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GaN heterostructures
Materials Science and Engineering B59 (1999) 395 – 400
Microwave electronics device applications of AlGaN/GaN heterostructures Q. Chen a,*, J.W. Yang a,1, M. Blasingame a, C. Faber a, A.T. Ping b, I. Adesida b b
a APA Optics, 2950 NE 84 th Lane, Blaine, MN 55449, USA Microelectronics Laboratory, Uni6ersity of Illinois, Urbana, IL 61801, USA
Abstract One thrust in the recent AlGaN/GaN based HFET development hinges on the use of SiC substrates for the growth of the AlGaN/GaN heterostructures. We have achieved Gm and maximum drain current (Imax) as high as 222 mS mm − 1 and 1.71 A mm − 1 for HFETs grown on n-SiC. The HFETs on p-SiC have also shown Gm and Imax of 230 mS mm − 1 and 1.43 A mm − 1. These devices exhibited cut-off frequency ( ft) and frequency of oscillation ( fmax) of 55 and 56 GHz for HFETs on p-SiC, further demonstrating the applicability of AlGaN/GaN-based HFETs in high power microwave frequency range. The availability of high quality AlGaN/GaN heterostructure has also permitted the implementation of such new device concept as metal – insulator–semiconductor FETs (MISFETs). Our MISFETs have shown low gate leakage in 96 V gate bias range with Gm as high as 86 mS mm − 1. © 1999 Elsevier Science S.A. All rights reserved. Keywords: HFET; Devices; MISFETs
1. Introduction With a direct band gap, GaN and related compounds (InGaN and AlGaN) were recognized earlier for their applications in light emitters covering the ultraviolet (UV) and visible spectral range. An examination of the basic material properties and several composite material figures of merits of semiconductors, such as shown in Table 1, brings out the fact that the GaN-related compounds possess excellent property for high power (to some extent related to high temperature) electronic device applications. A straightforward electronic device application of GaN material is the fabrication of metal semiconductor field effect transistors (MESFETs) [2]. In such MESFETs, the Schottky type gates are formed directly on the GaN channel. While a highly doped channel with a tight profile is desirable for high transconductance (Gm) of the MESFET, the formation of Schottky junction often requires a lightly doped channel. This compro* Corresponding author. Tel.: +1-612-7844995; fax: + 1-6127842038. 1 Currently with E and CE Department, University of South Carolina, Columbia, SC 29208, USA.
mise leads to thicker and lightly doped GaN channel. As a result, the MESFETs showed a low Gm of 23 mS mm − 1 which is to be compared to values of Gm \200 mS mm − 1 readily achievable in GaAs based MESFETs. In addition, the low electron mobility in the bulk GaN channel (350 cm2 V − 1 s − 1 typical) will give rise to a high series resistance, limiting DC and RF performance of the MESFETs. Electron mobility enhancement at AlGaN/GaN hetero-interface was first reported in 1991 [3]. Heterostructure field effect transistors (HFETs) were soon fabricated using such AlGaN/GaN heterostructures [4]. Despite the electron mobility enhancement, these early HFETs exhibited a DC Gm of 28 mS mm − 1, similar to that of the MESFETs. It must be noted, however, that the excellent potential of these AlGaN/GaN-based HFETs for microwave electronics application was illustrated by the early short gate (0.25 mm) HFETs which exhibited a cut-off frequency ( ft) and maximum frequency of oscillation ( fmax) of 11 and 35 GHz, respectively [5]. Drastic improvement in DC and RF performance of the AlGaN/GaN-based HFETs came several years later after further advancement in material quality, heterostructure design, and Ohmic contact formation.
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Table 1 Material properties and figure-of-merit of semiconductors for power electronic devices, after [1], otherwise noted Materials properties
GaN
SiC-3C (SiC-6H)
Diamond
Band gap (eV, 300 K) Electron mobility (cm2 V−1 s−1, 300 K) Saturation velocity (107 cm s−1) Thermal conductivity (W cm−1 deg−1) Breakdown field (106 V cm−1) EBVs/2p (V s−1), Johson’s st (CVs/4po)1/2 (W deg−1 s−1), Keyes’
3.39 900 2.7 1.3 5? 1.6×1013 12×107 on GaN 30–40×107 on SiC
2.2 (2.9) 1000 (600) 2 5 3 (4) 1.0×1013a (1.3×1013) 35×107 (39×107)
5.5 2200 2 20 10 3.2×1013a 190×107a
a
Calculated using the values available in [1].
These include the demonstration of HFETs with Gm of 120 mS mm − 1 and maximum drain current (Imax) of 350 mA mm − 1 [6]. This was soon followed by reports of HFETs with Gm of 210 mS mm − 1 [7], drain-gate breakdown voltage of 230 V [8] and maximum drain current of 1.02 A mm − 1 [9]. The excellent power capability of AlGaN/GaN HFETs in microwave frequency has recently been demonstrated by two groups with maximum power density of 1.7 W mm − 1 at 8.4 GHz [10] and 2.7 W mm at 10 GHz [11]. In this paper, we shall first deal with material aspect of the AlGaN/GaN heterostructures. The focus will then be turned to the recent development in the fabrication of HFETs grown on SiC substrate based on the work carried out at APA Optics. This approach takes advantage of the desirable transport properties of the two dimensional electron gas available at the AlGaN/ GaN hetero-interface and high thermal conductivity of the SiC substrates. This will greatly increase the power handling capability of GaN base HFETs in microwave frequencies. In order to increase high temperature stability of the gate in an HFET, we have also experimented with metal – insulator – semiconductor FET device concept using AlGaN/GaN heterostructures. These new results will also be discussed.
2. Alx Ga1 − x N/GaN heterostructures The basic heterostructure of consideration here is shown schematically in Fig. 1. It consists of a thick ( \ 1 mm) of GaN followed by a thin layer (500–1000 ˚ ) of lightly Si-doped GaN (n in the order of 3× 1017 A
cm − 3). The heterostructure is completed with the deposition of a layer of Alx Ga1 − x N. The substrates widely used are sapphire. SiC substrates have been used for this work. Compared to AlAs/GaAs heterostructure, the [in-plane, (0001)] lattice mismatch between AlN and GaN ( : 2.4%) is relatively large. Hetero-epitaxy of thick AlGaN on GaN or vice versa will result in visible cracks due to strain relaxation. when the capping Alx Ga1 − x N layer with x values up to 0.5 is made thin ˚ ), the overall structure shows n-type (below about 500 A conductivity with a typical room temperature electron mobility over 1000 cm2 V − 1 s − 1 and sheet carrier density of 1× 1013 cm − 2. Part of the high sheet carrier density comes from piezo-electric effect. The evidence that this mobility enhancement is due to the formation of a two dimensional electron gas (2DEG) are the following. First, the electron mobility increases monotonously as much as four fold from room temperature to 77 K while for a bulk GaN the mobility decreases after peaking due to ionized impurity scattering which is much lower in the 2DEG [12]. Second, we have observed Shubnikov-de-Hass (SdH) oscillation from these heterostructures [13]. The cyclotron resonance frequency is proportional to the magnetic field component that is perpendicular to the plane of the interface. This confirms the 2-dimensional character of the carriers. Other improvement in the transport property in 2DEG structures include the insertion of an undoped AlGaN spacer layer at the immediate interface [6] and the use of modulation doping [8]. The former permits higher doped sheet carrier density while keeping the electron mobility relatively high. The latter included an undoped AlGaN cap layer which will facilitate the formation of high quality Schottky barrier gates.
3. HFETs on SiC substrates
Fig. 1. Schematic of the basic AlGaN/GaN heterostructures for HFET fabrication.
From the Table 1 it can be seen that SiC is a prospective candidate for how power electronic device applications. However, the higher saturated electron drift velocity in GaN and the availability of the 2DEG
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Fig. 2. Drain I– V characteristics of HFETs on n-SiC. The device dimensions were Lsd =2 mm, Lg =0.25 mm with offset 0.6 mm from the source. From top Vgs =1 V at − 1 V step − 1. (a) Drain I –V characteristics of HFETs on n-SiC. The device dimensions were Lsd =2 mm, Lg = 0.25 mm with offset 0.6 mm from the source. From top Vgs = 1 V at −1 V step − 1. (b) Drain I – V characteristics of HFETs on p-SiC. The device dimensions were Lsd =2 mm, Lg = 0.25 mm. From top Vgs =1 V at −1 V step − 1.
at the AlGaN/GaN interface make the AlGaN/GaN HFET approach more attractive for high frequency high power device applications. At the several W mm − 1 power density already demonstrated on AlGaN/ GaN based HFETs [10,11] the low thermal conductivity (0.5 W cm − 1 K [1]) of the sapphire presents a serious challenge for high power device packaging. While flip-chip approach that dissipates the heat generated by the device from the top surface can alleviate this problem to some extent, long term reliability may be compromised due to thermally induced stress on the contacting pads which also serve as the heat conducting path. SiC has a high thermal conductivity (4.9 W cm − 1 K for 6H SiC) and is closely lattice matched to GaN. AlGaN/GaN high electron mobility transistors (HEMTs) have been reported [14] using SiC as the substrate with a transconductance of 70 mS mm − 1 and ft and fmax of 6 and 11 GHz, respectively. It has also been shown that the 2-dimensional electron gas (2DEG) transport property from the AlGaN/GaN heterostructures grown on SiC is comparable to or even better than those grown on sapphire substrates [15] in terms of the electron mobility and the sheet carrier density-times-mobility (nsm) product. Better HFET performance should be expected. In this section, we present the DC and RF characteristics of the greatly improved high transconductance HFETs fabricated with AlGaN/GaN grown on both n- and p-type SiC substrates. The layer structures used in this work consisted of a 0.15 mm AlN buffer on either n- and p-type c-plane SiC. This was followed by a nominally 1 mm semi-insulating GaN, 50 nm of n-GaN (n =5 −7 × 1017 cm − 3), 3 nm undoped Al0.2Ga0.8N spacer, and 30 nm of doped Al0.2Ga0.8N. Hall measurement resulted in room temperature electron mobility and sheet carrier concentra-
