He, Z.; Chen, Y.; He, J.; Su, W.; Fang, W.; En, Y.; Huang, Y.; Liu, Y. Hydrogen effects on AlGaN/GaN MISFET with LPCVD-SiNx gate dielectric. Semicond. Sci. Technol. 2019, 34, 035020.
14. Chen, Y.Q.; Zhang, Y.C.; Liu, Y.; Liao, X.Y.; En, Y.F.; Fang, W.X.; Huang, Y. Effect of Hydrogen on Defects of AlGaN/GaN HEMTs Characterized by Low-Frequency Noise. IEEE Trans. Electron Devices 2018, 65, 1321-1326.
15. McWhorter, A.L. 1/f Noise and Germanium Surface Properties in Semiconductor Surface Physics; University of Pennsylvania Press: Philadelphia, PA, USA, 1957; pp. 207-208.
16. Vandamme, L.K.J.; Hooge, F.N. What do we certainly know about1/f noise in MOSTs? IEEE Trans. Electron. Devices 2008, 55, 3070-3085.
17. Ohba, Y.; Hatano, A. A study on strong memory effects for Mg doping in GaN metalorganic chemical vapor deposition. J. Cryst. Growth 1994, 145, 214-218.
18. Heikman, S.; Keller, S.; DenBaars, S.P.; Mishra, U.K. Growth of Fe doped semi-insulating GaN by metalorganic chemical vapor deposition. Appl. Phys. Lett. 2002, 81, 439-441.
19. Kang, H.-S.; Won, C.-H.; Kim, Y.-J.; Kim, D.-S.; Yoon, Y.J.; Kang, I.M.; Lee, Y.S.; Lee, J.-H. Suppression of current collapse in AlGaN/GaN MISHFET with carbondoped GaN/undoped GaN multilayered buffer structure. Phys. Status Solidi A 2015, 212, 1116-1121.
20. Chroboczek, J.A.; Piantino, G. France Patent 15075, November 1999.
21. Ghibaudo, G.; Roux, O.; Nguyen-duc, C.; Balestra, F.; Brini, J. Improved analysis of low frequency noise in field-effect MOS transistors. Phys. Status Solidi A 1991, 124, 571-581.
22. Theodorou, C.G.; Fasarakis, N.; Hoffman, T.; Chiarella, T.; Ghibaudo, G.; Dimitriadis, C.A. Flicker noise in n-channel nanoscale tri-gate fin-shaped field-effect transistors. Appl. Phys. Lett. 2012, 101, 243512.
23. Im, K.-S.; Reddy, M.S.P.; Caulmilone, R.; Theodorou, C.G.; Ghibaudo, G.; Cristoloveanu, S.; Lee, J.-H. Low-frequency noise characteristics of GaN nanowire gate-all-around transistors with/without 2-DEG channel. IEEE Trans. Electron Devices 2019, 66, 1243-1248.
24. Theodorou, C.G.; Fasarakis, N.; Hoffman, T.; Chiarella, T.; Ghibaudo, G.; Dimitriadis, C.A. Origin of the low-frequency noise in n-channel FinFETs. Solid State Electron. 2013, 82, 21-24.
Ki-Sik Im1,*, Mallem Siva Pratap Reddy2, Yeo Jin Choi3, Youngmin Hwang1, Sung Jin An3 and Jea-Seung Roh3
1Advanced Material Research Center, Kumoh National Institute of Technology, Gumi 39177, Korea
2School of Electronics Engineering, Kyungpook National University, Daegu 41566, Korea
3Department of Advanced Materials Science and Engineering, Kumoh National Institute of Technology, Gumi 39177, Korea
*Author to whom correspondence should be addressed.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
© 2020. This work is licensed under http://creativecommons.org/licenses/by/3.0/ (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.
Abstract
A tetramethyl ammonium hydroxide (TMAH)-treated normally-off Gallum nitride (GaN) metal-insulator-semiconductor field-effect transistor (MISFET) was fabricated and characterized using low-frequency noise (LFN) measurements in order to find the conduction mechanism and analyze the trapping behavior into the gate insulator as well as the GaN buffer layer. At the on-state, the noise spectra in the fabricated GaN device were 1/fγ properties with γ ≈ 1, which is explained by correlated mobility fluctuations (CMF). On the other hand, the device exhibited Lorentzian or generation-recombination (g-r) noises at the off-state due to deep-level trapping/de-trapping into the GaN buffer layer. The trap time constants (τi) calculated from the g-r noises became longer when the drain voltage increased up to 5 V, which was attributed to deep-level traps rather than shallow traps. The severe drain lag was also investigated from pulsed I-V measurement, which is supported by the noise behavior observed at the off-state.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer