当前位置:首页 >> 电力/水利 >>

2001-Very-high power density AlGaN


586

IEEE TRANSACTIONS ON ELECTRONIC DEVICES, VOL. 48, NO. 3, MARCH 2001

Very-High Power Density AlGaN/GaN HEMTs
Yi-Feng Wu, Member, IEEE, David Kapolnek, James P. Ibbetson, Primit Parikh, Member, IEEE, Bernd P. Keller, and Umesh K. Mishra, Fellow, IEEE

Abstract—Research work focusing on the enhancement of large-signal current–voltage (I–V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMT’s. 100–150 m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited. 2-mm-wide devices flip-chip mounted on to AlN substrates produced 9.2–9.8 W output power at 8 GHz with 44–47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date. Index Terms—AlGaN, FET, flip-chip, GaN, HEMT, microwave power.

I. INTRODUCTION IDE-BANDGAP AlGaN/GaN high electron mobility transistors (HEMTs) have great promise in power generation at high frequencies, due to the high breakdown field and excellent electron transport properties. These devices are typically grown on either SiC or sapphire substrates. The former provides an excellent thermal conductivity of 3.5 to 4.5 W/cm C, while the latter is available at a lower cost and in larger wafer sizes. For a specific device technology, output power density is a major performance benchmark. This is usually evaluated using small devices with minimum thermal complications. For practical applications, however, the device periphery needs to be scaled up to obtain high total output power. Earlier demonstrations include devices grown on sapphire substrates with power density of 4.6 W/mm and a total power of 7.6 W [1], as well as devices on SiC with power density of 6.9 W/mm and a total power of 9.1 W [2]. In this paper, we present further improvement of the AlGaN/GaN HEMT technology focusing on enhancement of large-signal current–voltage (I–V) capabilities under rf operations. II. DEVICE TECHNOLOGY The devices in our laboratories were grown by metal organic chemical vapor deposition on either semi-insulating SiC substrates or sapphire substrates. The epi-layers consisted of a semiinsulating GaN buffer and a modulation-doped AlGaN layer
Manuscript received May 15, 2000; revised September 25, 2000. This work was primarily supported by the Office of Naval Research. The review of this paper was arranged by Editor U. K. Mishra. F.-Y. Wu, J. P. Ibbetson, P. Parikh, and B. P. Keller are with Cree Lighting Company, Goleta, CA 93117 USA. D. Kapolnek is with Ericsson Datacom Inc., Goleta, CA 93117 USA. U. K. Mishra is with Cree Lighting Company, Goleta, CA 93117 USA, and also with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA. Publisher Item Identifier S 0018-9383(01)01534-9.

W

Fig. 1. I–V characteristics under dc and ac gate drives for (a) an AlGaN/GaN HEMT with severe trapping effect or dispersion, and (b) an AlGaN/GaN HEMT with minimum trapping effect.

to supply charge for the two-dimensional (2-D) gas as well as to offer a Schottky-gate barrier. The basic device fabrication process included ohmic contacts by Ti/Al/Ni/Au [3], mesa isolation by Cl reactive ion etching and gate formation by Ni/Au. Important issues in device development are addressed below. A. Maximizing Channel Current and Breakdown Voltages The output power density of a field-effect-transistor (FET) is directly related to its current density and breakdown voltage. For an AlGaN/GaN HEMT, the maximum channel current is set by the conduction band discontinuity between the AlGaN donor/barrier layer and the GaN channel layer. Device optimization has to do with increasing this discontinuity by increasing

0018–9383/01$10.00 ? 2001 IEEE

WU et al.: AlGaN/GaN HEMTs

587

The AlGaN layers were modulation doped to cm by Si. Typical room temperature charge density and mobility are cm , respectively. 1200 cmV S and B. Reducing the Large-Signal Trapping Effect It has been observed that most previous AlGaN/GaN HEMTs were plagued with a discrepancy, or dispersion, between the I–V characteristics under dc gate drives and ac (or pulsed) gate drives as illustrated in Fig. 1(a) [1]. When measured at dc conditions, the device shows a high channel current and a low knee voltage. Under fast gate drives, however, the available drain current is significantly reduced along with a much higher knee voltage. This directly reduces the output power and efficiency under ac operation. The above phenomenon is caused by traps in the device structure. A power device operates along the load line as indicated in Fig. 1(a). In the off state the device supports a large drain voltage, which leads to a high gate-drain electric field. Electrons can then be excited and captured in the traps between the gate-drain region. The time constant for detrapping is too long for these trapped electrons to follow the ac signal and hence they are not available for conduction in the on state. This results in a lack of free electrons at the drain side of the device and forms a current choke, which both limits the channel current and increases the knee voltage. Potentially, all traps on the AlGaN surface, in the AlGaN layer, at the AlGaN/GaN interface or even in the GaN buffer can cause the above problem. A multitude of efforts have to be made to ensure high-quality epi-growth and low-damage device processing. A well-engineered device shows well-behaved I–V characteristics under ac gate drive as in Fig. 1(b). Reduction of this trapping effect is key to the improvement of device performance. III. DEVICE PERFORMANCE Fig. 2(a) shows the I–V characteristics of a 75- m wide Al Ga N HEMT grown on SiC substrates. The maximum current is greater than 1 A/mm. This compares favorably with the 0.68 A/mm value for the lower-Al-content Al Ga N/GaN HEMTs grown on the same substrates [2]. The ohmic contact resistance was measured to be 0.5–0.6 -mm, substantially lower than the 0.9–1.0 -mm value for the higher-Al-content Al Ga N HEMTs reported earlier [4]. This resulted in a low device on-resistance of 3 -mm. The Schottky gate turn-on is about 1.8 V and gate-drain break80 V. The channel pinch-off down voltage is typically 60 is about 5 V and the transconductance is as high as 230 mS/mm. I–V curves of a device grown on sapphire substrate are shown in Fig. 2(b) for comparison. The characteristics are quite similar except for the apparent negative output resistance at high dc power levels, which is attributed to the poor thermal conductivity of sapphire. Both devices exhibited minimal large-signal trapping effect similar to what is shown in Fig. 1(b). Small-signal current-gain and power-gain cutoff frequencies were measured as 25–30 GHz and 60–100 GHz, respectively, for these 0.5–0.6 m gate devices. The superior gain properties were maintained at high bias voltages well

Fig. 2. I–V characteristics of AlGaN/GaN HEMTs (a) grown on SiC substrates, and (b) grown on sapphire substrates. Device dimension: : m . V start: 1.5 V, step: 1 V.

+

0

0 5 2 75

the Al content of the AlGaN layer. Additional advantages of adopting a high Al-content include a higher breakdown field in the AlGaN and a higher Schottky barrier of the gate diode. These enhance both the voltage handling capability and temperature tolerance of the devices. However, the high Al content should be realized without unwanted compromises such as introduction of deep levels and reduction in the crystal quality of the epi-layers. Although not yet conclusive, research work to date has shown that high-current AlGaN/GaN HEMTs can be grown with Al-contents up to 50% [4]. However, achieving high crystal quality for high Al-content AlGaN layers is still a challenge. Also, difficulty in forming a low-resistance ohmic contact was observed when Al content exceeded 40–45%. Devices covered in this work used an Al-content of 38–40% for optimum overall performance with the present material and device technologies.

588

IEEE TRANSACTIONS ON ELECTRONIC DEVICES, VOL. 48, NO. 3, MARCH 2001

Fig. 3. Power performance of the AlGaN/GaN HEMTs for (a) a 150-m-wide devices grown on SiC substrate and (b) a 100-m-wide device grown on sapphire substrate.

Fig. 4. (a) Schematics of the flip-chip bonded AlGaN/GaN HEMT; (b) I–V characteristics of the 2-mm-wide AlGaN/GaN HEMT showing a maximum current of 1.94 A.

above 30 V for the devices on SiC without thermal complications, which is in agreement with the high electron velocity predicted for this material system [5]. An ATN load-pull system was used for large-signal continuous-wave (CW) characterization at 8 GHz. Fig. 3(a) shows the on-wafer measurement result of a 150- m-wide device on SiC substrate. The device was biased at 38 V and 487 mA/mm and tuned for maximum power. The saturation power is 31.7 dBm, translating into a power density of 9.8 W/mm. The associated power gain and power added efficiency (PAE) are 9.6 dB and 47%, respectively. The power density level is the highest for a field effect transistor (FET) to date and is about ten times greater than GaAs-based FETs in the same frequency band. The devices were also tested in class-AB mode for high efficiency with single-harmonic turning. The bias voltage was 24 V with a quiescent current of 120 mA/mm, which was self-adjusted with increasing input drive to 436 mA/mm. At an input power of 0.63 W/mm, an excellent PAE and power density combination of 60% and 6.5 W/mm was achieved. The same measurements were also performed on devices grown on sapphire substrates for comparison. Fig. 3(b) shows

