S ILICON carbide (SiC) has emerged as the most favorable

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1 IEEE TRANSACTIONS ON POWER ELECTRONICS Letters Photonic Compensation of Temperature-Induced Drift of SiC-DMOSFET Switching Dynamics Tirthajyoti Sarkar and Sudip Kumar Mazumder Q Abstract A potential mechanism for high-temperature integration of a recently developed GaAs-based optically triggered power transistor (OTPT) to a SiC DMOSFET for photonic control of power flow is outlined and experimentally demonstrated over a range of switching frequencies and duty cycles. It was found that the switching dynamics of the SiC DMOSFET varies with the case temperature due to a change in the conductance of the GaAs OTPT. This temperature-induced drift in the rise time of the SiC DMOS- FET is compensated by a nonlinear variation in the intensity of the triggering signal of the OTPT. Index Terms Compensation, GaAs optically triggered power transistor (OTPT), high temperature, modulation, photonic, silicon carbide (SiC) DMOSFET, switching dynamics. 18 I. INTRODUCTION S ILICON carbide (SiC) has emerged as the most favorable 20 choice of material for next generation power semiconduc- 21 tor devices (PSDs) because of its superior electrical and thermal 22 properties as compared to silicon [1]. One of the desirable fea- 23 tures of SiC power devices is their high-temperature operating 24 capability [2]. Some defense applications, such as fly-by-light 25 (FBL) program of U.S. Air Force, envision high-ambient tem- 26 perature up to 200 C [3], and therefore, SiC-based power elec- 27 tronics may be a viable option for such applications. Nonethe- 28 less, it is generally difficult to design a gate driver for such high- 29 ambient temperature [4] because the driver has to be in close 30 proximity of the SiC DMOSFET and may experience excessive 31 leakage. For Si-based gate drivers, silicon-on-insulator (SOI) 32 approach has been employed to address this issue [4], [5]. An- 33 other approach could be based on SiC-based driver electronics, 34 which is expected to have lower leakage at elevated tempera- 35 ture due to its low-intrinsic carrier concentration. For instance, 36 SiC-based CMOS power amplifiers have been demonstrated to 37 work over a wide temperature range [6]. Manuscript received September 16, 2009; revised February 10, 2010, and April 5, 2010; accepted May 13, Recommended for publication by Associate Editor K. Sheng. The authors are with the Electrical and Computer Engineering, University of Illinois, Chicago, IL USA ( tsarka1@uic.edu; mazumder@ece.uic.edu). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPEL However, applications such as FBL systems have the addi- 38 tional requirement of transmitting the control signals via optical 39 fiber to achieve high immunity from electromagnetic interfer- 40 ence (EMI). This optical-triggering requirement poses further 41 problem because one of the principal difficulties in realizing di- 42 rect photonic control of a SiC power device is the unavailability 43 of commercial short-wavelength (< 400 nm) semiconductor op- 44 tical source [7] that is required to create photogeneration inside 45 a SiC device. To address this issue, a suitable front-end optical 46 device needs to be developed, which on one hand, is highly 47 responsive to the wavelength range of commercially available 48 lasers, and on the other hand, seamlessly couples with SiC-based 49 power devices to control the switching operation. 50 In this letter, we demonstrate such an approach. It is shown 51 that, by coupling a GaAs-based optically triggered power tran- 52 sistor (OTPT) with a SiC DMOSFET, reliable switching op- 53 eration can be performed at a high temperature using a com- 54 mercially available, long-wavelength ( 800 nm near infrared 55 range) optical source. However, it is also found that the switch- 56 ing properties of a SiC DMOSFET may vary at such a high 57 temperature due to undesirable change in the conductance of 58 the GaAs OTPT. While temperature-induced variation of steady- 59 state properties (e.g., carrier mobility, threshold voltage, chan- 60 nel resistance, etc.) of SiC DMOSFET have been reported 61 in [8] [11], temperature-induced variation in the DMOSFET 62 switching dynamics with a high-temperature gate driver is not 63 well investigated. If the switching parameters vary with tem- 64 perature, then the dv/dt- and di/dt-induced device stresses will 65 also vary undesirably. Therefore, a mechanism for compensat- 66 ing the temperature-induced drift of SiC DMOSFET switching 67 dynamics is critical. In this regard, we further investigate a 68 photonic-compensation mechanism, which demonstrates that, 69 by nonlinearly adjusting the photonic intensity of the trigger- 70 ing signal of the OTPT, it is possible to compensate for the 71 variations in the switching dynamics over a range of temper- 72 ature. The basic idea of this mechanism is to compensate for 73 the progressively reducing rise time of the SiC DMOSFET with 74 increasing temperature by increasing the on-state resistance of 75 the series-connected OTPT (by reducing the power of its optical 76 excitation), which serves as the on-time gate resistance of the 77 DMOSFET. The aim of this letter is not to propose a switching- 78 loss reduction technique, but to demonstrate a mechanism for 79 regulating switching dynamics with increasing temperature be- 80 cause that results in a balance between switching loss and EM 81 noise, and device stress /$ IEEE

2 2 IEEE TRANSACTIONS ON POWER ELECTRONICS Fig. 1. Structure and packaged prototype of the GaAs OTPT. 83 II. GaAs-Based OTPT for Photonic Compensation OTPT is the principal optically activated element for the photonic compensation scheme. GaAs-based OTPT [12], [13] has been developed and is shown in Fig. 1 with structural and packaged representations. GaAs is selected as the base material because it possesses high optical absorption coefficient of 10 4 cm 1 near 800 nm wavelength range, high electron mobility, and low-carrier recombination lifetime. These properties of GaAs enable the OTPT to attain rapid turn-on and turn-off capabilities (thereby minimizing the delay between the optical excitation of the OTPT and the actuation of the PSD) and good optical efficiency (thereby minimizing optical-triggering power requirement). The N + collector, P-body, and N-GaAs drift and P-GaAs epitaxial layers are grown using metal organic chemical vapor deposition. Shallow ion implantation with Si atoms is done to realize high-doped N + emitter region. E-beam evaporated Au Ge Ni alloyed metal layers form the electrodes. Optical window is defined by plasma-enhanced CVD (PECVD) deposited Si 3 N 4 antireflecting (AR) coating. The thickness and the refractive index of the AR coating have been chosen such as to minimize reflection from the top surface of the device so that maximum utilization of the incident optical beam is possible. Fig. 2. Schematic of the photonic modulation scheme III. PHOTONIC INTENSITY MODULATION OF SIC DMOSFET SWITCHING DYNAMICS AND ON-STATE IMPEDANCE Schematic of the photonic-modulation scheme is shown in Fig. 2, while the mechanism for the photonic modulation is illustrated in Fig. 3. OTPT is activated using the laser output, which modulates the conductivity of the OTPT. Photogeneration and subsequent transistor action increases the conductivity of the OTPT and reduces its on-resistance (R OTPT ), as described by the following relation [13]: C 1 R OTPT = C 1 + C 1 P opt where C 1,C 1,C 1,C 2,C 2,C + C 2 C 2 + C 2 P opt 2 represent constant design parameters for a given OTPT design and P opt represents OTPT optical power. Correspondingly, a gate-excitation current from the V Bias flows through the OTPT and charges the DMOS- FET gate up to the level of V Bias. When the light is shut OFF, the OTPT goes back to its high-resistance mode and the R gate (1) Fig. 3. Illustration of the photonic modulation of the SiC DMOSFET switching dynamics. acts as a discharging element and pulls down the gate potential 120 of the DMOSFET close to the ground level, thereby turning 121 the DMOSFET OFF. Fig. 3 illustrates the correlation between 122 the optical input to OTPT and the OTPT on-resistance, and 123 gate-to-source and drain-to-source voltages of SiC DMOSFET. 124 Optical signals of two different intensity levels are shown. A 125 higher intensity of the optical signal results in an enhanced pho- 126 togeneration rate inside the OTPT. Therefore, the steady-state 127

3 IEEE TRANSACTIONS ON POWER ELECTRONICS 3 Fig. 4. Leakage current of the GaAs OTPT with varying case temperature at 50 V reverse bias. Fig. 5. OTPT. Photonic intensity modulated on-resistance measurement setup for the on-resistance of the OTPT decreases with increasing optical intensity [13], which leads to faster charging/discharging of the input capacitance of the DMOSFET, thereby leading to a smaller rise time. The rise time of the SiC DMOSFET (t r ) can be written as follows [14]: ( ) V Bias t r = C iss R OTPT ln V Bias (V Th + I M /g m ) where I M is the DMOSFET current and V Th and g m are the threshold voltage and transconductance of the DMOSFET, respectively. From (1), because R OTPT is a nonlinear function of P opt, and t r is also a function of P opt. (2) Fig. 6. Experimentally measured modulation of the OTPT on-resistance with varying photonic intensity under the following conditions: V Bias = 10 V, R Load = 50 Ω, frequency = 5 khz, and duty cycle = 50%. IV. EXPERIMENTAL RESULTS: PHOTONIC INTENSITY 137 MODULATION, PHOTONIC COMPENSATION, AND NOMINAL 138 CHARACTERIZATIONS 139 First, the GaAs OTPT is characterized for high-temperature 140 operation. Design of the OTPT is done in such a manner so 141 as to ensure a reliable operation at 200 C. To demonstrate its 142 high-temperature robustness, the leakage current of the OTPT 143 (without optical illumination) was measured over a temperature 144 range of 20 C 200 C at 50 V reverse bias. The leakage, as 145 showninfig.4,at25 C is under 1 µa, whereas it increases 146 slightly to 2.5 µa at 200 C. However, this level of leakage is 147 fairly acceptable for nominal operation, and therefore, it can be 148

4 4 IEEE TRANSACTIONS ON POWER ELECTRONICS Fig. 7. Experimental setup, following Fig. 2, for high-temperature switching characterizations. A setup outline is shown in top right. Fig. 8. Uncompensated and photonically compensated rise-time profiles of the SiC DMOSFET with varying case temperature. Compensated profile shows a considerably flatter response. Fig. 10. Nonlinear variation in the optical power required to compensate the change in the rise time of SiC DMOSFET with varying case temperature Fig. 9. Photonic intensity modulation of the slope of the drain source voltage of the SiC DMOSFET during turn-on. This is the basis for rise-time compensation. concluded that the careful design and optimization for the GaAs OTPT ensure its reliable operation at elevated temperatures. Next, photonic-intensity-based modulation of the on-state resistance of the OTPT is experimentally demonstrated. The OTPT on-state resistance is measured by a point-by-point method us- ing a resistive load circuit (see Fig. 5). In this method, voltage 154 drop across the OTPT during steady-state conduction is mea- 155 sured for different optical power levels. Resistance values are 156 extracted from those data points and they are plotted to show the 157 variation (see Fig. 6). Low bias voltage of 10 V is used for this 158 experimentation so as to achieve high level of resolution in the 159 measurement of the on-state voltage drop of the OTPT. Switch- 160 ing frequency is also lowered to 5 khz so as to allow the OTPT 161 to remain in the steady-state conduction mode for a larger time. 162 Fig. 6 shows a sharp drop in the on-resistance with variation in 163 optical power, and then, enters a relatively flat saturation region. 