High Electron Confinement under High Electric Field in RF GaN-on
electronics Article
High Electron Confinement under High Electric Field in RF GaN-on-Silicon HEMTs Farid Medjdoub *, Riad Kabouche, Ezgi Dogmus, Astrid Linge and Malek Zegaoui Institute of Electronics, Microelectronics and Nanotechnology (IEMN), UMR-CNRS 8520, 59652 Villeneuve d’Ascq, France; [email protected] (R.K.); [email protected] (E.D.); [email protected] (A.L.); [email protected] (M.Z.) * Correspondence: [email protected]; Tel.: +33-320-19-7840 Academic Editor: Geok Ing Ng Received: 28 December 2015; Accepted: 14 March 2016; Published: 18 March 2016
Abstract: We report on AlN/GaN high electron mobility transistors grown on silicon substrate with highly optimized electron confinement under a high electric field. The fabricated short devices (sub-10-nm barrier thickness with a gate length of 120 nm) using gate-to-drain distances below 2 µm deliver a unique breakdown field close to 100 V/µm while offering high frequency performance. The low leakage current well below 1 µA/mm is achieved without using any gate dielectrics which typically degrade both the frequency performance and the device reliability. This achievement is mainly attributed to the optimization of material design and processing quality and paves the way for millimeter-wave devices operating at drain biases above 40 V, which would be only limited by the thermal dissipation. Keywords: GaN-on-Si; high breakdown voltage; low leakage current
1. Introduction As a consequence to the rapid development of Radio Frequency (RF) power electronics, wide bandgap materials have been introduced due to their potential in high output power density, high operation voltage and high input impedance. Gallium Nitride (GaN)-based RF power devices have made substantial progress in the last decade, which will enable new applications demanding higher output power and efficiency at higher frequencies, especially in the Ka band (26–40 GHz) and beyond, with the aim of replacing or complementing traveling wave tube amplifiers. Satellite and broadband wireless communications as well as advanced radars are only a few of the many applications that would greatly benefit from the increased reliability, reduced size and reduced noise of these solid state-based amplifiers. In order to achieve the goal of operating at mm-wave frequencies and beyond, new process technologies and device structures have to be developed. Indeed, the device dimensions, such as the gate-to-channel distance, the gate length or the gate-to-drain distance, need to be reduced in order to increase the frequency performance. However, GaN device downscaling is usually achieved at the expense of a much lower breakdown voltage as compared to devices with larger dimensions [1–4]. This is due to the significant increase of the leakage current in these highly scaled devices that can result from many parameters such as bulk defects, interface and surface traps, as well as a poor electron confinement due to the epilayer design or process-induced imperfections. Recently, sub-10-nm ultrathin barrier GaN transistors have been proposed [5] in order to avoid gate recessing which is commonly used to reduce the gate-to-channel distance while shrinking the gate length but generally causes reliability issues due to plasma damage under the gate [6]. In this frame, we have demonstrated the possibility of preventing gate tunneling through highly scaled Aluminum Nitride (AlN) barrier thickness (below 5 nm) by showing an extremely low gate leakage current [7,8]. On top of the material quality, the in situ grown SiN cap layer has been a key feature for controlling Electronics 2016, 5, 12; doi:10.3390/electronics5010012
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Electronics 2016, 5, 12 2 of 5 the surface parasitic leakage current and achieving high performance [9–11], namely by preventing the strain relaxation of the barrier layer. current [7,8]. On top of the material quality, the in situ grown SiN cap layer has been a key feature for In this paper, report on theleakage high breakdown highly scaled GaN transistors grown controlling the we surface parasitic current and voltage achievinginhigh performance [9–11], namely by on a silicon substrate, showing that all the above-mentioned issues can be overcome in these emerging preventing the strain relaxation of the barrier layer. types of devices. In this paper, we report on the high breakdown voltage in highly scaled GaN transistors grown
on a silicon substrate, showing that all the above-mentioned issues can be overcome in these
2. Experimental Section emerging types of devices. The AlN/GaN heterostructures were grown by metal organic chemical vapor deposition 2. Experimental Section (MOCVD) on a highly resistive 4 in Si (111) substrate. The High Electron Mobility Transistor (HEMT) AlN/GaN heterostructures were grown metal organicbuffer chemical deposition structure The consists of nucleation and transition layers,by a 1.5-µm-thick layervapor including undoped (MOCVD) on a highly resistive 4 in Si (111) substrate. The High Electron Mobility Transistor graded AlGaN back barrier layers, followed by a 100 nm GaN channel and a 4.0 nm ultrathin AlN (HEMT) structure consists of nucleation and transition layers, a 1.5-µm-thick buffer layer including barrier layer as well as a 5.0-nm-thick in situ Si3 N4 cap layer (Figure 1). The in situ SiN layer is used both undoped graded AlGaN back barrier layers, followed by a 100 nm GaN channel and a 4.0 nm as early passivation as well as as to well prevent relaxation Hall measurements ultrathin AlN barrier layer as a strain 5.0-nm-thick in situ[12]. Si3NRoom-temperature 4 cap layer (Figure 1). The in situ SiN 13 cm´2 with a mobility of 1250 cm2 /Vs in the showed a high electron sheet concentration of 1.5 ˆ 10 layer is used both as early passivation as well as to prevent strain relaxation [12]. Room-temperature heterostructure. Sheet resistance (Rsh )electron measurements revealed aofhigh over the 4 in of wafers Hall measurements showed a high sheet concentration 1.5 ×uniformity 1013 cm−2 with a mobility 2/Vs with Rsh 310 Ω/δ ˘ 3% in the HEMT structure. A Ti/Al/Ni/Au metalrevealed stack annealed at 875 ˝ C has 1250 =cm in the heterostructure. Sheet resistance (Rsh) measurements a high uniformity in wafers with Rsh = 310 directly Ω/δ ± 3%on in top the of HEMT structure. A Ti/Al/Ni/Au metal the stack been over usedthe to 4form the ohmic contacts the AlN barrier layer by etching in situ annealed at 875 °C has been used to form the ohmic contacts directly on top of the AlN barrier layer (Rc ) Si3 N4 layer. Device isolation was achieved by nitrogen implantation. Ohmic contact resistance by etching the in transmission situ Si3N4 layer. Device was achieved nitrogen implantation. extracted from linear line modelisolation (TLM) structures wasby0.35 Ω mm. Then, a 120Ohmic nm Ni/Au contact resistance (Rc) extracted from linear transmission line model (TLM) structures was 0.35 Ω mm. T-gate was defined by e-beam lithography (Figure 1). The SiN underneath the gate was fully removed Then, a 120 nm Ni/Au T-gate was defined by e-beam lithography (Figure 1). The SiN underneath the by SF6 plasma etching through the ebeam lithography. The gate-source was 0.3 µm and gate-drain gate was fully removed by SF6 plasma etching through the ebeam lithography. The gate-source was spacings were and 1.8 spacings µm, respectively. The width wasThe 50 µm. 0.3 µm and0.8 gate-drain were 0.8 and 1.8device µm, respectively. device width was 50 µm.
Figure 1. Cross-section of the fabricated AlN/GaN-on-Si HEMTs (left) and SEM images of the 120 nm
Figure 1. Cross-section of the fabricated AlN/GaN-on-Si HEMTs (left) and SEM images of the 120 nm gate technology (right). gate technology (right).
3. Results and Discussion
3. Results and Discussion 3.1. DC Characteristics
3.1. DC Characteristics
Typical output characteristics of 2 × 25 µm AlN/GaN-on-Si HEMT are shown in Figure 2a. The
maximum DC current density at Vof GS = is about 1 A/mm with a peak extrinsic Typical output characteristics 2+2 ˆV25 µm AlN/GaN-on-Si HEMT are transconductance shown in Figure 2a. of 390 mS/mm at V DS = 4 V. In Figure 2b, the semi-log scale corresponding transfer characteristics at The maximum DC current density at VGS = +2 V is about 1 A/mm with a peak extrinsic VDS = 4 and 10 V It can be noticed that the transconductance decreases as a function of the transconductance ofare 390plotted. mS/mm at V DS = 4 V. In Figure 2b, the semi-log scale corresponding transfer drain bias due to the self-heating effect. In spite of a deep sub-micrometer gate length as well as characteristics at VDS = 4 and 10 V are plotted. It can be noticed that the transconductance decreases short gate-drain distances and an ultrathin barrier layer, generating an extremely high electric field, as a function of the drain bias due to the self-heating effect. In spite of a deep sub-micrometer gate a sub-threshold drain leakage current well below 1 µA/mm is reproducibly observed (within the length as well as shorttenths gate-drain distances an ultrathin barrier layer, generating an extremely range of several of nA/mm) withand no shift of the threshold voltage nor an increase of the high electric field, a sub-threshold drain leakage current well below 1 µA/mm is reproducibly observed leakage current as a function of VDS. This clearly shows an optimum electron confinement combined (within the range of several of nA/mm) with no shift of thecurrent. threshold voltage nor an increase with high material qualitytenths and controlled surface parasitic leakage of the leakage current as a function of VDS . This clearly shows an optimum electron confinement combined with high material quality and controlled surface parasitic leakage current.
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Figure 2. 2. (a) (a)DC DCoutput output(a) ID-V -VDS characteristics characteristics of of aa22ˆ× 25 25 µm µm AlN/GaN-on-Si AlN/GaN-on-Si HEMT.VVGS swept swept from from Figure ID (b)HEMT. DS GS −2 up to +2 V in 1 V steps; (b) Transfer characteristics of the 2 × 25 µm AlN/GaN-on-Si HEMT at ´2 up to +2 V in 1 V steps; (b) Transfer characteristics of the 2 ˆ 25 µm AlN/GaN-on-Si HEMT at Figure 2. (a) DC output ID-VDS characteristics of a 2 × 25 µm AlN/GaN-on-Si HEMT. VGS swept from and 10 V) using L ==0.12 variousVVDS (4 various using(b) LGGTransfer 0.12µm. µm. −2 upDSto(4 +2and V in101 V) V steps; characteristics of the 2 × 25 µm AlN/GaN-on-Si HEMT at various VDS (4 and 10 V) using LG = 0.12 µm.
Thebenefit benefitofof this configuration, enabling to prevent both short-channel effects and the The this configuration, enabling us tous prevent both short-channel effects and the electron electron injection into the buffer layers under high electric field, is absolutely obvious of this configuration, enabling to prevent both short-channel and thein the injection The into benefit the buffer layers under high electricusfield, is absolutely obvious ineffects the sub-threshold sub-threshold region and consequently reflected in the three-terminal breakdown voltage of these electron injection into is the bufferinlayers under high electric field, voltage is absolutely obvious in Figure the region and is consequently reflected the three-terminal breakdown of these devices. 3 devices. Figure 3 indeed shows the off-state characteristics at V GS = −5 V of devices with sub-threshold region and is consequently reflected in the three-terminal breakdown voltage of these indeed shows the off-state characteristics at VGS = ´5 V of devices with a gate-to-drain distance of 0.8a devices. indeed shows off-state characteristics at V GS extremely = −5 V devices with a gate-to-drain distance of 0.8 that and an 1.8the µm. It canlow be leakage noticed current that an low leakage current and 1.8 µm. ItFigure can be3 noticed extremely below 100ofnA/mm is observed gate-to-drain distance of 0.8 and 1.8 µm. It can be noticed that an extremely low leakage current below is observed up to athree-terminal high bias, yielding outstanding three-terminal breakdown up to a 100 highnA/mm bias, yielding outstanding breakdown voltages defined at 1 µA/mm of below 100 nA/mm is observed up to 80 a high bias, yielding outstandingThis three-terminal breakdown voltages defined at 1 µA/mm of about V and 160 V, respectively. translates to a breakdown aboutvoltages 80 V and 160 V, respectively. This translates to a breakdown field ranging from 90 to 100 V/µm, defined at 1 µA/mm of about 80 V and 160 V, respectively. This translates to a breakdown field ranging from comparable 90 to 100 V/µm, whichreported is favorably comparable to typical such reported values in the which is favorably to typical values in the literature devices, field ranging from 90 to 100 V/µm, which is favorably comparable to typicalfor reported short valuesRF in the literature for such short RF devices, especially in terms of leakage current level [13]. especially in terms of leakage current level [13]. literature for such short RF devices, especially in terms of leakage current level [13]. -3
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DS current at V Figure 3. Off-state characteristics and gate leakage GS = ´5 V of the 2 ˆ 25 µm AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) and 1.8 µm (blue circles). AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) 1.8of µmthe (blue Figure 3. Off-state characteristics and gate leakage current at VGS = and −5 V 2 ×circles). 25 µm
AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) and 1.8 µm (blue circles). 3.2. RF Characteristics
3.2. RF Characteristics The S-parameters of a 0.12 × 50 µm2 AlN/GaN-on-Si HEMT were measured from 1 to 110 GHz. 3.2. RF Characteristics The S-parameters of cut-off a 0.12 frequency ˆ 50 µmfT2 =AlN/GaN-on-Si HEMToscillation were measured A current gain extrinsic 78 GHz and a maximum frequency from fmax = 1 to 2 AlN/GaN-on-Si 190 (showngain in Figure 4a)×have been extrapolated the and current gain H21 and the unilateral 110 GHz. A current cut-off frequency fT = from 78 GHz a maximum oscillation TheGHz S-parameters ofextrinsic a 0.12 50 µm HEMT were measured from 1 tofrequency 110 GHz. power gain (U) at V DS = 20 V. The strong reduction of the short-channel effects by using a short fA = 190 GHz (shown in Figure 4a) have been extrapolated from the current gain H the= current gain extrinsic cut-off frequency fT = 78 GHz and a maximum oscillation frequency max 21 andfmax gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas unilateral power gain (U) at4a) VDShave = 20been V. The strong reduction of the short-channel effects using a 190 GHz (shown in Figure extrapolated from the current gain H21 and theby unilateral power gain (U) at VDS = 20 V. The strong reduction of the short-channel effects by using a short gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas
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short gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas (2DEG) explains excellent power gain close 200GHz. GHz.It Ithas hastotobe be pointed pointed out out that these (2DEG) explains the the excellent power gain close to to 200 frequency been achieved despite non-optimized high RF losses aboutof2 dB/mm frequency performances performanceshave have been achieved despite non-optimized high RF of losses about 2 as can beas seen 4b.Figure The RF losses transmission lines reflectlines a parasitic dB/mm caninbeFigure seen in 4b. The extracted RF losses from extracted from transmission reflectconduction a parasitic at the buffer/Si substrate due to the intermixing between the GaN buffer layerbuffer and the Si conduction at the buffer/Siinterface substrate interface due to the intermixing between the GaN layer substrate and the Si[14]. substrate [14].
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Figure 4. 4. (a) (a) RF RF performance performanceof ofthe the0.12 0.12ˆ× 50 50 µm µm22 AlN/GaN AlN/GaN HEMT substrate at at V VDS DS = Figure HEMT on on silicon silicon substrate = 20 20 V; V; (b) RF RF losses losses extracted extracted from from transmission transmission lines lines up up to to 67 67 GHz. GHz. (b)
Further reduction reductionof ofthe thecontact contactresistances resistancesasaswell wellasasthe theRFRFlosses losses should result increase Further should result in in anan increase of of the extrinsic transconductance and a significant improvement of the frequency performance. The the extrinsic transconductance and a significant improvement of the frequency performance. The use of use of sub-100-nm gate lengths in the AlN/GaN DHFETtherefore should therefore pavefor thehigher way for higher sub-100-nm gate lengths in the AlN/GaN DHFET should pave the way frequency frequency of operation of GaN-based devices grownresistive on highly resistive Si substratewith combined with operation GaN-based devices grown on highly Si substrate combined high power high power (e.g., high current density and high voltage). (e.g., high current density and high voltage). 4. Conclusions Conclusions
We have developed grown onon silicon substrate delivering an We developed high high frequency frequencyAlN/GaN AlN/GaNHEMTs HEMTs grown silicon substrate delivering outstanding breakdown field close to to 100100 V/µm while using highly scaled dimensions such as an outstanding breakdown field close V/µm while using highly scaled dimensions such sub-10-nm barrier thickness, 120 nm gate length and gate-to-drain distances below 2 µm. This as sub-10-nm barrier thickness, 120 nm gate length and gate-to-drain distances below 2 µm. achievement is mainly attributed to the to optimization of bothofmaterial design design and processing quality This achievement is mainly attributed the optimization both material and processing enablingenabling high electron confinement underunder a high electric quality high electron confinement a high electricfield. field.These Thesedata data show show that millimeter-wave GaN thethe Ka K band andand above can operate at unique drain biases GaNdevices devicesoperating operatinginin above can operate at unique drain a band above above 40 V. 40 V. biases Acknowledgments: This Thiswork workwas was supported by French Defense Procurement (DGA)theunder the Acknowledgments: supported by French Defense Procurement AgencyAgency (DGA) under National National Research project (ANR-13-ASTR-0022: CROCUS project), the French RENATECH network as well as Research project (ANR-13-ASTR-0022: CROCUS project), the French RENATECH network as well as the company the company EpiGaNdelivery. for material delivery. EpiGaN for material All authors authors have participated to the design, the fabrication and the electrical Author Author Contributions: Contributions: All electrical characterization characterization of of the the presented presented devices. devices. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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Shinohara, K.; Regan, D.; Corrion, A.; Brown, D.; Burnham, S.; Willadsen, P.J.; Alvarado-Rodriguez, I.; Shinohara, K.; Regan, D.; Corrion, A.; Brown, D.; Burnham, S.; Willadsen, P.J.; Alvarado-Rodriguez, I.; Cunningham, M.; Butler, C.; Schmitz, A.; et al. Deeply-Scaled Self-Aligned-Gate GaN DH-HEMTs with Cunningham, M.; Butler, C.; Schmitz, A.; et al. Deeply-Scaled Self-Aligned-Gate GaN DH-HEMTs with Ultrahigh Cutoff Frequency. In Proceedings of the IEEE International Electron Devices Meeting (IEDM), Ultrahigh Cutoff Frequency. In Proceedings of the IEEE International Electron Devices Meeting (IEDM), Washington, DC, USA, 5–7 December 2011; Volume 19, pp. 1–4. Washington, DC, USA, 5–7 December 2011; Volume 19, pp. 1–4. Shinohara, K.; Regan, D.C.; Tang, Y.; Corrion, A.L.; Brown, D.F.; Wong, J.C.; Robinson, J.F.; Fung, H.H.; Shinohara, K.; Regan, D.C.; Tang, Y.; Corrion, A.L.; Brown, D.F.; Wong, J.C.; Robinson, J.F.; Fung, H.H.; Schmitz, A.; Oh, T.C.; et al. Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Schmitz, A.; Oh, T.C.; et al. Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Applications. IEEE Trans. Electron Devices 2013, 60, 2982–2996. [CrossRef] Applications. IEEE Trans. Electron Devices 2013, 60, 2982–2996. Tirelli, S.; Marti, D.; Sun, H.; Alt, A.R.; Carlin, J.-F.; Grandjean, N.; Bolognesi, C.R. Fully Passivated AlInN/GaN HEMTs with fT/fMAX of 205/220 GHz. IEEE Electron Device Lett. 2011, 32, 1364–1366.