tion of 2019 cm2 V − 1 s − 1 and 1.3×1013 cm − 2 on the n-type SiC substrate and 1904 cm2 V − 1 s − 1 and 9.8× 1012 cm − 2 on the p-type SiC substrates. The HFETs had nominal source to drain separation of 2 mm and gate length of 0.25 mm. For device on the n-SiC, a gate width of 45 mm was used. Helium (He) ion implantation was used for isolation. For devices on the p-SiC, a gate width of 100 mm was used. Since the p-SiC is much more resistive than the n-SiC, both RIE etching and ion implantation isolation was experimented. A Ti/Al metal scheme was used for the Ohmic contacts and a Ni/Au bi-layer was used for the gate Schottky contacts. Fig. 2a shows the drain current–voltage (I –V) characteristics of a 0.25 mm-gate HFET with 2 mm S-D spacing and an offset gate that is 0.6 mm from the source. The device exhibited complete pinch-off at Vgs = − 9 V in spite of the highly conducting n-SiC substrate. Similar drain I–V characteristics was also displayed by the devices fabricated on the p-SiC substrate as is shown in Fig. 2b for a 0.25 mm-long and centered gate HFET with 2 mm S-D spacing. The lower pinch-off voltage (− 8 V) is consistent with the lower nsm product of the layer structure. An important feature to note from both I–V curves is the nearly absence of the current drop with increasing drain bias voltage at high current and high voltage levels that is often evident for HFETs grown on sapphire substrates [7,8]. A possible explanation for this is the thermal modulation of the channel electron drift velocity which decreases at high power dissipation level. The large thermal conductivity of the SiC substrates has alleviated such problem. The drain current and the transconductance as a function of the gate bias voltage are plotted in Fig. 3a and Fig. 3b for HFETs grown on n- and p-type SiC, respectively. The HFET on the n-SiC reached a maxi-
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Fig. 3. Transfer characteristics of HFETs on n-SiC with offset gate (solid line) and symmetric gate (dotted line), at Vsd =8 V. (b) Transfer characteristics of HFETs on p-SiC at Vsd = 8 V.
mum drain current (Imax) of 1.71 A mm − 1 at a gate bias of 1 V. To our knowledge, this is the highest value ever reported for AlGaN/GaN base HFETs. The Imax of the HFETs on p-SiC is also a high value of 1.43 A mm − 1. The DC transconductance for these devices were 222 and 229 mS mm − 1 for the HFETs on n-SiC and p-SiC, respectively. These values are also the highest reported for HFETs on SiC substrates. From the gate to drain I – V characteristics for the HFET fabricated on the n-SiC, we estimate a drain breakdown voltage about 30 V under fully pinched off condition. The maximum power dissipation for the device is confined by the ID,max and the VD,max. In a Class A application, it can be estimated as :1/8 × (ID,max × VD,max). Our data thus show that the power density achievable by these HFETs on SiC substrate is as high as 6.4 W mm − 1. The small signal RF performance of these devices was investigated on-wafer by the standard S-parameter measurement using an HP 8510B network analyzer. Due to the close proximity of a conducting plane to the HFET devices, the attempt to perform the S-parameter measurement with the HFETs on the n-SiC was futile. For the HFETs on the less conductive p-SiC, a padscorrection can be used to yield the current gain (h21) and maximum available gain as a function frequency as shown in Fig. 4. An ft and fmax as high as 53 and 58 GHz were obtained at a drain bias of 8 V and 0.4 A mm − 1. For high power microwave application, short channel HFETs are to be operated at relatively high voltage. It is relevant to investigate the RF performance of the HFETs under different drain bias. Shown in Fig. 5 is the ft and fmax as a function of drain bias voltage for a fixed drain bias current of 500 mA mm − 1. As seen, the ft only decreased slightly upon further increase in the drain voltage past the optimal biasing point. The ft is related to the effective saturated electron drift velocity
by ft = Veff/2pLg (or conversely, to the transit time of the carrier passing across the gate). In the GaN, the peak saturation velocity occurs at higher field and drops less dramatically than that in the GaAs. Good thermal conduction on the SiC substrate will allow for a lower operating junction temperature of the HFETs. It is expect that the ft of the HFETs on SiC will have less degradation upon quiescent bias point change than that of the HFETs on sapphire substrates.
4. Metal–insulator–semiconductor FETs on AlGaN/GaN In some applications, however, extremely low gate leakage is desirable. This will favor the insulated-gate rather than a Schottky barrier gate as used in the HFETs published to date. An added advantage of the metal-insulator-semiconductor FETs (MISFETs) is the thermal stability of the gate characteristics at high
Fig. 4. Current gain and maximum available gain as a function of frequency.
