Present Status and Future Prospects for Power Semiconductors Ken’ya Sakurai 1. Introduction (1) Devices related to multimedia ① High-voltage silicon diodes and damper diodes From the viewpoint of a highly information-orient- with high-speed switching performance to im- ed society in the coming 21st century, the social prove the picture quality of the CRT (cathode infrastructure will undergo rapid repairs and reforma- ray tube) display monitors and televisions tions. What will bring us to a society where computers ② Low on-resistance SOP-8 power MOSFETs and communications are closely intertwined? Techni- that extend the battery life of portable elec- cal innovations have always brought us advantages as tronic appliances such as notebook computers well as disadvantages. Any future technical innova- (2) Vehicles and rolling stock tions must definitely exclude disadvantages. ① Intelligent power MOSFETs that decrease the A highly information-oriented society will result in size and improve reliability of car electronics a great increase in electric energy consumption. Prob- systems lems of the global environment, social environment, ② High-voltage, high-power NPT (non punch- and energy resources must be improved through more through)-IGBT modules and flat IGBTs that serious consideration, with electrical manufactures reduce rolling stock size, weight, and energy leading these technical innovations. Development of consumption high power generation and conversion efficiency and (3) Power conversion (inverter control) energy-saving technology for electron devices are core ① Molded IGBTs, IGBT modules, and IGBT-IPMs technologies. More specifically, power electronics that for applications including NC (numerical con- control electric energy increases in importance, and trol) equipment, general-purpose inverters, especially power semiconductor devices as the key servo mechanisms, welding machines, and devices are required for further advances in perfor- UPSs (uninterruptible power system) mance and functions. The major directions of the research and development are: 2. Technical Trends of Power Semiconductor (1) low power dissipation devices Devices (reduction in conduction and switching losses) (2) system-integrated devices These applications extend over a wide range of (system-on-a-chip devices and system-integrated equipment including power systems, transportation, modules) industries, information, communications, and house- Under these circumstances, Fuji Electric plans to hold appliances. Major technical developments follow work with a leading company of power devices for the trend toward decreased power loss, high speed, intelligent motion control and intelligent power man- high reliability, and advanced functions. agement. In particular, power MOSFETs (metal oxide semi- 2.1 Technical trends conductor field effect transistors), IGBTs (insulated Figure 1 shows the progress of improvement in gate bipolar transistors), IPMs (intelligent power mod- power MOSFET on-resistance. In low-voltage devices ules), and power ICs (integrated circuits) with ad- (100V or less), their on-resistance has been greatly vanced performance and functions will rapidly enlarge reduced by fine patterning technology for LSIs (large- the market. scale integrated circuits). It is further reduced by This special issue describes the present status and adopting trench gate technology. On the other hand, future prospects for Fuji Electric’s power semiconduc- no great improvement has been made in high-voltage tor devices in the highly information-oriented society of devices. The reason is that the main on-resistance the future. Listed below are the device items classified component of high-voltage MOSFETs is in the drift by application fields. region and therefore, great reductions cannot be ex- 2 Vol. 44 No. 1 FUJI ELECTRIC REVIEW Fig. 1 Improvements in on-resistance Fig. 3 Carrier distribution of typical devices Carrier density Gate p (IGBT, Thyristor) T ≒n ) hy p 2 p ristor,GTO –1 MOSFET cm 10 n n (MOSFET) p≒ IGBT Ω・ IGBT n Emitter MOSFET n Collector 10–2 n p n p (n 1982 ) 1986 On resistance ( 1990 10–3 1996 Silicon unipolar limit nology to power devices was delayed by about two 50 100 500 1,000 generations. The fine patterning level of the power Breakdown voltage (V) devices has greatly advanced by application of the trench process. In particular, the on-resistance of low- voltage MOSFETs has been greatly reduced by the Fig. 2 Development of process design rules trench gate application. As for the new concept devices, the application of trench gates will also result in further improvement. The application of various LSI process technology as well as the submicron-level 50 Thyristor patterning technology to power semiconductors will be Bipolar transistor 1k (bits) rapidly promoted in the future, and a positive influ- 10 ence on their performance is expected. m) IGBT µ Next we will describe the merger of action physics. 