Monolithic Integration of Lateral Field-Effect Rectifier with Normally-off HEMT for GaN-on-Si Switch-mode Power Supply Converters Wanjun Chen, King-Yuen Wong, and Kevin J. Chen Dept. of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong * Corresponding author:
[email protected], Tel: (852) 2358-8969, Fax: (852)2358-1485 Abstract A lateral field-effect rectifier (L-FER) that can be fabricated with normally-off transistor on the same AlGaN/GaN HEMT with the same fabrication process has been demonstrated. The L-FER exhibits low turn-on voltage, low specific on-resistance and high reverse breakdown. A prototype of switch-mode dc-dc Boost converter that features monolithically integrated L-FER and normally-off HEMT is demonstrated for the first time using industry-standard GaNon-Si epitaxial wafers to prove the feasibility of GaN power inegrated technology. Introduction AlGaN/GaN high electron mobility transistors (HEMTs) are attractive for high-efficiency power electronics because of their favorable device characteristics including high breakdown voltage, low on-resistance as well as high switching speed [1-2]. Proof-of-concept switch-mode power converters have been demonstrated using a normally-on AlGaN/GaN HEMT switch, and a silicon p-i-n diode or a SiC Schottky barrier diode (SBD) [3-4]. However, for the development of low-cost GaN-based integrated power converters that require both HEMT switches and rectifiers, it is desirable to integrate high-performance power transistors and rectifiers on the same epitaxial wafer with the same fabrication process. Up to now, while major efforts are being made in the development of GaN-based discrete high voltage power transistors and rectifiers, they have not been developed in the context of monolithic integration for power integrated circuits. This is mainly due to the structure incompatibility between HEMT and diodes. In this work, we propose a lateral field-effect rectifier (L-FER) that is compatible with AlGaN/GaN normally-off HEMT [5]. The normally-off transistors are preferred as power switches used in power electronics systems because of their fail-safe nature, reduced circuit complexity, and higher noise margin. By monolithically integrating the L-FER and normally-off HEMT using an industry-standard GaN-on-Si wafer, a prototype of boost converter with 1 MHz switching frequency is demonstrated for the first time to prove the feasibility of GaN integrated switch-mode power converters. Device design and fabrication
L-FER
Normally-off HEMT LGS LG
LD
L
C
A
S
G
AlGaN
Ohmic contact
LGD
AlGaN
D
Schottky contact
Fig. 1. The schematic cross-section of the monolithic integration of lateral field-effect rectifier (L-FER) with a normally-off HEMT. Where, L and LD are length of the Schottky contact region (CF4 plasma treatment region) and the drift length of the L-FER, respectively. LGS, LG and LGD are gate-source distance, gate length and gate-drain distance of HEMT, respectively.
The schematic cross-section of the monolithically integrated L-FER and normally-off HEMT is shown in Fig. 1. The design of L-FER is based on a conventional AlGaN/GaN normally-off HEMT structure [6]. The cathode electrode (C) of the L-FER is made of an electrode in Ohmic contact with the 2DEG, while the anode electrode (A) is made of electrically shorted Schottky contact and ohmic contact. By tying up the Schottky gate and anode together, the forward turn-on voltage of the rectifier is determined by the threshold voltage of the channel instead of the on-voltage of Schottky
(a)
(c)
(b)
(d)
Fig. 2. Schematics showing the main process flows of monolithic integration of lateral field-effect rectifier and normally-off HEMT: (a) Mesa etching; (b) Ohmic contacts with metal layers of Ti/Al/Ni/Au; (c) Definition of L-FER’s anode and HEMT’s gate followed by CF4 plasma treatment with a RF power of 135 W and time of 170 seconds; (d) Schottky contacts for L-FER and HEMT gate with conventional Ni/Au gate metallization.
Authorized licensed use limited to: Hong Kong University of Science and Technology. Downloaded on November 2, 2009 at 23:03 from IEEE Xplore. Restrictions apply.