164 This behavior is expected from the theoretical analysis [13] and 165 (1), which outlines the nonlinear relation between the OTPT 166 on-resistance and the intensity of its optical signal. 167 The switching test setup, following Fig. 2, is shown in Fig FAP600 laser (from Coherent Inc.) was used as the optical 169 source. This is an 808 nm wavelength laser with spherically 170 homogenized output terminated in a multimode fiber bundle. 171 PCX-7410 (from Directed Energy) was used as the pulsating 172 laser driver. The optical fiber is a multimode index type with sil- 173 ica core (featuring a 600 µm core diameter and a 0.37 numerical 174

5 IEEE TRANSACTIONS ON POWER ELECTRONICS 5 Fig. 11. Switching waveforms at 400 V bias, 20 khz, and at 200 C case temperature with duty cycles of: (a) 5% and (b) 90%. Fig. 12. Switching waveforms at 400 V bias, 10% duty cycle, and at 200 C case temperature with frequencies of: (a) 10 khz and (b) 50 khz. Time periods, corresponding to various frequencies, are shown in the figures aperture). Both GaAs OTPT and the SiC DMOSFET are firmly attached to the hot plate by thermal pad and are internally connected. High voltage power bias of 400 V is applied to the SiC MOSFET through a load resistor (R Load ) of 166 Ω. The GaAs OTPT is found to be able to drive the SiC DMOS- FET successfully from 20 C to 200 C. However, when the photonic intensity was kept fixed at 1.5 W, the rise time of the SiC DMOSFET is found to decrease from 649 ns at 20 Cto 410 ns at 200 C (see Fig. 8). As explained earlier, variation in the photonic intensity affects the slope of the drain source voltage of the SiC DMOSFET (see Fig. 9). Therefore, to compensate for the large variation of rise time (of the SiC DMOSFET) with temperature, photonic triggering intensity is varied from 1.5 to 0.65 W for each discrete temperature point so that the rise time at that temperature is close to the value at the room temperature. The resulting optical-power profile is shown in Fig. 10 that yields, as shown in Fig. 8, an almost flat rise-time response with varying temperature for the SiC DMOSFET. The difference in the rise-time responses corresponding to the compensated and uncompensated cases in Fig. 8 is due to nonlinear variation in the photonic intensity of the triggering signal for the OTPT. This is because, for the compensated case, reduced photonic intensity increases the OTPT resistance [as evident from (1)] leading to slower charging of input capacitance of the SiC DMOSFET, thereby maintaining [following (2)] the SiC DMOSFET rise 199 time close to that at the room temperature. 200 Further, in a standard power electronic converter SiC DMOS- 201 FET is expected to work under varying pulsewidth and duty 202 cycle. Therefore, apart from demonstrating the photonic com- 203 pensation, following experiments were carried out at 200 C: 204 1) duty cycle of the optical signal is varied from 5% to 90%. 205 Switching operation is verified at 200 C (see Fig. 11), and 206 2) frequency of operation is varied from 10 to 50 khz (keeping 207 the duty cycle fixed at 10%). Switching operation is verified at C (see Fig. 12). This demonstrates the photonic control of 209 SiC DMOSFET at high temperature over a range of switching 210 conditions. 211 V. SUMMARY 212 GaAs-based OTPT is fabricated and coupled to a SiC DMOS- 213 FET for photonic control. Design of the OTPT is done such a 214 way to minimize leakage even at elevated temperature. High- 215 temperature (200 C) driving capability of the GaAs OTPT is 216 demonstrated. However, it is found that, the turn-on dynamics 217 of the SiC DMOSFET changes with temperature because of the 218 temperature-induced change in OTPT conductance. However, 219 modulation in the photonic intensity can produce variation in 220

6 6 IEEE TRANSACTIONS ON POWER ELECTRONICS the switching dynamics of the SiC DMOSFET due to nonlinear modulation in the OTPT resistance, which affects the rate of charging of the input capacitance of the DMOSFET. Therefore, temperature-induced drift of SiC DMOSFET rise time can be compensated by nonlinear photonic intensity variation. A considerably flatter rise-time profile over the temperature range of 25 C 200 C is obtained by varying the optical triggering power from 1.5 to 0.65 W. Although this letter demonstrates the optical controllability of SiC DMOSFET rise time at high temperature, the concept is extendable, following [15], for controllability of fall time and turn-on and turn-off delays. Further, direct photogeneration of OTPT also enables one to achieve switching control of faster PSDs, as illustrated in [15]. The potential applications of this photonic control are in emerging high-temperature power electronics, such as in automotive, defense, and aerospace systems. It is noted that, for traditional power electronics applications [15], low-cost fiber-coupled lasers with dedicated current drivers are being explored. In either case, the photonic control mechanism does not require any changes in the power electronics topology, only the mechanism of excitation is different. REFERENCES [1] J. A. Cooper, Jr. and A. Agarwal, SiC power-switching devices The second electronics revolution? Proc. IEEE, vol. 90, no. 6, pp , Jun [2] P. G. Neudeck, R. S. Okojie, and L. Chen, High-temperature electronics A role for wide bandgap semiconductors? Proc. IEEE, vol. 90, no. 6, pp , Jun [3] T. L. Weaver, Fly-by-light aircraft system cable plants, in Proc. IEEE Digital Avionics Syst. Conf., 1997, pp [4] M. A. Huque, R. Vijayaraghavan, M. Zhang, B. J. Blalock, L. M. Tolbert, 251 and S. K. Islam, An SOI-based high-voltage, high-temperature gate- 252 driver for SiC FET, in Proc. IEEE Power Electron. Spec. Conf., 2007, 253 pp [5] J. S. Chen and K. T. Kornegay, Class-AB SiC CMOS power opamp 255 with stable voltage gain over wide temperature range, in Proc. IEE Proc. 256 Circuits, Devices, Syst., 1999, pp [6] S. Waffler, S. D. Round, and J. W. Kolar, High temperature (> 200 C) 258 isolated gate drive topologies for silicon carbide (SiC) JFET, in Proc. 259 Annu. Conf. IEEE Ind. Electron., 2008, pp [7] S. K. Mazumder and T. Sarkar, SiC-based optically-gated high-power 261 solid-state switch for pulsed-power application, J. Mater. Sci. Forum, 262 vol , pp , [8] T. R. McNutt, A. R. Hefner, Jr., H. A. Mantooth, D. Berning, and S. 264 H. Ryu, Silicon carbide power MOSFET model and parameter extraction 265 sequence, IEEE Trans. Power Electron., vol.22,no.2,pp ,Mar [9] M. Hasanuzzaman, S. K. Islam, and L. M. Tolbert, Effects of temperature 268 variation ( K) in MOSFET modeling in 6 H silicon carbide, 269 Solid-State Electron., vol. 48, pp , [10] M. Chinthavali, B. Ozpineci, and L. M. Tolbert, High-temperature and 271 high-frequency performance evaluation of 4 H-SiC unipolar power de- 272 vices, in Proc. IEEE Appl. Power Electron. Conf., 2005, pp [11] S. L. Rumyantsev, M. S. Shur, M. E. Levinshtein, P. A. Ivanov, J. W. Pal- 274 Q2 mour, A. K. Agarwal, B. A. Hull, and S. Ryu, Channel mobility and 275 on-resistance of vertical double implanted 4 H-SiC MOSFETs at elevated 276 temperatures, Semicond. Sci. Technol., vol. 24, p , [12] S. K. Mazumder and T. Sarkar, Optically-modulated active gate control 278 for switching electrical power conversion systems, in Proc. IEEE Electr. 279 Ship Technol. Symp., 2009, pp [13] T. Sarkar and S. K. Mazumder, Dynamic power density, wavelength, and 281 switching time modulation of optically-triggered power transistor (OTPT) 282 performance parameters, Microelectron. J., vol. 38, pp , [14] V. Benda, J. Gowar, and D. A. Grant, Power Semiconductor Devices. 284 West Sussex, U.K.: Wiley, [15] S. K. Mazumder and T. Sarkar, Optical modulation for high power sys- 286 tems: Potential for electromagnetic-emission, loss, and stress control by 287 switching dynamics variation of power semiconductor devices, in Proc. 288 IEEE Energy2030 Conf., 2008, pp