IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 4, APRIL 2002
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Fast-Switching and Shallow Saturation Bipolar Power Transistors Using Corrugated Base Junctions Chanho Park and Kwyro Lee, Senior Member, IEEE
Abstract—A fast-switching and shallow saturation bipolar power transistor fabrication technology using corrugated base junctions, which does not require additional process steps, is proposed in this paper. Computer simulation shows that less excess minority and majority carriers stored in the base and the collector drift region cause the shallow saturation phenomena of the corrugated base transistors at the conduction stage, and that the corrugated base transistors have lateral built-in electric fields under the base electrode, which accelerate the movement of the minority carriers from the bulk to the surface and promote the recombination of excess electrons and holes in the base region. The turn-off times and the saturation voltages between the collector and the emitter are studied systematically as a function of the base masking oxide widths of the corrugated base region, which agree well with the simulation results. Index Terms—Bipolar transistor, corrugated base junctions, fast-switching, shallow saturation.
I. INTRODUCTION
T
HERE have been more and more needs for high-frequency fast-switching devices in power converting electronic systems such as switch mode power supplies, inverters, and choppers. The switching bipolar power transistors have been one of the most popular switching power devices that are used widely in power converting systems [1]–[3]. But it has been known that the bipolar transistors have a slow turn-off transition time due to the excess majority and minority carriers stored in the base and the collector region, which has limited the application of the device to the fast switching systems. In order to enhance turn-off transition speed, various techniques have been introduced, which are classified into process and design approaches. The typical process approaches include lifetime control methods using such technologies as gold diffusion [4]–[8], platinum diffusion [9], [10], and electron irradiation [10], [11]. The design approaches include Schottky diode or low-loss diode clamping between the base and the collector [12], [13] and new emitter structure design such as two-step emitter and ring emitter transistors [14]–[17]. Even though those techniques have shown tremendous improvements in the switching characteristics, they have not only required a few additional fabrication steps, an additional silicon die area and/or elaborate process steps but also shown serious deterioration in Manuscript received July 3, 2001; revised January 8, 2002. The review of this paper was arranged by Editor M. A. Shibib. C. Park is with PDD Division, Fairchild Semiconductor International, Inc., Puchon, Korea (e-mail:
[email protected]). K. Lee is with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Taejeon, Korea. Publisher Item Identifier S 0018-9383(02)03051-4.
other static characteristics such as the increase of the reverse bias leakage currents and of the on-state saturation voltages. To overcome these drawbacks, we have recently proposed a new bipolar power transistor with corrugated base junctions [18], [19]. The bipolar transistors with corrugated base junctions have been fabricated without any additional process steps and have shown shorter storage times at turn-off stage than the conventional bipolar transistors with parallel plane base junctions. In this paper, we investigate the mechanism for the fast switching and shallow saturation characteristics of the bipolar power transistor using corrugated base junctions through simulation and experiments. We also discuss the relationship between the variations of the base masking oxide widths and electrical characteristics such as saturation voltages and turn-off times, and explain the differences between the measured data and the simulated data for the corrugated base transistors and the conventional base transistors.
II. DEVICE STRUCTURE AND FABRICATION The conventional bipolar junction transistors are doped uniformly along the lateral direction in planar process, so that they have parallel plane junctions except for the junction termination edge region. On the other hand, the corrugated base transistors have many corrugated base junctions below the base electrodes, and the base electrodes contact both to high-doped and low-doped base regions simultaneously. Fig. 1 shows the fabrication steps and the cross sectional structures of the bipolar transistor using corrugated base junctions [18], [19]. We have adopted perforated emitter or hollow emitter [20] as an emitter pattern. To form the corrugated junctions, we leave masking oxide in the surface as shown in the first figure of Fig. 1. The only difference between the corrugated base transistor and the conventional base transistor lies in whether we leave base-masking oxides in the base window or not. The base dopants such as boron diffuse into the bulk laterally as well as vertically. Since the lateral diffusion is about 75 85% of the vertical diffusion, we can form corrugated base junctions properly by adjusting the widths of the base masking oxides. To study the effects of the base masking oxide width qualitatively, we first fixed the base junction depth of 10 m and then raised the width of the base masking oxide from 1 to 7 m in the unit structure. The contact window width is chosen at 10 m so that the base electrodes contact both to the highly doped and to the corrugated base regions at the same time. The
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Fig. 3. Cross-sectional structure of the fabricated transistor using the corrugated base junctions.
total width and the depth of the simulated structures are 40 m and 45 m, respectively. Fig. 2 shows lateral distributions of the net doping concentration from the edge of the emitter to the next edge of the emitter along the line of emitter–base junction for the conventional base transistor and the corrugated base transistors with the base masking oxide widths of 3 m and 5 m. The conventional base transistor corresponds to the case with the base masking oxide width of 0 m. The doping level of the p-base region is about 3 10 cm at the surface except for the corrugated region. Fig. 3 shows the cross sectional structure of the fabricated bipolar transistor, which clearly delineates corrugated base junctions under the base contact region. The fabricated transistors have die area of 3.24 mm and die thickness of 280 m, so that the simulated structures are appropriately scaled down. Fig. 1. Fabrication steps of the bipolar transistor with corrugated base junctions.
III. ELECTRICAL CHARACTERISTICS AND DISCUSSION The Gummel plots obtained through simulation using Atlas of Silvaco International [21] for the proposed structures are shown in Fig. 4(a) and (b). We find the base currents of the corrugated base transistor increase at the high-level injection region more than those of the conventional base transistor. The increases of the base currents of the corrugated base transistor cause steep current gain fall-off at the high current region, which are in good agreement with the measured data as will be shown later. Furthermore, the corrugated base transistors have lateral built-in electric fields under the base electrode region since the corrugated base region has a doping gradient laterally as well as vertically. The magnitude of the electric field can be calculated from the current equations at equilibrium condition. is the sum of the drift current The hole current density density and the diffusion current density. That is (1)
Fig. 2. Lateral distributions of the net doping concentration from the edge of the emitter to the next edge of the emitter along the line of emitter–base junction for the conventional and the corrugated base transistors.
the hole mobility, the hole Here, is the electronic charge, the electric field, and the hole diffusion concentration,
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(a) Fig. 5. Lateral electric field distributions from the edge of the emitter to the next edge of the emitter along the line of emitter–base junction for the 2 V and V conventional and the corrugated base transistors at V 0.75 V.
=
Fig. 6.
=
Basic switching circuit diagram.
(b) Fig. 4. (a) Gummel plots of the conventional and the corrugated base transistors at V 2 V and (b) enlarged Gummel plots for the voltages of the base and emitter from 0.7 V to 0.85 V.
=
coefficient. We take positive direction as the lateral direction from the start point of the base masking oxides. By equating (1) to zero and using the Einstein relationship, we obtain the electric field (2) is Boltzmann’s constant, and is the absolute where temperature. is negative in this case, the direction Since the sign of of the built-in electric field is negative so that the built-in field acts as an accelerating field [22], [23] for the minority carriers. That is, the built-in electric field accelerates the movement of the electrons in the lateral direction from the internal region of the base to the surface base electrode and promotes the recombination of the excess carriers in the base region, which results in the increase of the base currents at the high-level injection region as shown in the Fig. 4(b). Fig. 5 shows the lateral electric field distributions from the edge of the emitter to the next edge of the emitter along the line of emitter–base junction at the collector and emitter voltage of 2 V and the base and emitter
Fig. 7. Electron concentration distributions of the conventional and the corrugated base transistors at the saturation state.
voltage of 0.75 V. It is noted that there is no lateral built-in field in the base region for the conventional base transistor and the magnitude of the lateral built-in field increases as the width of the base masking oxide widens. The basic switching circuit diagram is shown in Fig. 6. Figs. 7 and 8 show the simulated results of the electron concentration distributions below the emitter electrode region and the
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Fig. 8. Simulated turn-off transients for several base-masking oxide widths.
Fig. 10.
Measured switching waveforms at the turn-off stage.
Fig. 9. Turn-off times and the collector–emitter saturation voltages versus base-masking oxide widths.
Fig. 11.
Measured and simulated dc current gains.
switching characteristics at turn-off stage, respectively, when the external switching conditions are identical. We find that the wider the base-masking oxide becomes, the faster switching speed we obtain. It would also be noteworthy that the electron concentration stored in the base and the collector region decreases considerably as the base-masking oxide width increases while the slopes of the electron concentration are identical. These shallow saturation phenomena would be arisen from the larger base spreading resistances from the base electrode to the base–emitter junction in the base region for the corrugated base transistors. Shallow saturation results in the fast switching speed at turn-off stage and the increased saturation voltages at on-state. Fig. 9 shows the simulated turn-off times and the saturation voltages between the collector and the emitter as a function of the base masking oxide width. From the figure we can optimize the widths of the base masking oxides in terms of the power dissipation for each specific application. Fig. 10 shows the measured switching waveforms of the collector currents and the base currents at turn-off stage. The switching conditions are at the collector currents of 1 A, the forward base
currents of 0.2 A and the reverse base currents of 0.2 A. The storage time of the fabricated transistors with corrugated base junctions has shown 1.0 s whereas that of the conventional base transistor 1.4 s. Figs. 11 and 12 show the measured and the simulated dc current gains and the saturation voltages between the collector and the emitter respectively. As discussed earlier, the corrugated base transistor has steeper current gain fall-off at high current region in agreement with Fig. 4. The discrepancies between the measured data and the simulated data at the high current region are due to the differences in the parasitic resistances of the fabricated devices and the simulated structures because the fabricated devices have much thicker collector region. It would be noted that there are no discrepancies in the measured breakdown voltages between the corrugated base transistors and the conventional base transistors, which would be thought that the concentration of the corrugated base region of the fabricated transistors is still high enough to block the extension of the space charges region in the base, and the breakdown voltages might decrease for the case of extremely wide base-masking oxide width.
PARK AND LEE: FAST-SWITCHING AND SHALLOW SATURATION BIPOLAR POWER TRANSISTORS
Fig. 12. Measured and simulated collector–emitter saturation voltages.
IV. CONCLUSION The bipolar transistors using the corrugated base junctions have been proposed, which have not only shallow saturations at conduction state but also lateral built-in electric fields accelerating the recombination of excess electrons and holes in the base region. The turn-off times and the saturation voltages between the collector and the emitter are described as a function of the base-masking oxide widths. The wider the base-masking oxide width becomes, the faster switching speed we obtain at the expense of the saturation voltages. The bipolar power transistors with corrugated base junctions have been fabricated without any additional process steps and have shown much faster turn-off switching characteristics than the conventional bipolar transistors with parallel plane base junctions. The corrugated base transistors would be more suitable for fast switching applications than the conventional base transistors.
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[6] M. Hill, M. Lietz, and R. Sittig, “Diffusion of gold in silicon,” J. Electrochem. Soc., vol. 129, no. 7, pp. 1579–1587, 1982. [7] U. Gosele and F. Morehead, “Diffusion of gold in silicon: A new model,” Appl. Phys. Lett., vol. 38, no. 3, pp. 157–159, 1981. [8] W. M. Bullis, “Properties of gold in silicon,” Solid-State Electron., vol. 9, pp. 143–168, 1966. [9] S. D. Brotherton and J. E. Lowther, “Electron and hole capture at Au and Pt center in silicon,” Phys. Rev. Lett., vol. 44, no. 9, pp. 606–609, 1980. [10] J. Baliga and E. Sun, “Comparison of gold, platinum, and electron irradiation for controlling life time in power rectifiers,” IEEE Trans. Electron Devices, vol. ED-24, pp. 685–688, June 1977. [11] A. O. Evwaraye and B. J. Baliga, “The dominant recombination centers in electron-irradiated semiconductor devices,” J. Electrochem. Soc., vol. 124, pp. 913–916, June 1977. [12] Y. Amemiya, T. Sugeta, and Y. Mizushima, “Novel low-loss and high speed diode utilizing an ideal ohmic contact,” IEEE Trans. Electron Devices, vol. ED-29, pp. 236–243, Feb. 1982. [13] J. Narain, “A novel method of reducing the storage time of transistors,” IEEE Electron Device Lett., vol. EDL-6, pp. 578–579, Nov. 1985. [14] M. S. Adler, K. W. Owyang, B. J. Baliga, and R. A. Kokosa, “The evolution of power device technology,” IEEE Trans. Electron Devices, vol. ED-31, pp. 1570–1591, Nov. 1984. [15] L. Lorenz, “The dynamic behavior of the SIRET and its advantages in the application,” in Proc. Int. Symp. Power Semiconductor Devices, Tokyo, Japan, 1988, pp. 58–67. [16] G. Miller, A. Porst, K. G. Oppermann, and H. Strack, “Turn-off dynamics of a new very fast switching 1000V, 50A bipolar power transistor,” in IEDM Tech. Dig., 1986, pp. 106–109. [17] Y. Nakatani, H. Nakazawa, Y. Nawata, K. Ono, M. Kobayashi, and M. Kohno, “A new ultra-high speed high voltage switching transistor,” in Proc. Powercon7, 1980, pp. J3-1–J3-8. [18] C. H. Park, Y. S. Yoon, D. J. Kim, and K. Lee, “A new high speed switching bipolar power transistor with corrugated base junctions,” in 2000 Solid State Devices and Materials, Sendai, Japan, 2000, pp. 390–391. [19] C. Park and K. Lee, “A new high speed switching bipolar power transistor with corrugated base junctions,” Jpn. J. Appl. Phys., pt. 1, vol. 40, no. 4B, pp. 2717–2720, 2001. [20] V. Sukumar, “Transistors for horizontal deflection in televisions and monitors,” ST Microelectronics, Italy, Applicat. Note, 1999. [21] Atlas User’s Manual. Santa Clara, CA: Silvaco International, 1997. [22] U. Zugelder and D. J. Roulston, “Analytic results for the base region of bipolar transistors based on computer simulations,” Solid-State Electron., vol. 30, no. 9, pp. 895–900, 1987. [23] D. J. Roulston, Bipolar Semiconductor Devices. New York: McGrawHill, 1990, pp. 220–226.