5 16k MOSFET Figure 3 shows the on-state carrier distribution of 64k Trench MOS various power devices. The carrier distribution of a 1 256k 1M device determines its on-voltage drop and switching 4M Design rule ( 0.5 16M speed performance. The next goal is the merger of 64M MOSFET and thyristor actions. More than ten years have passed since an MCT (MOS-controlled thyristor) 1970 1975 1980 1985 1990 1995 2000 was announced. However, because of low controllable (Year) current density, its weakest point, it has not been widely accepted. Various new-concept devices for improvement have been announced, such as the EST pected by adopting finely patterned cells. To reduce (emitter switched thyristor), IGCT (insulated gate the on-resistance of high-voltage devices, an IGBT was controlled thyristor), and BRT (base resistance con- developed. As shown in the figure, the IGBT on- trolled thyristor). In addition, an attempt at easier resistance is much lower than the silicon unipolar limit application of the thyristor and attainment of high due to the conductivity modulation effect. From the performance by introducing dual gate control has viewpoint of on-resistance, the application range is begun. However, it is not yet marketable. It is thought to be less than 150V for power MOSFETs and difficult to efficiently remove excess carriers at turn- more than 150V for IGBTs. off, and the excess carriers necessarily prolong the The performance of power semiconductors will switching time. To improve these problems, various improve with the following technical innovations: means have been contrived in the LSI process technol- (1) A merger of semiconductor action physics and the ogy and the parasitic thyristor for prevention of latch- application of new concept devices up. (2) A breakthrough in a trade-off with intelligent Another movement is toward intelligent devices. functions (drive, protection, sensor functions, etc.) The rapidly developed ICs were motive power to the (3) Application of LSI process technology current social reformation. In the future, they are (4) Notable advances in the performance of devices expected to continue to play a role. However, in the using new semiconductor material mid-1980s, Dr. Adler et al. of General Electric Co. Figure 2 shows the transition of design rules for predicted that a second reformation would be caused power devices in comparison with that for DRAMs by intelligent power devices. We feel that the technical (dynamic random access memories). When compared innovation toward them has been quite remarkable. with DRAMs, the application of fine patterning tech- These power ICs have developed remarkably, especial- Present Status and Future Prospects for Power Semiconductors 3 ly toward large capacities and advanced functions. On voltage MOSFETs is greatly reduced by adopting the market are high-side and low-side switching, high trench gates. However, trench gate MOSFETs have cost performance, advanced function power MOSFETs the following problems: for automobiles and power ICs for igniters up to 400V ① a complicated process (high process cost) and tens of amperes. The one-chip inverter IC that ② the reliability of MOS gates and the yield rate incorporates a power supply, various protective cir- of chips cuits, and PWM control circuits has also been commer- Many efforts have been made to solve these cialized. The biggest problem of these devices is high- problems. Table 1 shows a comparison of power cost isolation technology. Recently, direct bonded SOI MOSFET performances. Overall, judging from these, (silicon on insulator) technology has gained attention. manufacturers are promoting commercialization under The age of on-a-chip systems with an integrated power their own technical strategies. device is not very far in the future. System-integrated Intelligent low-voltage MOSFETs using power modules will be mainstream in comparatively large MOSFETs as output devices are expected to be widely capacities. Fuji Electric plans to complete them all used for automobiles and power supply systems. In with silicon devices and silicon sensors concurrently response to this, we are promoting development and realizing compactness, advanced functions, and high manufacture of the smart discrete (such as advanced cost performance. We intend to promote the commer- function MOSFETs and IGBTs). These have driver, cialization of the next generation IPM series and power overcurrent and overheating protection functions and supply devices. Figure 4 shows a power system block are expected to be used in many applications due to diagram. The target of