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High Electron Confinement under High Electric Field in RF GaN-on-Silicon HEMTs Farid Medjdoub *, Riad Kabouche, Ezgi Dogmus, Astrid Linge and Malek Zegaoui Institute of Electronics, Microelectronics and Nanotechnology (IEMN), UMR-CNRS 8520, 59652 Villeneuve d’Ascq, France; [email protected] (R.K.); [email protected] (E.D.); [email protected] (A.L.); [email protected] (M.Z.) * Correspondence: [email protected]; Tel.: +33-320-19-7840 Academic Editor: Geok Ing Ng Received: 28 December 2015; Accepted: 14 March 2016; Published: 18 March 2016
Abstract: We report on AlN/GaN high electron mobility transistors grown on silicon substrate with highly optimized electron confinement under a high electric field. The fabricated short devices (sub-10-nm barrier thickness with a gate length of 120 nm) using gate-to-drain distances below 2 µm deliver a unique breakdown field close to 100 V/µm while offering high frequency performance. The low leakage current well below 1 µA/mm is achieved without using any gate dielectrics which typically degrade both the frequency performance and the device reliability. This achievement is mainly attributed to the optimization of material design and processing quality and paves the way for millimeter-wave devices operating at drain biases above 40 V, which would be only limited by the thermal dissipation. Keywords: GaN-on-Si; high breakdown voltage; low leakage current
1. Introduction As a consequence to the rapid development of Radio Frequency (RF) power electronics, wide bandgap materials have been introduced due to their potential in high output power density, high operation voltage and high input impedance. Gallium Nitride (GaN)-based RF power devices have made substantial progress in the last decade, which will enable new applications demanding higher output power and efficiency at higher frequencies, especially in the Ka band (26–40 GHz) and beyond, with the aim of replacing or complementing traveling wave tube amplifiers. Satellite and broadband wireless communications as well as advanced radars are only a few of the many applications that would greatly benefit from the increased reliability, reduced size and reduced noise of these solid state-based amplifiers. In order to achieve the goal of operating at mm-wave frequencies and beyond, new process technologies and device structures have to be developed. Indeed, the device dimensions, such as the gate-to-channel distance, the gate length or the gate-to-drain distance, need to be reduced in order to increase the frequency performance. However, GaN device downscaling is usually achieved at the expense of a much lower breakdown voltage as compared to devices with larger dimensions [1–4]. This is due to the significant increase of the leakage current in these highly scaled devices that can result from many parameters such as bulk defects, interface and surface traps, as well as a poor electron confinement due to the epilayer design or process-induced imperfections. Recently, sub-10-nm ultrathin barrier GaN transistors have been proposed [5] in order to avoid gate recessing which is commonly used to reduce the gate-to-channel distance while shrinking the gate length but generally causes reliability issues due to plasma damage under the gate [6]. In this frame, we have demonstrated the possibility of preventing gate tunneling through highly scaled Aluminum Nitride (AlN) barrier thickness (below 5 nm) by showing an extremely low gate leakage current [7,8]. On top of the material quality, the in situ grown SiN cap layer has been a key feature for controlling Electronics 2016, 5, 12; doi:10.3390/electronics5010012
www.mdpi.com/journal/electronics
Electronics 2016, 5, 12
2 of 5
Electronics 2016, 5, 12 2 of 5 the surface parasitic leakage current and achieving high performance [9–11], namely by preventing the strain relaxation of the barrier layer. current [7,8]. On top of the material quality, the in situ grown SiN cap layer has been a key feature for In this paper, report on theleakage high breakdown highly scaled GaN transistors grown controlling the we surface parasitic current and voltage achievinginhigh performance [9–11], namely by on a silicon substrate, showing that all the above-mentioned issues can be overcome in these emerging preventing the strain relaxation of the barrier layer. types of devices. In this paper, we report on the high breakdown voltage in highly scaled GaN transistors grown
on a silicon substrate, showing that all the above-mentioned issues can be overcome in these