ACKNOWLEDGMENT The authors would like to thank to Dr. D. J. Kim, Y. S. Yoon, and the members in the Bipolar Power Transistor Team, Fairchild Semiconductor International, Inc., for their encouragement and cooperation. REFERENCES [1] B. K. Bose, “Evaluation of modern power semiconductor devices and future trends of converters,” IEEE Trans. Ind. Applicat., vol. 28, pp. 403–413, Mar./Apr. 1992. [2] P. L. Hower, “Power semiconductor devices: An overview,” Proc. IEEE, vol. 76, pp. 335–342, Apr. 1988. [3] M. S. Adler and S. R. Westbrook, “Power semiconductor switching devices—A comparison based on inductive switching,” IEEE Trans. Electron Devices, vol. ED-29, pp. 947–952, June 1982. [4] C. Boit, F. Lau, and R. Sittig, “Gold diffusion in silicon by rapid optical annealing: A new insight into gold and silicon interstitial kinetics,” Appl. Phys. A, vol. 50, pp. 197–205, 1990. [5] G. B. Bronner and J. D. Plummer, “Gettering of gold in silicon: A tool for understanding the properties of silicon interstitials,” J. Appl. Phys., vol. 61, pp. 5286–5298, Dec. 1987.
Chanho Park was born in Taejeon, Korea, in January 2, 1964. He received the B.S. degree in physics from Seoul National University, Seoul, Korea, in 1986, and the M.S. degree in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejeon, in 1996, where he is currently pursuing the Ph.D. degree. He had been working for Samsung Electronics, Inc., Puchon, Korea, in the Power Semiconductor Division from 1986 to 1999. He was a Project Leader in the Research and Development Section and was in charge of the development of new devices and new processes. He developed a lot of high-voltage and high-speed switching power semiconductor devices for motor controls, inverters, electronic ballasts, and switch-mode power supplies. He has been a Principal Engineer in the Power Device Division, Fairchild Semiconductor International, Inc., Puchon, since 1999. He received several patents, including U.S. patents and Japanese patents regarding high-voltage and fast switching power semiconductor design. His current interests lie in the fields of power semiconductor devices, device physics, design of high voltage and high speed switching power semiconductor devices, process architecture integration, RF power devices and systems.
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Kwyro Lee (S’80–M’83–SM’90) received the B.S. degree in electronics engineering from Seoul National University, Seoul, Korea, in 1976, and the M.S. and Ph.D. degrees from the University of Minnesota, Minneapolis, in 1981 and 1983, respectively, where he did many pioneering works for modeling heterojunction field effect transistors. He worked as an Engineering General Manager with Gold Star Semiconductor, Inc., Korea, from 1983 to 1986, where he was responsible for the development of the first polysilicon CMOS products in Korea. He joined the Korea Advanced Institute of Science and Technology (KAIST), Taejeon, Korea, in 1987 as an Assistant Professor in the Department of Electrical Engineering, where he is currently a Professor. His research interests are focused on RF and base-band circuits for mobile multimedia. He is the first author of the book Semiconductor Device Modeling for VLSI (Prentice-Hall: New York, 1993). He has also been working as the Director of the Micro Information and Communication Remote-Object Oriented Systems (MICROS) Research Center since 1997, which is an Engineering Research Center supported by Korea Science and Engineering Foundation. Dr. Lee is a Life Member of KIEE and served as the Chairman of the IEEE Electron Device Society Korea chapter.
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