COVER CONTENT STORY Enhanced Trench IGBTs and Field Charge Controlled Diode The Next Leap in IGBT and Diode Performance Future generations of IGBT modules will employ Enhanced Trench ET-IGBTs and Field Charge Extraction FCE diodes capable of providing higher level of electrical performance in terms of low losses, good controllability, high robustness and soft diode reverse recovery. By Liutauras Storasta, Chiara Corvasce, Maxi Andenna, Sven Matthias, Raffael Schnell and Munaf Rahimo, ABB semiconductors demonstrated that soft recovery performance under extreme switching conditions combined with low losses could be achieved while having no drawbacks on other electrical parameters. IC=75A, VDC=1800V, Lσ=2400nH, Tj=150 C 200 190 Turn-off losses (mJ) Despite the fact that the Insulated Gate Bipolar Transistor (IGBT) and antiparallel diode have experienced over the past two decades important breakthroughs with respect to the device process and design concepts which resulted in clear leaps in device overall performance, further development work is underway to achieve the next level in terms of higher power densities, improved controllability and robustness. In this article, we will first briefly discuss the current IGBT and diode development trends while focusing on the next generation technologies; namely the Enhanced Trench IGBT (ET-IGBT) and Field Charge Extraction (FCE) diode. Then, the new device concepts and their electrical performance will be demonstrated for the 3.3 kV voltage class. 180 170 160 150 For the fast diode part, the losses and reverse recovery softness remain as a critical performance target for matching the performance of the new ET-IGBTs. The Field Charge Extraction (FCE) concept 24 Bodo´s Power Systems® gate gate IGBT and diode future development trends ET-IGBT 140 The main three IGBT development trends today are targeting higher EP-IGBT 130 power densities with (a) Enhanced Trench ET-IGBTs, (b) higher operating temperatures above the traditional 125°C mark and (c) IGBT/ 120 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 Diode integration solutions referred to as Reverse Conducting RCIGBT or Bi-mode Insulated Gate Transistor (BIGT). In the BIGT case, On-state voltage drop (V) the single chip approach provides improved performance especially Figure 1: 3.3kV IGBT trade-off curve between on-state voltage drop with respect to the limitations due to the restriction in available diode Fig. 1. 3.3kV IGBT trade-off curve between on-state voltage drop and turn-off losses. and turn-off losses. Comparison between the enhanced trench (ET) area depending on the given application requirements. Nevertheless, Comparison between the enhanced trench (ET) and enhanced planar (EP) structures. and enhanced planar (EP) structures the traditional IGBT/Diode two chip approach remains as an important development path for many mainstream applications. Today, state-ofThe ET-IGBT concept the-art high voltage devices with a similar loss performance employ The main approach followed for the realization of the ET-IGBT conEnhanced Planar IGBT (EP-IGBT) or Trench IGBT MOS cell concepts cept with the targeted enhanced carrier concentration near the trench on Soft Punch Through (SPT) structures. However, for lower voltage emitter for lower losses is based on the introduction of a striped active devices rated below 2 kV, in addition to trench IGBTs, advanced Trench MOS Cell with an n-enhancement layer. In order to reduce the ET-IGBTs are already an established technology. Furthermore, the effective input capacitance for improved switching controllability, the ET-IGBT concept is also capable of providing the next step in loss focus is on the optimization of the regions between the active cells, reduction for high voltage IGBTs. Figure 1 demonstrates the on-state which contribute strongly to the device effective input capacitance Vce(sat) loss reduction of a 3300 V IGBT for the same turn-off losses value during switching. By eliminating gate regions between the active cells as shown in the cross section in figure 2, we allow for a Eoff achieved with the new ET-IGBT MOS cell on the same bulk SPT low effective gate emitter input capacitance compared to state-of-theplatform. However, it is important to point out that the trench based IGBTs, especially for higher voltage ratings, exhibit an inherently high N-source effective gate input capacitance when compared to planar based devices which results in less controllability for optimum switching P performance during IGBT turn-on. Overcoming this negative aspect n-enhancement layer combined with the lower losses of the ET-IGBT will provide an ideal solution for the next generation high voltage IGBTs. Figure 2: ET-IGBT MOS Cell concept Fig. 2: ET-IGBT MOS Cell concept. December 2014 www.bodospower.com COVER CONTENT STORY 3.3 kV ET-IGBT module prototypes 3.3 kV ET-IGBT and FCE-diode chips were manufactured with an active area of approximately 1 cm2 per chip with a defined rating of 75 A for the IGBT and 150 A for the diode. The chips were employed in a standard high voltage insulated module (140 x 70) mm2 having a dual configuration as shown in the inset of figure 4. Each IGBT/diode part in the dual package consists of a single substrate containing 4 x ETIGBTs and 2 x FCE-diodes. The resulting current rating of the module is 300 A compared to today’s 250 A for an equivalent EP-IGBT. art trench IGBT designs while providing optimum reverse blocking capability. The lower on-state losses of the 3.3 kV ET-IGBT provides potentially a 20% increase in the rated current capability compared to the EP-IGBT generation. Nominal T = 25°C T = 150°C ET-IGBT Current (A) 200 EP-IGBT 100 0 0 1 2 3 4 Voltage (V) Figure 4: 300A/3.3kV ET-IGBT module output I-V characteristics at 25°C and 150°C. Fig. 4: 300A/3.3kV ET-IGBT module output I-V characteristics at 25 C and 150 C. The modules were tested electrically under static and dynamic conditions. Figure 4 shows the on-state characteristics for the ET-IGBT at Figure 3: The combination of FCE and FSA concepts (a) and (b) 25°C and 150°C and compared to the EP-IGBT. The ET-IGBT module cross sections, (c) doping profile exhibits much lower static losses compared to the EP-IGBT together combination of FCE and FSA concepts (a) and (b) cross sections, doping profile. with(c) strong positive temperature coefficient for safe paralleling of chips. At the rated current of 300 A, the ET-IGBT design has a Vce(sat) The FCE diode concept For the new diode, a combination of the Field Charge Extraction of 2.75 V compared to 3.55 V for the EP-IGBT at 150°C. (FCE) concept and the well-established Field Shielded Anode (FSA) design is utilized as shown in figure 3 when compared to a convenFigure 5 show the nominal turn-off and turn-on switching waveforms tional design. The thickness of the n-base plays a key-role for the for both ET-IGBT and the reference EP-IGBT, respectively. The test overall loss generation where low-loss diodes require a thin n-base conditions were kept the same to better evaluate the device perfordesign. However, further reductions of the thickness of the n-base mance. The devices were switched against an applied DC-link voltage region have been typically restricted by the snappy reverse recovery of 1800 V and a rated current of 300 A at 150°C with a gate emitter behavior of the resulting diodes. By introducing small p-doped areas capacitance of 47 nF. The stray inductance was 600 nH and the turnat the cathode side of the diodes as shown in figure 3, a field-induced off gate resistance was 9Ω while the turn-on gate resistance varied carrier injection process is enabled during the recovery phase, which per design as indicated. The turn-off losses Eoff of the ET-IGBT were generates inherently soft diodes. Therefore, the n-base of a 3.3 kV at around 650mJ compared to 600 mJ for the EP-IGBT. However, rated diode can be thinned by 10% while the blocking capability is larger variations were obtained for the turn-on losses Eon where the maintained by increasing the 3000 600 3000 resistivity without compromis75 75 EP-IGBT (R =10) EP-IGBT Current ET-IGBT ET-IGBT (R =18) ing soft reverse recovery. The 300 2500 2500 500 60 60 benefit of this approach is 2000 Voltage 45 45 a 20% improvement on the 2000 400 Voltage Current technology curve. Moreover, 30 30 200 1500 1500 300 the robustness of these inherGate Gate 15 15 ently soft diodes has been 1000 1000 200 improved due to the absence 0 0 100 500 of large overshoot voltages 500 100 -15 -15 during reverse recovery. 0 G -30 -30 0 0 1 2 3 4 5 6 -500 Time (s) Current (A) Voltage (V) Gate voltage (V) Voltage (V) Current (A) Gate voltage (V) G 0 0 0 2 4 6 8 Time (s) Figure 5: 300A/3.3kV module Turn-off (left) and Turn-on (right) waveforms (1800V, 300A, 150°C). Fig. 5: 300A/3.3kV module Turn-off (left) and Turn-on (right) waveforms (1800V, 300A, 150 C). 26 Bodo´s Power Systems® December 2014 www.bodospower.com COVER CONTENT STORY ET-IGBT Turn-off and Short Circuit SOA Performance The turn-off (RBSOA) behavior was tested for two paralleled chips under high current and voltage conditions. For the RBSOA, the ETIGBT was tested against a high DC-link voltage of 2500 V and the maximum achieved switchable current is approximately 5x and 4x the nominal current at 25°C and 125°C, respectively as shown in figure 8. The device enters and withstands both stress conditions known as dynamic avalanche and Switching Self Clamping Mode SSCM at 25°C. At a temperature of 125°C, the device experiences stronger dynamic avalanche as expected due to the higher levels of carrier concentrations which results in a lower but still sufficient turn-off capability. ET-IGBT was at 860 mJ albeit with a different gate resistor compared to 910 mJ for the EP-IGBT. The total switching losses for all tested devices were approximately at the same level just below 1.5 J. The FCE-diode reverse recovery performance can also be seen in the IGBT turn-on waveforms. The controllability of the ET-IGBT is illustrated in figure 6 when plotting the turn-on parameters (Icmax, Eon and di/dt) against the variation in the gate resistance. 550 500 1500 EP-IGBT ET-IGBT 1200 1360 680 0 20 900 Current (A) 0 600 600 -20 -40 -60 -80 400 1.5 2.0 2.5 Time (s) 3.0 3.5 Figure 7: 300A/3.3kV module reverse recovery (1800V, 15A, 25°C). 200 5 10 15 20 25 Fig. 7: 300A/3.3kV module recovery 15A, 25carried C). out at 1800 V The singlereverse chip ET-IGBT short(1800V, circuit test was Gate resistor () and 25°C and the resulting waveforms are shown in figure 8. At a short circuit current level of around 300 A, a smooth and stable beFigure 6: Effect of varying the gate resistance on the turn-on paramhavior the gate resistance on the turn-on parameters (1800V, 300A, 150was C).obtained for pulse widths of at least 15 us. eters (1800V, 300A, 150°C). With the above SOA performance, it is encouraging that the improvements achieved for lowering the on-state losses of the ET-IGBT have not compromised the device robustness, which is strongly required especially when targeting higher power densities for the next generation HiPak and LinPak modules. The FCE diode softness was also tested under the same circuit setup but at the critical softness conditions with a lower current of 15 A and a lower temperature of 25°C as shown in figure 7. The FCE diode clearly shows very soft recovery performance under these extreme conditions when compared to the standard diodes exhibiting a typical current snap-off along with the associated high overshoot voltage. 3600 3000 600 2400 4×Inom 1800 300 1200 150 600 0 voltage Voltage (V) 750 450 600 2400 Voltage (V) Current (A) 900 750 3000 4200 25°C 150°C www.abb.com/semiconductors 450 1800 300 current 1200 Current (A) varying Standard diode FCE diode 2040 450 Voltage (V) Average di/dt (A/s) Turn-on losses (mJ) IC max (A) 600 150 600 0 0 0 1 2 3 4 5 Time (s) 6 0 0 2 4 6 8 10 12 14 16 18 -150 Time (s) Figure 8: 3300V ET-IGBT Turn-off RBSOA (2500V, Rg=33Ω, Ls=2400nH, Vge=20V) and short circuit SOA (1800V, tsc=15us, Rg=33Ω, Ls=2400nH, T=25°C) waveforms Fig. 8: 3300V ET-IGBT Turn-offVge=15V, RBSOA (2500V, Rg=33Ω, Ls=2400nH, Vge=20V) and short circuit 28 SOA (1800V, tsc=15us, Vge=15V, Rg=33Ω, Ls=2400nH, T=25 C) waveforms Bodo´s Power Systems® December 2014 www.bodospower.com