2. Experimental Section emerging types of devices. The AlN/GaN heterostructures were grown by metal organic chemical vapor deposition 2. Experimental Section (MOCVD) on a highly resistive 4 in Si (111) substrate. The High Electron Mobility Transistor (HEMT) AlN/GaN heterostructures were grown metal organicbuffer chemical deposition structure The consists of nucleation and transition layers,by a 1.5-µm-thick layervapor including undoped (MOCVD) on a highly resistive 4 in Si (111) substrate. The High Electron Mobility Transistor graded AlGaN back barrier layers, followed by a 100 nm GaN channel and a 4.0 nm ultrathin AlN (HEMT) structure consists of nucleation and transition layers, a 1.5-µm-thick buffer layer including barrier layer as well as a 5.0-nm-thick in situ Si3 N4 cap layer (Figure 1). The in situ SiN layer is used both undoped graded AlGaN back barrier layers, followed by a 100 nm GaN channel and a 4.0 nm as early passivation as well as as to well prevent relaxation Hall measurements ultrathin AlN barrier layer as a strain 5.0-nm-thick in situ[12]. Si3NRoom-temperature 4 cap layer (Figure 1). The in situ SiN 13 cm´2 with a mobility of 1250 cm2 /Vs in the showed a high electron sheet concentration of 1.5 ˆ 10 layer is used both as early passivation as well as to prevent strain relaxation [12]. Room-temperature heterostructure. Sheet resistance (Rsh )electron measurements revealed aofhigh over the 4 in of wafers Hall measurements showed a high sheet concentration 1.5 ×uniformity 1013 cm−2 with a mobility 2/Vs with Rsh 310 Ω/δ ˘ 3% in the HEMT structure. A Ti/Al/Ni/Au metalrevealed stack annealed at 875 ˝ C has 1250 =cm in the heterostructure. Sheet resistance (Rsh) measurements a high uniformity in wafers with Rsh = 310 directly Ω/δ ± 3%on in top the of HEMT structure. A Ti/Al/Ni/Au metal the stack been over usedthe to 4form the ohmic contacts the AlN barrier layer by etching in situ annealed at 875 °C has been used to form the ohmic contacts directly on top of the AlN barrier layer (Rc ) Si3 N4 layer. Device isolation was achieved by nitrogen implantation. Ohmic contact resistance by etching the in transmission situ Si3N4 layer. Device was achieved nitrogen implantation. extracted from linear line modelisolation (TLM) structures wasby0.35 Ω mm. Then, a 120Ohmic nm Ni/Au contact resistance (Rc) extracted from linear transmission line model (TLM) structures was 0.35 Ω mm. T-gate was defined by e-beam lithography (Figure 1). The SiN underneath the gate was fully removed Then, a 120 nm Ni/Au T-gate was defined by e-beam lithography (Figure 1). The SiN underneath the by SF6 plasma etching through the ebeam lithography. The gate-source was 0.3 µm and gate-drain gate was fully removed by SF6 plasma etching through the ebeam lithography. The gate-source was spacings were and 1.8 spacings µm, respectively. The width wasThe 50 µm. 0.3 µm and0.8 gate-drain were 0.8 and 1.8device µm, respectively. device width was 50 µm.
Figure 1. Cross-section of the fabricated AlN/GaN-on-Si HEMTs (left) and SEM images of the 120 nm
Figure 1. Cross-section of the fabricated AlN/GaN-on-Si HEMTs (left) and SEM images of the 120 nm gate technology (right). gate technology (right).
3. Results and Discussion
3. Results and Discussion 3.1. DC Characteristics
3.1. DC Characteristics
Typical output characteristics of 2 × 25 µm AlN/GaN-on-Si HEMT are shown in Figure 2a. The
maximum DC current density at Vof GS = is about 1 A/mm with a peak extrinsic Typical output characteristics 2+2 ˆV25 µm AlN/GaN-on-Si HEMT are transconductance shown in Figure 2a. of 390 mS/mm at V DS = 4 V. In Figure 2b, the semi-log scale corresponding transfer characteristics at The maximum DC current density at VGS = +2 V is about 1 A/mm with a peak extrinsic VDS = 4 and 10 V It can be noticed that the transconductance decreases as a function of the transconductance ofare 390plotted. mS/mm at V DS = 4 V. In Figure 2b, the semi-log scale corresponding transfer drain bias due to the self-heating effect. In spite of a deep sub-micrometer gate length as well as characteristics at VDS = 4 and 10 V are plotted. It can be noticed that the transconductance decreases short gate-drain distances and an ultrathin barrier layer, generating an extremely high electric field, as a function of the drain bias due to the self-heating effect. In spite of a deep sub-micrometer gate a sub-threshold drain leakage current well below 1 µA/mm is reproducibly observed (within the length as well as shorttenths gate-drain distances an ultrathin barrier layer, generating an extremely range of several of nA/mm) withand no shift of the threshold voltage nor an increase of the high electric field, a sub-threshold drain leakage current well below 1 µA/mm is reproducibly observed leakage current as a function of VDS. This clearly shows an optimum electron confinement combined (within the range of several of nA/mm) with no shift of thecurrent. threshold voltage nor an increase with high material qualitytenths and controlled surface parasitic leakage of the leakage current as a function of VDS . This clearly shows an optimum electron confinement combined with high material quality and controlled surface parasitic leakage current.
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Figure 2. 2. (a) (a)DC DCoutput output(a) ID-V -VDS characteristics characteristics of of aa22ˆ× 25 25 µm µm AlN/GaN-on-Si AlN/GaN-on-Si HEMT.VVGS swept swept from from Figure ID (b)HEMT. DS GS −2 up to +2 V in 1 V steps; (b) Transfer characteristics of the 2 × 25 µm AlN/GaN-on-Si HEMT at ´2 up to +2 V in 1 V steps; (b) Transfer characteristics of the 2 ˆ 25 µm AlN/GaN-on-Si HEMT at Figure 2. (a) DC output ID-VDS characteristics of a 2 × 25 µm AlN/GaN-on-Si HEMT. VGS swept from and 10 V) using L ==0.12 variousVVDS (4 various using(b) LGGTransfer 0.12µm. µm. −2 upDSto(4 +2and V in101 V) V steps; characteristics of the 2 × 25 µm AlN/GaN-on-Si HEMT at various VDS (4 and 10 V) using LG = 0.12 µm.
Thebenefit benefitofof this configuration, enabling to prevent both short-channel effects and the The this configuration, enabling us tous prevent both short-channel effects and the electron electron injection into the buffer layers under high electric field, is absolutely obvious of this configuration, enabling to prevent both short-channel and thein the injection The into benefit the buffer layers under high electricusfield, is absolutely obvious ineffects the sub-threshold sub-threshold region and consequently reflected in the three-terminal breakdown voltage of these electron injection into is the bufferinlayers under high electric field, voltage is absolutely obvious in Figure the region and is consequently reflected the three-terminal breakdown of these devices. 3 devices. Figure 3 indeed shows the off-state characteristics at V GS = −5 V of devices with sub-threshold region and is consequently reflected in the three-terminal breakdown voltage of these indeed shows the off-state characteristics at VGS = ´5 V of devices with a gate-to-drain distance of 0.8a devices. indeed shows off-state characteristics at V GS extremely = −5 V devices with a gate-to-drain distance of 0.8 that and an 1.8the µm. It canlow be leakage noticed current that an low leakage current and 1.8 µm. ItFigure can be3 noticed extremely below 100ofnA/mm is observed gate-to-drain distance of 0.8 and 1.8 µm. It can be noticed that an extremely low leakage current below is observed up to athree-terminal high bias, yielding outstanding three-terminal breakdown up to a 100 highnA/mm bias, yielding outstanding breakdown voltages defined at 1 µA/mm of below 100 nA/mm is observed up to 80 a high bias, yielding outstandingThis three-terminal breakdown voltages defined at 1 µA/mm of about V and 160 V, respectively. translates to a breakdown aboutvoltages 80 V and 160 V, respectively. This translates to a breakdown field ranging from 90 to 100 V/µm, defined at 1 µA/mm of about 80 V and 160 V, respectively. This translates to a breakdown field ranging from comparable 90 to 100 V/µm, whichreported is favorably comparable to typical such reported values in the which is favorably to typical values in the literature devices, field ranging from 90 to 100 V/µm, which is favorably comparable to typicalfor reported short valuesRF in the literature for such short RF devices, especially in terms of leakage current level [13]. especially in terms of leakage current level [13]. literature for such short RF devices, especially in terms of leakage current level [13]. -3
10 10-3
IDI for µm forGD GD== 0.8 0.8 µm D
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V current (V) at VGS = −5 V of the 2 × 25 µm Figure 3. Off-state characteristics and gate leakage
DS current at V Figure 3. Off-state characteristics and gate leakage GS = ´5 V of the 2 ˆ 25 µm AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) and 1.8 µm (blue circles). AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) 1.8of µmthe (blue Figure 3. Off-state characteristics and gate leakage current at VGS = and −5 V 2 ×circles). 25 µm
AlN/GaN-on-Si HEMTs for gate-drain distances of 0.8 µm (black squares) and 1.8 µm (blue circles). 3.2. RF Characteristics
3.2. RF Characteristics The S-parameters of a 0.12 × 50 µm2 AlN/GaN-on-Si HEMT were measured from 1 to 110 GHz. 3.2. RF Characteristics The S-parameters of cut-off a 0.12 frequency ˆ 50 µmfT2 =AlN/GaN-on-Si HEMToscillation were measured A current gain extrinsic 78 GHz and a maximum frequency from fmax = 1 to 2 AlN/GaN-on-Si 190 (showngain in Figure 4a)×have been extrapolated the and current gain H21 and the unilateral 110 GHz. A current cut-off frequency fT = from 78 GHz a maximum oscillation TheGHz S-parameters ofextrinsic a 0.12 50 µm HEMT were measured from 1 tofrequency 110 GHz. power gain (U) at V DS = 20 V. The strong reduction of the short-channel effects by using a short fA = 190 GHz (shown in Figure 4a) have been extrapolated from the current gain H the= current gain extrinsic cut-off frequency fT = 78 GHz and a maximum oscillation frequency max 21 andfmax gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas unilateral power gain (U) at4a) VDShave = 20been V. The strong reduction of the short-channel effects using a 190 GHz (shown in Figure extrapolated from the current gain H21 and theby unilateral power gain (U) at VDS = 20 V. The strong reduction of the short-channel effects by using a short gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas
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short gate-to-channel distance combined with high electron mobility in the two-dimensional electron gas (2DEG) explains excellent power gain close 200GHz. GHz.It Ithas hastotobe be pointed pointed out out that these (2DEG) explains the the excellent power gain close to to 200 frequency been achieved despite non-optimized high RF losses aboutof2 dB/mm frequency performances performanceshave have been achieved despite non-optimized high RF of losses about 2 as can beas seen 4b.Figure The RF losses transmission lines reflectlines a parasitic dB/mm caninbeFigure seen in 4b. The extracted RF losses from extracted from transmission reflectconduction a parasitic at the buffer/Si substrate due to the intermixing between the GaN buffer layerbuffer and the Si conduction at the buffer/Siinterface substrate interface due to the intermixing between the GaN layer substrate and the Si[14]. substrate [14].
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Figure 4. 4. (a) (a) RF RF performance performanceof ofthe the0.12 0.12ˆ× 50 50 µm µm22 AlN/GaN AlN/GaN HEMT substrate at at V VDS DS = Figure HEMT on on silicon silicon substrate = 20 20 V; V; (b) RF RF losses losses extracted extracted from from transmission transmission lines lines up up to to 67 67 GHz. GHz. (b)
Further reduction reductionof ofthe thecontact contactresistances resistancesasaswell wellasasthe theRFRFlosses losses should result increase Further should result in in anan increase of of the extrinsic transconductance and a significant improvement of the frequency performance. The the extrinsic transconductance and a significant improvement of the frequency performance. The use of use of sub-100-nm gate lengths in the AlN/GaN DHFETtherefore should therefore pavefor thehigher way for higher sub-100-nm gate lengths in the AlN/GaN DHFET should pave the way frequency frequency of operation of GaN-based devices grownresistive on highly resistive Si substratewith combined with operation GaN-based devices grown on highly Si substrate combined high power high power (e.g., high current density and high voltage). (e.g., high current density and high voltage). 4. Conclusions Conclusions
We have developed grown onon silicon substrate delivering an We developed high high frequency frequencyAlN/GaN AlN/GaNHEMTs HEMTs grown silicon substrate delivering outstanding breakdown field close to to 100100 V/µm while using highly scaled dimensions such as an outstanding breakdown field close V/µm while using highly scaled dimensions such sub-10-nm barrier thickness, 120 nm gate length and gate-to-drain distances below 2 µm. This as sub-10-nm barrier thickness, 120 nm gate length and gate-to-drain distances below 2 µm. achievement is mainly attributed to the to optimization of bothofmaterial design design and processing quality This achievement is mainly attributed the optimization both material and processing enablingenabling high electron confinement underunder a high electric quality high electron confinement a high electricfield. field.These Thesedata data show show that millimeter-wave GaN thethe Ka K band andand above can operate at unique drain biases GaNdevices devicesoperating operatinginin above can operate at unique drain a band above above 40 V. 40 V. biases Acknowledgments: This Thiswork workwas was supported by French Defense Procurement (DGA)theunder the Acknowledgments: supported by French Defense Procurement AgencyAgency (DGA) under National National Research project (ANR-13-ASTR-0022: CROCUS project), the French RENATECH network as well as Research project (ANR-13-ASTR-0022: CROCUS project), the French RENATECH network as well as the company the company EpiGaNdelivery. for material delivery. EpiGaN for material All authors authors have participated to the design, the fabrication and the electrical Author Author Contributions: Contributions: All electrical characterization characterization of of the the presented presented devices. devices. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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