Current (mA/mm)
200
(a)
LD=15 μm
150 100
470 V
50 0
-500 -400 -300 -200 -100 0 Voltage ( V ) 250 10 (b) 200 10 Vk=0.1 V 150 100
Current (mA/mm)
Current (mA/mm)
3
50 0 -3
1
10
-1
10
-3
10
-5
-3
-2
-1 0 1 Voltage ( V )
2
3
-2 -1 0 1 2 Forward Voltage ( V )
3
Fig. 3. Measured output characteristics of L-FER (a), and an enlarged view for forward bias region (b). The inserted semi-logarithmic scale figure shows Vk, which defined as the anode bias at a forward current of 1 mA/mm.
at higher temperatures as a result of increased phonon scattering. The characteristics of a normally-off AlGaN/GaN HEMT fabricated with the same process run are shown in Fig. 7. The HEMT with a 1.5 μm gate-length and a 12 μm gate-drain distance exhibits a threshold voltage (Vth) of 0.9 V, a maximum drain current (Imax) of 350 mA/mm at VGS=3 V and VDS=10 V, and a peak transconductance (Gm) of 175 mS/mm. The off-state breakdown voltage is 370 V at a drain current leakage of 1 mA/mm and the specific on-resistance (RON, sp) is about 1.34 mΩ·cm2 at VGS=3 V. The DC characteristics of the HEMT at 250 ℃, as plotted in Fig. 8,
500 400
L=2 μm L=1 μm
4 3
300
2
200
1
100 4
0 6 8 10 12 14 16 Drift length LD (μm)
2
The output characteristic of an L-FER is plotted in Fig. 3 (a). The L-FER exhibits the breakdown voltage (BV) of 470 V. An enlarged view for the forward bias of L-FER is shown in Fig. 3 (b). The knee voltage (Vk) of the proposed L-FER is 0.1 V, which is much lower than that of SBDs and p-i-n rectifiers that are typically ~ 1 V. The specific on-resistance (RON,sp) of the L-FER is 2.04 mΩ·cm2, leading to a figure of merit (BV2/RON,sp) of 108 MW· cm-2. The dependence of RON,sp and BV on the drift length are plotted in Fig. 4. Both RON, sp and BV are increased as the drift length increases. Figure 5(a) shows the forward current density of L-FERs with different drift length. The forward turn-on voltages (VF, 2 ON) at a forward current density (JF) 100 A/cm are shown in Fig. 5(b), significantly lower than those reported in the vertical SBDs and p-i-n rectifiers rectifiers [7-10]. The much lower VF, ON is attributed to the turn-on control mechanism of the field-effect action, which is similar to the synchronous rectifier based on power MOSFET, instead of Schottky junction or p-i-n junction. In addition, the high 2DEG density and mobility in the drift region also leads to the low VF, ON in L-FER. The temperature characteristics of L-FERs are shown in Fig. 6. Vk shows little temperature dependence as the temperature rises to 250 ℃ although the current decreases with the increasing temperature. Main reason for the current decrease is that the 2DEG mobility of AlGaN/GaN degrades
L=2 μm
RON,sp (mΩ.cm )
Device characteristics
250
BV ( V )
or p-n junctions. The key fabrication process of the L-FER is the incorporation of fluorine ions under the Schottky contact by CF4 plasma treatment that effectively depletes the 2DEG under the Schottky contact region and then pinches off the conduction path. The sample used in this work is a commercial AlGaN/GaN HEMT wafer grown by MOCVD on (111) silicon substrate. The epitaxial structure includes an undoped GaN buffer, an AlN interface enhancement layer, a ~18 nm AlGaN barrier and a 2 nm GaN cap. The starting wafer features a sheet resistance of 330 ohm/square and a threshold voltage of -2.1 V. The specific contact resistance of ~0.7 Ω.mm is obtained using a standard transfer length method (TLM) procedure. To realize the normally-off operations for both L-FER and HEMT, our previously developed fluorine plasma treatment technique (CF4 plasma treatment) is used. The CF4 plasma treatment and Schottky contact evaporation are self-aligned. The device fabrication (Fig. 2(a)-(d)) started with the mesa isolation etching followed by ohmic contact formation. The Schottky contacts are then defined for both the rectifiers and HEMTs. Prior to the e-beam evaporation of Ni/Au, Schottky contact regions are treated by CF4 plasma at 135 W for 170 seconds in an RIE system. The negatively charged fluorine ions can effectively deplete the 2DEG channel and shifts the channel threshold voltage from -2.1 V to +0.9 V. A post-gate annealing at 400 ºC for 10 minutes was carried out to repair the plasma-induced lattice damages and defects.
Fig. 4. Dependence of specific on-resistance (RON, sp) and breakdown voltage (BV) on the drift length (LD).
Authorized licensed use limited to: Hong Kong University of Science and Technology. Downloaded on November 2, 2009 at 23:03 from IEEE Xplore. Restrictions apply.
LD=10 μm
2 0 -3
-2
-1 0 1 Voltage ( V )
2
0.8
VF,on ( V )
0.6
JF=100 A/cm
(b)
2
0.4 L=1 μm
0.2 0.0 4
L=2 μm 6 8 10 12 14 16 Drift length LD ( μm )
Fig. 5. (a) The current density versus the forward bias, and (b) dependence of forward turn-on voltage (VF, ON) on the LD.
Monolithically integrated Boost converter demonstration
2
Current density (KA/cm )
A Boost converter, a major component for switchmode power supply, was demonstrated using the integrated L-FER/HEMT pair as shown in Fig. 9. In this demonstration, an L-FER (L=1 μm, LD=15 μm and anode width W=1 mm) and a normally-off HEMT (LGS=1.5 μm, LG=1.5, LGD=12 μm and gate length W=2 mm) are integrated on AlGaN/GaN heterostructure using the same process run. The chip size is 0.36 mm2 including an active device area of 0.0625 mm2. In addition, off-chip components including a capacitor of 47 nF, an inductor of 330 μH and a load resistor of 5 kΩ, are used in
1.5 1.2 0.9 0.6 0.3 0.0 -3
o
o
T=RT, 50 C, 100 C o o o 150 C, 200 C, 250 C LD=15 μm L=2 μm
-2
-1 0 1 2 Voltage ( V )
0 3 6 9 -3 -2 -1 0 1 2 3 VDS ( V ) VGS ( V )
3
3
3
40
1.0 V
0 0
Current ( mA/mm)
1
80
1.5 V
90
LD=15 μm
120
2.0 V V =0.9 V th
180
Gm (mS/mm)
3
LD=5 μm
160
VDS=10 V
370 V @ 1mA/mm
2
(b)
LGS=1.5 μm, LG=1.5 μm
1
LGD=12 μm
0 -1 0
100 200 300 Voltage ( V )
400
Fig. 7. DC characteristics of the normally-off HEMT with LGS=1.5 μm, LG=1.5 μm, and LGD=12 μm. (a) on-state characteristics: (left) IDS vs. VDS, (right) transfer characteristics, (b) off-state characteristics.
200 150 100 50 0 0
VGS=3 V V =10 V DS o T=250 C 2.5 V 2.0 V V =0.9 V th 1.5 V 1.0 V
100 75 50 25
Gm (mS/mm)
4 L= 2 μm
2.5 V
270
(a)
200
(a)
VGS=3 V
360 ID (mA/mm)
5
450
ID (mA/mm)
2
Current density (kA/cm )
show the normally-off HEMT with a robust high temperature performance.
0 3 6 9 -3 -2 -1 0 1 2 3 VDS ( V ) VGS ( V )
Fig. 8. DC characteristics of the normally-off HEMT at 250 oC: (left) IDS vs. VDS, (right) transfer characteristics.
this demonstration. Measured waveforms operating at a switching frequency (fsw) of 1 MHz, a duty cycle (D) of 55% and an input voltage (Vin) of 10 V are shown in Fig. 10. An output voltage (Vout) of 21 V and a power efficiency of 84% are obtained. Figure 11 shows the dependence of the power efficiency and output voltage on input voltage. The power efficiency decreases with the input voltage due to an increase of the on-state resistances of HETT (RDS_on) and L-FER (Ron) devices. The main reason for the large on-state resistance is the increased dynamic on-resistance caused by current collapse at higher switching amplitude and high switching
Fig. 6. Temperature dependence of the forward characteristics of an L-FER.
Authorized licensed use limited to: Hong Kong University of Science and Technology. Downloaded on November 2, 2009 at 23:03 from IEEE Xplore. Restrictions apply.
L
+
-
VRec
-
3.0
VRec ( V ) VHEMT ( V ) VGS ( V )
+
Vout
1.5
L-FER
2.5V 0V
+ VHEMT -
0.0
C
Normally-off HEMT
Monolithic Integration
20 10 0 0
-10
Vout ( V )
-20 22
Fig. 9. Demonstrated monolithic GaN-based Boost converter and chip micrograph of the integrated device with interdigital layout, where the drain (D) of normally-off HEMT and the anode (A) of L-FER sharing the same electrode.
frequency, which can be reduced using passivation technique and field-plate technique [4].
Conclusions High-performance lateral field-effect rectifiers and normally-off HEMTs are successfully integrated using conventional GaN-on-Si wafers. A boost converter that features a single active device chip has been demonstrated for proof-of-concept of GaN-based integrated switch-mode power supply. Acknowledgement This work is supported by Hong Kong Research Grant Council through the grant 611706 and Innovation Technology Fund ITS/096/07. References (1)
(2)
W. Saito, Y. Takada, M. Kuraguchi, K. Tsuda, I. Omura, T. Ogura, and H. Ohashi, “High breakdown voltage AlGaN-GaN power-HEMT design and high current density switching behavior,” IEEE Trans. Electron Devices vol. 50, pp. 2528-2531, Dec., 2003. Y. Dora, A. Chakraborty, L. McCarthy, S. Keller, S. P. DenBaars, and U. K. Mishra, “High breakdown voltage achieved on AlGaN/GaN HEMTs with integrated slant field plates,” IEEE Electron Device Lett. Vol. 27, pp. 713-715, Sep., 2006.
20
0
1
2 Time ( μs )
3
4
Fig. 10. Measured voltage waveforms of the fabricated monolithic GaNbased Boost converter operating at the switching frequency (fsw) of 1 MHz, duty cycle (D) of 55% and input voltage (Vin) of 10 V, where, the VGS, VHEMT, VRec and Vout are gate-source voltage of HEMT, drain-source voltage of HEMT, anode-cathode voltage of L-FER and output voltage, respectively. (3)
W. Satio, Y. Takada, M. Kuraguchi, K. Tsuda, I. Omura and T. Ogura, “600 V AlGaN/GaN power-HEMT: design, fabrication and demonstration on high voltage DC-DC converter,” in IEDM tech. Dig., Dec., 2003. (4) W. Saito, T. Nitta, Y. Kakiuchi, Y. Saito, K. Tsuda, I. Omura, M. Yamaguchi, “A 120-W boost converter operation using a high-voltage Gan-HEMT,”IEEE Elec. Dev. Lett., vol. 29, pp. 8-10, Jan., 2008. (5) W. Chen, W. Huang, K. Y. Wong, and K. J. Chen, “High performance AlGaN/GaN lateral field-effect rectifiers compatible with high electron mobuility transistors,”Appl. Phys, Lett., vol. 92, 253501, Jun., 2008 (6) Y. Cai, Y. Zhou, K. M. Lau, and K. J. Chen, “Control of threshold voltage of AlGaN/GaN HEMTs by Fluoride-based plasma treatment: from depletion mode to enhancement mode,” IEEE Trans. Electron Devices vol. 53, pp. 2207-2215, Sep., 2006. (7) Z. Z. Bandić, P. M. Bridger, E. C. Piquette, and T. C. McGill, R. P. Vaudo, V. M. Phanse and J. M. Redwing, “High voltage (450 V) GaN Schottky rectifiers,” Appl. Phys. Lett. Vol. 74, pp. 1266-1268, Mar., 1999. (8) A. P. Zhang, G. T. Dang, F. Ren, H. Cho, K. Lee, S. J. Pearton, J.-I Chyi, T.-E. Nee, C.-M. Lee, and C.-C. Chuo, “Comparison of GaN p-in and Schottky rectifier performance,” IEEE Trans. Electron Devices vol. 48, pp. 407-411, Mar., 2001. (9) J. B. Limb, D. Yoo, J. –H. Ryou, S. –C. Snen, and R. D. Dupuis, “High performance GaN pin rectifiers grown on free-standing GaN substrates” IEE Electronics Lett. vol. 43, pp. 1313-1314, Oct., 2006. (10) S. Yoshida, N. Ikeda, J. Li, T. Wada, and H. Takehara, “Low onvoltage operation AlGaN/GaN schottky barrier diode with a dual Schottky structure” IEICE Trans. Electron. E88-C, pp. 690-693, 2005.
Authorized licensed use limited to: Hong Kong University of Science and Technology. Downloaded on November 2, 2009 at 23:03 from IEEE Xplore. Restrictions apply.