the power sweep of a 100- m-wide device when biased at 26 V, 427 mA/mm and tuned for maximum power. The saturation power density is 6.5 W/mm along with associated gain of 9.1 dB and PAE of 51%. Due to the poor substrate thermal conductivity, the device was not able to withstand higher bias, which limited its output power. Nonetheless, it demonstrated significant improvement over previous devices on sapphire [1]. When operated in class-AB mode at 20 V, the device achieved 60% PAE with 4.4 W/mm power density. The performance improvement for both AlGaN/GaN HEMTs on SiC and sapphire substrates is attributed to adoption of a high Al content and reduction of the trapping effect, which greatly enhance large-signal current–voltage capabilities under rf operations. Large-periphery devices were also fabricated and flip-chip bonded on to AlN carriers as shown in Fig. 4(a). The Au source bonds serve as low-inductance ground connection as well as paths for heat dissipation. A 2-mm-wide device exhibited a high current level of 1.94 A as seen in Fig. 4(b). The corresponding current density is 0.97 A/mm, indicating excellent current scaling. Due to both the thermal cross talk inherent in a multifinger device and the lower thermal conductivity of AlN (1.8 W/cm C) compared with SiC, a relatively low bias of 22 V was used during the power measurement. Nonetheless, it

WU et al.: AlGaN/GaN HEMTs

589

Fig. 5. Power performance of the 2-mm-wide AlGaN/GaN HEMTs when biased at 22 V, showing an output power level of 9.2–9.8 W with PAE 44–47% at 8 GHz.

Fig. 7. Power performance of the AlGaN/GaN amplifier IC showing a CW output power of 14 W at 8 GHz.

3-mm-wide Al Ga N/GaN HEMTs [2] in all aspects including shear output power, PAE and operation frequency. At higher biases than 22 V, the output power exceeded 10 W. However, both the gain and PAE were significantly reduced, which is attributed to the limited thermal conductivity of the AlN circuit substrate mentioned earlier. Owing to the low input impedance for devices larger than 2-mm wide, it is difficult to realize optimum input matching at X band using passive turners. For this reason, a flip-chip amplifier IC employing a 4-mm device was designed and constructed, where all matching elements were fabricated on the AlN circuit substrate as seen in Fig. 6(a). Small-signal characteristics of the amplifier are shown in Fig. 6(b). The bandwidth is 6–10 GHz with better than 10 dB input return loss. The mid-band linear gain is greater than 9 dB. When biased at 25 V, the amplifier generated 14 W at 8 GHz, as seen in Fig. 7, which sets the highest CW power generation using a GaN-based FET to date. Optimization of both the circuit design and thermal management is needed for improving both the power density and efficiency of large-periphery devices. Reliability studies are also yet to be done before a viable product is introduced. IV. CONCLUSION Development of AlGaN/GaN HEMTs focusing on adopting high Al-content AlGaN layers and reduction of large-signal trapping effect has resulted in significant performance improvement. Devices grown on SiC substrates showed a very-high power density of 9.8 W/mm at 8 GHz, which is the highest for a FET to date and is ten times higher than GaAs-based FETs at the same frequency band. Devices on sapphire also generated 6.5 W/mm in spite of the poor substrate thermal conductivity. Initial large periphery devices were fabricated. A 2-mm-wide device produced 9.2 to 9.8 W at 8 GHz with 44 to 47% PAE. Initial high-power amplifier ICs were also demonstrated with a flip-chip IC scheme. The amplifier using a 4-mm-wide device generated 14 W at 8 GHz. This is the highest CW power level obtained using a GaN-based integrated circuit to date.

Fig. 6. (a) Photograph and (b) small-signal characteristics of the flip-chip power amplifier IC using a 4-mm-wide AlGaN/GaN HEMT.

generated an output power of 9.8 W at 8 GHz with 44% PAE as seen in Fig. 5. The peak PAE is 47% at an output power level of 9.2 W. This 2-mm gate periphery device outperforms previous

590

IEEE TRANSACTIONS ON ELECTRONIC DEVICES, VOL. 48, NO. 3, MARCH 2001

ACKNOWLEDGMENT Y.-F. Wu would like to thank Prof. R.A. York at UCSB for EM simulation and valuable ideas on circuit design. REFERENCES
[1] Y.-F. Wu et al., “GaN-based FET’s for microwave power amplification,” IEICE Trans. Electron., vol. E-82-C, pp. 1895–1905, Nov. 1999. [2] S. T. Sheppard et al., “High power microwave GaN/AlGaN HEMT’s on SiC substrates,” IEEE Electron Device Lett., vol. 20, pp. 61–163, June 1999. [3] Z. Fan et al., “Very-low resistance ohmic contact to n-GaN,” Appl. Phys. Lett., vol. 68, pp. 1672–1674, Mar. 1996. [4] Y.-F. Wu et al., “High Al-content AlGaN/GaN MODFET’s for ultra-high performance,” IEEE Electron Device Lett., vol. 19, pp. 50–53, Feb. 1998. [5] B. Gelmont, K. Kim, and M. Shur, “Monte Carlo simulation of electron transport in gallium nitride,” J. Appl. Phys., vol. 74, pp. 1818–1821, August 1, 1993.