Q. Chen et al. / Materials Science and Engineering B59 (1999) 395–400
Fig. 5. The ft and fmax versus drain bias voltage at a fixed drain current of 500 mA mm − 1.
temperature. The difficulty in obtaining high quality insulator in the present and other III – V semiconductors at large has prevented this device concept being implemented successfully. Binari et al. [16] reported on the fabricate MISFETs directly on GaN which resulted in a transconductance of Gm =16 mS mm − 1. In this section, we report on high transconductance MISFETs fabricated on AlGaN/GaN heterostructures. The layer structure used in this work consisted of a 1 mm semi-insulating GaN on a sapphire substrate. This is followed by a 50 nm of n-GaN (n =5 −7 ×1017 cm − 3), 3 nm undoped Al0.2Ga0.8N spacer, and 30 nm of doped Al0.2Ga0.8N. The doping level in the AlGaN had resulted in a measured sheet carrier density-timesmobility product (nsm) of 0.7×1016 V − 1-s. The MISFETs had nominal source-drain separation of 3 mm and gate width of 150 mm. Ti/Al metal scheme was used for the Ohmic contacts and the contacts were annealed at 750°C for 2 min. The isolation between devices was achieved by etching down to the i-GaN layer. SiO2 was used as the gate insulator. The gates of
Fig. 6. Drain current–voltage characteristics of MISFET with 3 mm source-drain spacing. From top to bottom, the gate bias was 5 V at −1 V step − 1.
399
Fig. 7. Transfer characteristics of a MISFET under Vsd of 8 V. A transconductance value as high as 86 mS mm − 1 was observed at Vgs =0.5 V.
the MISFETs covered the entire source-drain opening and overlapped with both the source and the drain by 0.25 mm, which made the effective gate length of the devices to be 3 mm. Fig. 6 shows the drain current–voltage (I–V) characteristics. From top to bottom, the gate biases were 5 V at − 1 V step − 1. As seen the MISFET can be fully pinched off at about −7 V of gate bias. The benefit of an insulating gate is evidenced by equally good transistor characteristics at large positive gate bias (up to +6 V). A high transconductance at large positive gate bias is shown more clearly in the transfer characteristics of the MISFET (Fig. 7). The maximum transconductance Gm = 86 mS mm − 1 was obtained at a gate bias of + 0.5 V. To our knowledge, this is the highest Gm value reported in the literature for a MISFET fabricated on GaN based materials. This dramatic improvement in
Fig. 8. Gate leakage current as a function of Vgs for a 3 mm-long and 150 mm-wide gate
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the DC transconductance of the MISFET is a result of the superior lateral transport property of the AlGaN/ GaN heterostructure as compared to that of the bulk GaN. It can also be seen from the Fig. 7 that the drain current density reaches a value of 640 mA mm − 1 at a gate bias of +6 V. these results demonstrate the potential of these MISFETs for high power operation. The gate electrical characteristics is shown in Fig. 8. In the scale as shown (1 mA div − 1), the gate leakage appears flat within 9 6 V. The gate leakage current is measured to be about 0.1 nA within 92 V. This is much lower than the 0.2 mA value reported in [16]. The RF performance of these MISFETs was also investigated on-wafer. The cutoff frequency ( ft) and the maximum frequency of oscillation ( fmax) were measured to be 2.9 and 7.1 GHz, respectively. It is obvious that the overlap between the gate and the drain (and source) plays a major role in limiting the ft and fmax since the overlap at the drain side serves as a large negative feedback capacitance that reduces the RF gain at high frequency. A different device layout is necessary to optimize the RF performance.
5. Conclusions The use of SiC substrates has brought great improvement in the device performance of HFETs based on AlGaN/GaN heterostructures. With a maximum drain current as high as 1.71 A mm − 1, Gm \200 mS mm − 1, drain-gate breakdown better than 30 V, ft and fmax better than 50 GHz, these data further establish the viability of the AlGaN/GaN based HFETs for high power and high frequency device applications. We have also demonstrated for the first time MISFETs employing AlGaN/GaN heterostructure as the active channel layer and SiO2 as the gate insulator. Transconductance as high as 86 mS mm − 1 was obtained with a permissible gate operating voltage range of 9 6 V.
.
Acknowledgements The work presented here was partially supported by BMDO under the US Army Contract No. DASG60-96C-0009 and has been partially supported by the US Air Force (WPAFB-AADO) under contract number F33615-96-C-1926.
References [1] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (3) (1994) 1363. [2] M.A. Khan, J.N. Kuznia, A.R. Bhattarai, D.T. Olson, Appl. Phys. Lett. 62 (15) (1993) 1786. [3] M.A. Khan, J.M. Van Hove, J.N. Kuznia, D.T. Olson, Appl. Phys. Lett. 58 (21) (1991) 2408. [4] M.A. Khan, A. Bhattarai, J.N. Kuznia, D.T. Olson, Appl. Phys. Lett. 63 (9) (1993) 1214. [5] M.A. Khan, J.N. Kuznia, D.T. Olson, W.J. Schaff, J.W. Burn, M.S. Shur, Appl. Phys. Lett. 65 (9) (1994) 1121. [6] Q. Chen, M.A. Khan, J.W. Yang, C.J. Sun, M. Shur, H. Park, Appl. Phys. Lett. 69 (6) (1996) 794. [7] S.N. Mohammad, Z.-F. Fan, A. Salvador, O. Aktas, A.E. Botchkarev, W. Kim, H. Morkoc, Appl. Phys. Lett. 69 (10) (1996) 1420. [8] Y.-F. Wu, B.P. Keller, S. Keller, D. Kapolnek, P. Kozodoy, S.P. Denbaars, U.K. Mishra, Appl. Phys. Lett. 69 (10) (1996) 1438. [9] Q. Chen, R. Gaska, M. Asif Khan, et al., Electron. Lett. 33 (7) (1997) 637. [10] Q. Chen, J.W. Yang, R. Gaska, et al., IEEE Electron Device Lett. 19 (2) (1998) 44. [11] Y.-F. Wu, B.P. Keller, S. Keller, D. Kapolnek, P. Kozodoy, S.P. DenBaars, U.K. Mishira, Electron. Lett. 33 (1997) 1742. [12] M.A. Khan, Q. Chen, C.J. Sun, M. Shur, B. Gelmont, Appl. Phys. Lett. 67 (10) (1995) 1429. [13] W. Knap, S. Contreras, H. Alause, et al., Appl. Phys. Lett. 70 (16) (1997) 2123. [14] S.C. Binari, J.M. Redwing, G. Kelner, W. Krupta, Electron. Lett. 33 (3) (1997) 242. [15] C.F. Lin, H.C. Cheng, J.A. Huang, M.S. Feng, J.D. Guo, G.C. Chi, Appl. Phys. Lett. 70 (19) (1997) 2583. [16] S. Binari, K. Doverspike, G. Kelner, H.B. Dietrich, A.E. Wickenden, Solid-State Electron. 41 (2) (1997) 177.
Microwave electronics device applications of AlGaN/GaN heterostructures Q. Chen a,*, J.W. Yang a,1, M. Blasingame a, C. Faber a, A.T. Ping b, I. Adesida b b
a APA Optics, 2950 NE 84 th Lane, Blaine, MN 55449, USA Microelectronics Laboratory, Uni6ersity of Illinois, Urbana, IL 61801, USA
Abstract One thrust in the recent AlGaN/GaN based HFET development hinges on the use of SiC substrates for the growth of the AlGaN/GaN heterostructures. We have achieved Gm and maximum drain current (Imax) as high as 222 mS mm − 1 and 1.71 A mm − 1 for HFETs grown on n-SiC. The HFETs on p-SiC have also shown Gm and Imax of 230 mS mm − 1 and 1.43 A mm − 1. These devices exhibited cut-off frequency ( ft) and frequency of oscillation ( fmax) of 55 and 56 GHz for HFETs on p-SiC, further demonstrating the applicability of AlGaN/GaN-based HFETs in high power microwave frequency range. The availability of high quality AlGaN/GaN heterostructure has also permitted the implementation of such new device concept as metal – insulator–semiconductor FETs (MISFETs). Our MISFETs have shown low gate leakage in 96 V gate bias range with Gm as high as 86 mS mm − 1. © 1999 Elsevier Science S.A. All rights reserved. Keywords: HFET; Devices; MISFETs
1. Introduction With a direct band gap, GaN and related compounds (InGaN and AlGaN) were recognized earlier for their applications in light emitters covering the ultraviolet (UV) and visible spectral range. An examination of the basic material properties and several composite material figures of merits of semiconductors, such as shown in Table 1, brings out the fact that the GaN-related compounds possess excellent property for high power (to some extent related to high temperature) electronic device applications. A straightforward electronic device application of GaN material is the fabrication of metal semiconductor field effect transistors (MESFETs) [2]. In such MESFETs, the Schottky type gates are formed directly on the GaN channel. While a highly doped channel with a tight profile is desirable for high transconductance (Gm) of the MESFET, the formation of Schottky junction often requires a lightly doped channel. This compro* Corresponding author. Tel.: +1-612-7844995; fax: + 1-6127842038. 1 Currently with E and CE Department, University of South Carolina, Columbia, SC 29208, USA.
mise leads to thicker and lightly doped GaN channel. As a result, the MESFETs showed a low Gm of 23 mS mm − 1 which is to be compared to values of Gm \200 mS mm − 1 readily achievable in GaAs based MESFETs. In addition, the low electron mobility in the bulk GaN channel (350 cm2 V − 1 s − 1 typical) will give rise to a high series resistance, limiting DC and RF performance of the MESFETs. Electron mobility enhancement at AlGaN/GaN hetero-interface was first reported in 1991 [3]. Heterostructure field effect transistors (HFETs) were soon fabricated using such AlGaN/GaN heterostructures [4]. Despite the electron mobility enhancement, these early HFETs exhibited a DC Gm of 28 mS mm − 1, similar to that of the MESFETs. It must be noted, however, that the excellent potential of these AlGaN/GaN-based HFETs for microwave electronics application was illustrated by the early short gate (0.25 mm) HFETs which exhibited a cut-off frequency ( ft) and maximum frequency of oscillation ( fmax) of 11 and 35 GHz, respectively [5]. Drastic improvement in DC and RF performance of the AlGaN/GaN-based HFETs came several years later after further advancement in material quality, heterostructure design, and Ohmic contact formation.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 5 6 - 0
Q. Chen et al. / Materials Science and Engineering B59 (1999) 395–400
396
Table 1 Material properties and figure-of-merit of semiconductors for power electronic devices, after [1], otherwise noted Materials properties
GaN
SiC-3C (SiC-6H)
Diamond
Band gap (eV, 300 K) Electron mobility (cm2 V−1 s−1, 300 K) Saturation velocity (107 cm s−1) Thermal conductivity (W cm−1 deg−1) Breakdown field (106 V cm−1) EBVs/2p (V s−1), Johson’s st (CVs/4po)1/2 (W deg−1 s−1), Keyes’
3.39 900 2.7 1.3 5? 1.6×1013 12×107 on GaN 30–40×107 on SiC
2.2 (2.9) 1000 (600) 2 5 3 (4) 1.0×1013a (1.3×1013) 35×107 (39×107)
5.5 2200 2 20 10 3.2×1013a 190×107a
a
Calculated using the values available in [1].
These include the demonstration of HFETs with Gm of 120 mS mm − 1 and maximum drain current (Imax) of 350 mA mm − 1 [6]. This was soon followed by reports of HFETs with Gm of 210 mS mm − 1 [7], drain-gate breakdown voltage of 230 V [8] and maximum drain current of 1.02 A mm − 1 [9]. The excellent power capability of AlGaN/GaN HFETs in microwave frequency has recently been demonstrated by two groups with maximum power density of 1.7 W mm − 1 at 8.4 GHz [10] and 2.7 W mm at 10 GHz [11]. In this paper, we shall first deal with material aspect of the AlGaN/GaN heterostructures. The focus will then be turned to the recent development in the fabrication of HFETs grown on SiC substrate based on the work carried out at APA Optics. This approach takes advantage of the desirable transport properties of the two dimensional electron gas available at the AlGaN/ GaN hetero-interface and high thermal conductivity of the SiC substrates. This will greatly increase the power handling capability of GaN base HFETs in microwave frequencies. In order to increase high temperature stability of the gate in an HFET, we have also experimented with metal – insulator – semiconductor FET device concept using AlGaN/GaN heterostructures. These new results will also be discussed.
2. Alx Ga1 − x N/GaN heterostructures The basic heterostructure of consideration here is shown schematically in Fig. 1. It consists of a thick ( \ 1 mm) of GaN followed by a thin layer (500–1000 ˚ ) of lightly Si-doped GaN (n in the order of 3× 1017 A
cm − 3). The heterostructure is completed with the deposition of a layer of Alx Ga1 − x N. The substrates widely used are sapphire. SiC substrates have been used for this work. Compared to AlAs/GaAs heterostructure, the [in-plane, (0001)] lattice mismatch between AlN and GaN ( : 2.4%) is relatively large. Hetero-epitaxy of thick AlGaN on GaN or vice versa will result in visible cracks due to strain relaxation. when the capping Alx Ga1 − x N layer with x values up to 0.5 is made thin ˚ ), the overall structure shows n-type (below about 500 A conductivity with a typical room temperature electron mobility over 1000 cm2 V − 1 s − 1 and sheet carrier density of 1× 1013 cm − 2. Part of the high sheet carrier density comes from piezo-electric effect. The evidence that this mobility enhancement is due to the formation of a two dimensional electron gas (2DEG) are the following. First, the electron mobility increases monotonously as much as four fold from room temperature to 77 K while for a bulk GaN the mobility decreases after peaking due to ionized impurity scattering which is much lower in the 2DEG [12]. Second, we have observed Shubnikov-de-Hass (SdH) oscillation from these heterostructures [13]. The cyclotron resonance frequency is proportional to the magnetic field component that is perpendicular to the plane of the interface. This confirms the 2-dimensional character of the carriers. Other improvement in the transport property in 2DEG structures include the insertion of an undoped AlGaN spacer layer at the immediate interface [6] and the use of modulation doping [8]. The former permits higher doped sheet carrier density while keeping the electron mobility relatively high. The latter included an undoped AlGaN cap layer which will facilitate the formation of high quality Schottky barrier gates.
3. HFETs on SiC substrates
Fig. 1. Schematic of the basic AlGaN/GaN heterostructures for HFET fabrication.
From the Table 1 it can be seen that SiC is a prospective candidate for how power electronic device applications. However, the higher saturated electron drift velocity in GaN and the availability of the 2DEG
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Fig. 2. Drain I– V characteristics of HFETs on n-SiC. The device dimensions were Lsd =2 mm, Lg =0.25 mm with offset 0.6 mm from the source. From top Vgs =1 V at − 1 V step − 1. (a) Drain I –V characteristics of HFETs on n-SiC. The device dimensions were Lsd =2 mm, Lg = 0.25 mm with offset 0.6 mm from the source. From top Vgs = 1 V at −1 V step − 1. (b) Drain I – V characteristics of HFETs on p-SiC. The device dimensions were Lsd =2 mm, Lg = 0.25 mm. From top Vgs =1 V at −1 V step − 1.
at the AlGaN/GaN interface make the AlGaN/GaN HFET approach more attractive for high frequency high power device applications. At the several W mm − 1 power density already demonstrated on AlGaN/ GaN based HFETs [10,11] the low thermal conductivity (0.5 W cm − 1 K [1]) of the sapphire presents a serious challenge for high power device packaging. While flip-chip approach that dissipates the heat generated by the device from the top surface can alleviate this problem to some extent, long term reliability may be compromised due to thermally induced stress on the contacting pads which also serve as the heat conducting path. SiC has a high thermal conductivity (4.9 W cm − 1 K for 6H SiC) and is closely lattice matched to GaN. AlGaN/GaN high electron mobility transistors (HEMTs) have been reported [14] using SiC as the substrate with a transconductance of 70 mS mm − 1 and ft and fmax of 6 and 11 GHz, respectively. It has also been shown that the 2-dimensional electron gas (2DEG) transport property from the AlGaN/GaN heterostructures grown on SiC is comparable to or even better than those grown on sapphire substrates [15] in terms of the electron mobility and the sheet carrier density-times-mobility (nsm) product. Better HFET performance should be expected. In this section, we present the DC and RF characteristics of the greatly improved high transconductance HFETs fabricated with AlGaN/GaN grown on both n- and p-type SiC substrates. The layer structures used in this work consisted of a 0.15 mm AlN buffer on either n- and p-type c-plane SiC. This was followed by a nominally 1 mm semi-insulating GaN, 50 nm of n-GaN (n =5 −7 × 1017 cm − 3), 3 nm undoped Al0.2Ga0.8N spacer, and 30 nm of doped Al0.2Ga0.8N. Hall measurement resulted in room temperature electron mobility and sheet carrier concentra-
tion of 2019 cm2 V − 1 s − 1 and 1.3×1013 cm − 2 on the n-type SiC substrate and 1904 cm2 V − 1 s − 1 and 9.8× 1012 cm − 2 on the p-type SiC substrates. The HFETs had nominal source to drain separation of 2 mm and gate length of 0.25 mm. For device on the n-SiC, a gate width of 45 mm was used. Helium (He) ion implantation was used for isolation. For devices on the p-SiC, a gate width of 100 mm was used. Since the p-SiC is much more resistive than the n-SiC, both RIE etching and ion implantation isolation was experimented. A Ti/Al metal scheme was used for the Ohmic contacts and a Ni/Au bi-layer was used for the gate Schottky contacts. Fig. 2a shows the drain current–voltage (I –V) characteristics of a 0.25 mm-gate HFET with 2 mm S-D spacing and an offset gate that is 0.6 mm from the source. The device exhibited complete pinch-off at Vgs = − 9 V in spite of the highly conducting n-SiC substrate. Similar drain I–V characteristics was also displayed by the devices fabricated on the p-SiC substrate as is shown in Fig. 2b for a 0.25 mm-long and centered gate HFET with 2 mm S-D spacing. The lower pinch-off voltage (− 8 V) is consistent with the lower nsm product of the layer structure. An important feature to note from both I–V curves is the nearly absence of the current drop with increasing drain bias voltage at high current and high voltage levels that is often evident for HFETs grown on sapphire substrates [7,8]. A possible explanation for this is the thermal modulation of the channel electron drift velocity which decreases at high power dissipation level. The large thermal conductivity of the SiC substrates has alleviated such problem. The drain current and the transconductance as a function of the gate bias voltage are plotted in Fig. 3a and Fig. 3b for HFETs grown on n- and p-type SiC, respectively. The HFET on the n-SiC reached a maxi-
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Fig. 3. Transfer characteristics of HFETs on n-SiC with offset gate (solid line) and symmetric gate (dotted line), at Vsd =8 V. (b) Transfer characteristics of HFETs on p-SiC at Vsd = 8 V.
mum drain current (Imax) of 1.71 A mm − 1 at a gate bias of 1 V. To our knowledge, this is the highest value ever reported for AlGaN/GaN base HFETs. The Imax of the HFETs on p-SiC is also a high value of 1.43 A mm − 1. The DC transconductance for these devices were 222 and 229 mS mm − 1 for the HFETs on n-SiC and p-SiC, respectively. These values are also the highest reported for HFETs on SiC substrates. From the gate to drain I – V characteristics for the HFET fabricated on the n-SiC, we estimate a drain breakdown voltage about 30 V under fully pinched off condition. The maximum power dissipation for the device is confined by the ID,max and the VD,max. In a Class A application, it can be estimated as :1/8 × (ID,max × VD,max). Our data thus show that the power density achievable by these HFETs on SiC substrate is as high as 6.4 W mm − 1. The small signal RF performance of these devices was investigated on-wafer by the standard S-parameter measurement using an HP 8510B network analyzer. Due to the close proximity of a conducting plane to the HFET devices, the attempt to perform the S-parameter measurement with the HFETs on the n-SiC was futile. For the HFETs on the less conductive p-SiC, a padscorrection can be used to yield the current gain (h21) and maximum available gain as a function frequency as shown in Fig. 4. An ft and fmax as high as 53 and 58 GHz were obtained at a drain bias of 8 V and 0.4 A mm − 1. For high power microwave application, short channel HFETs are to be operated at relatively high voltage. It is relevant to investigate the RF performance of the HFETs under different drain bias. Shown in Fig. 5 is the ft and fmax as a function of drain bias voltage for a fixed drain bias current of 500 mA mm − 1. As seen, the ft only decreased slightly upon further increase in the drain voltage past the optimal biasing point. The ft is related to the effective saturated electron drift velocity
by ft = Veff/2pLg (or conversely, to the transit time of the carrier passing across the gate). In the GaN, the peak saturation velocity occurs at higher field and drops less dramatically than that in the GaAs. Good thermal conduction on the SiC substrate will allow for a lower operating junction temperature of the HFETs. It is expect that the ft of the HFETs on SiC will have less degradation upon quiescent bias point change than that of the HFETs on sapphire substrates.
4. Metal–insulator–semiconductor FETs on AlGaN/GaN In some applications, however, extremely low gate leakage is desirable. This will favor the insulated-gate rather than a Schottky barrier gate as used in the HFETs published to date. An added advantage of the metal-insulator-semiconductor FETs (MISFETs) is the thermal stability of the gate characteristics at high
Fig. 4. Current gain and maximum available gain as a function of frequency.
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Fig. 5. The ft and fmax versus drain bias voltage at a fixed drain current of 500 mA mm − 1.
temperature. The difficulty in obtaining high quality insulator in the present and other III – V semiconductors at large has prevented this device concept being implemented successfully. Binari et al. [16] reported on the fabricate MISFETs directly on GaN which resulted in a transconductance of Gm =16 mS mm − 1. In this section, we report on high transconductance MISFETs fabricated on AlGaN/GaN heterostructures. The layer structure used in this work consisted of a 1 mm semi-insulating GaN on a sapphire substrate. This is followed by a 50 nm of n-GaN (n =5 −7 ×1017 cm − 3), 3 nm undoped Al0.2Ga0.8N spacer, and 30 nm of doped Al0.2Ga0.8N. The doping level in the AlGaN had resulted in a measured sheet carrier density-timesmobility product (nsm) of 0.7×1016 V − 1-s. The MISFETs had nominal source-drain separation of 3 mm and gate width of 150 mm. Ti/Al metal scheme was used for the Ohmic contacts and the contacts were annealed at 750°C for 2 min. The isolation between devices was achieved by etching down to the i-GaN layer. SiO2 was used as the gate insulator. The gates of
Fig. 6. Drain current–voltage characteristics of MISFET with 3 mm source-drain spacing. From top to bottom, the gate bias was 5 V at −1 V step − 1.
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Fig. 7. Transfer characteristics of a MISFET under Vsd of 8 V. A transconductance value as high as 86 mS mm − 1 was observed at Vgs =0.5 V.
the MISFETs covered the entire source-drain opening and overlapped with both the source and the drain by 0.25 mm, which made the effective gate length of the devices to be 3 mm. Fig. 6 shows the drain current–voltage (I–V) characteristics. From top to bottom, the gate biases were 5 V at − 1 V step − 1. As seen the MISFET can be fully pinched off at about −7 V of gate bias. The benefit of an insulating gate is evidenced by equally good transistor characteristics at large positive gate bias (up to +6 V). A high transconductance at large positive gate bias is shown more clearly in the transfer characteristics of the MISFET (Fig. 7). The maximum transconductance Gm = 86 mS mm − 1 was obtained at a gate bias of + 0.5 V. To our knowledge, this is the highest Gm value reported in the literature for a MISFET fabricated on GaN based materials. This dramatic improvement in
Fig. 8. Gate leakage current as a function of Vgs for a 3 mm-long and 150 mm-wide gate
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the DC transconductance of the MISFET is a result of the superior lateral transport property of the AlGaN/ GaN heterostructure as compared to that of the bulk GaN. It can also be seen from the Fig. 7 that the drain current density reaches a value of 640 mA mm − 1 at a gate bias of +6 V. these results demonstrate the potential of these MISFETs for high power operation. The gate electrical characteristics is shown in Fig. 8. In the scale as shown (1 mA div − 1), the gate leakage appears flat within 9 6 V. The gate leakage current is measured to be about 0.1 nA within 92 V. This is much lower than the 0.2 mA value reported in [16]. The RF performance of these MISFETs was also investigated on-wafer. The cutoff frequency ( ft) and the maximum frequency of oscillation ( fmax) were measured to be 2.9 and 7.1 GHz, respectively. It is obvious that the overlap between the gate and the drain (and source) plays a major role in limiting the ft and fmax since the overlap at the drain side serves as a large negative feedback capacitance that reduces the RF gain at high frequency. A different device layout is necessary to optimize the RF performance.
5. Conclusions The use of SiC substrates has brought great improvement in the device performance of HFETs based on AlGaN/GaN heterostructures. With a maximum drain current as high as 1.71 A mm − 1, Gm \200 mS mm − 1, drain-gate breakdown better than 30 V, ft and fmax better than 50 GHz, these data further establish the viability of the AlGaN/GaN based HFETs for high power and high frequency device applications. We have also demonstrated for the first time MISFETs employing AlGaN/GaN heterostructure as the active channel layer and SiO2 as the gate insulator. Transconductance as high as 86 mS mm − 1 was obtained with a permissible gate operating voltage range of 9 6 V.
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Acknowledgements The work presented here was partially supported by BMDO under the US Army Contract No. DASG60-96C-0009 and has been partially supported by the US Air Force (WPAFB-AADO) under contract number F33615-96-C-1926.
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