Primit Parikh received the B.Tech. degree in electrical engineering from Indian Institute of Technology, Bombay, India, in June 1993, and the M.S. and Ph.D. degrees in electrical engineering from University of California, Santa Barbara, in 1994 and 1998, respectively. His dissertation topic was oxide based electronics in GaAs, which included record high efficiency operation, at C-band, from a GaAs MESFET at low drain voltages, using the GaAs on insulator technology. He has authored and co-authored 25 technical publications, conference and invited presentations, and two patents pending. He has over seven years experience in semiconductor fabrication technology, high-speed device, circuit design and characterization. He has been with Cree Lighting Company, Goleta, CA, since May 1998, and is currently leading the Cree Lighting Electronics and Microwave program.

Yi-Feng Wu (M’99) received the B.E. degree in engineering thermal physics in 1985 from Tsinghua University, Beijing, China, and the M.S. degree in mechanical engineering and Ph.D. degree in electrical engineering from the University of California, Santa Barbara, in 1994 and 1997, respectively. His Ph.D. dissertation was on microwave power AlGaN/GaN high-mobility-transistors. His experience ranges from traditional thermal engineering to submicrometer thermal imaging, from solid-state devices to microwave amplifiers. He has been with Cree Lighting Company (previously called Witech and then Nitre, Inc.), Goleta, CA, since November 1997, conducting researcher and development on GaN-based devices and microwave circuits. He has authored about 35 technical journal articles and conference presentations in the GaN electronics area.

Bernd P. Keller received the Dipl. degree in chemistry in 1987 and the Ph.D. degree in 1992, both from the University of Leipzig, Germany. His thesis work included the investigation of MOVPE of InP and GaInAs on planar and nonplanar substrates. In 1991, he began scientific research on the MOVPE-growth of AlInAs/InP with alternative precursors. In 1993, he joined the Department of Electrical and Computer Engineering as an Associate Research Engineer and has started research in the growth of GaN and its alloys with indium and aluminum by metal organic chemical vapor deposition (MOCVD). He is now with Cree Lighting Company, Goleta, CA. He has authored about 50 technical journal articles and conference presentations.

David Kapolnek received the B.S. degree from the University of Illinois, Urbana, in 1987, the M.S. degree from the University of California, Berkeley in 1990, and the Ph.D. degree from the University of California, Santa Barbara in 1998. He has contributed to the development of ceramic composite materials, nitride materials and related facilities. He was with Nitre, Inc. from 1996 to 1999, and is currently with Ericsson Datacom Inc., Goleta, CA.

James P. Ibbetson received the B.S. degree in applied physics from the California Institute of Technology in June 1990 and the Ph.D. degree in materials, with an emphasis in solid state, from the University of California, Santa Barbara in March 1997. He has over ten years experience in growth and characterization of III–V semiconductor materials and has authored or co-authored over 40 journal and conference papers on the subject. He is currently with Cree Lighting Company, Goleta, CA, where he is involved with MOCVD growth of III–N materials for electronic and opto-electronic device applications.

Umesh K. Mishra (S’80–M’83–SM’90–F’95) received the B.Tech. degree from the Indian Institute of Technology, Kanpur, India, the M.S. degree from Lehigh University, Bethlehem, PA, and the Ph.D. degree from Cornell University, Ithaca, NY, in 1979, 1980, and 1984, respectively, all in electrical engineering. He has worked in various laboratory and academic institutions, including Hughes Research Laboratories, Malibu, CA, University of Michigan, Ann Arbor, and General Electric, Syracuse, NY, where he has made major contributions to the development of AlInAs–GaInAs HEMTs and HBTs. He is now a Professor in the Department of Electrical and Computer Engineering, University of California, Santa Barbara. His current research interests are in oxide based III–V electronics and III–V nitride electronics and opto-electronics. He has authored or co-authored over 400 papers in technical journals and conferences and holds six patents. Dr. Mishra was a co-recipient of the Hyland Patent Award given by Hughes Aircraft, and the Young Scientist Award presented at the International Symposium on GaAs and Related Compounds.


赞助商链接
相关文章:
更多相关标签: