Advances in power semiconductor devices: New materials

Transcription

1 Advances in power semiconductor devices: New materials For both unipolar and bipolar devices, the larger limitation of the devices in power applications comes out from the need of a thick and low doped layer that should be inserted in any device structure to sustain a large blocking voltage in off state. The use of semiconductor materials different from the Silicon could introduce a new freedom on the basic trade-off experienced by the silicon devices. For the unipolar devices, like the MOS or JFET, the presence of this low doped layer will increase the R ON resistance. A possibility in this case is offered by the use of Gallium Arsenide (GaAs) in place of Si as semiconductor. GaAs presents a much larger electron mobility: µ n = 8500 cm 2 /Vs, with respect to the one of the Si. Then for the same thickness and doping of the low doped region, the R ON presented by the GaAs device is 5.6 times lower than the one of comparable Si device. GaAs power devices are mainly used for power RF applications as output power amplifiers.

2 A further possibility is offered by the use of semiconductor materials that present a larger Bandgap Energy E G : the most important to date is the Silicon Carbide (SiC) that has a Bandgap energy E GSiC = 3.2 ev. This larger bandgap imply that the critical field E CRIT for the breakdown is larger: for SiC the critical field is E crsic = 4 MV/cm The larger value of E CRIT presented by SiC allows to use a thinner layer to sustain the same breakdown voltage as for the Si. The use of a thinner layer will lead to an improvement of the trade-off both for unipolar devices like the MOS or Schottky diodes and bipolar ones, like the BJT or PIN (and also the IGBT). Even if the lifetime and the mobility of SiC are lower than the typical values of Si, the improvement for very high voltage devices is significant. Moreover the larger SiC bandgap allows a larger max temperature of operation. Actually the SiC devices are extending the voltage ratings up to the 10KV and more, making these devices attractive for very high voltages applications. The problems are related up to now to the material quality (non as good as for Si), and to the large cost of the SiC wafers even of small dimensions. 2

3 SiC The critical electric field E CRIT for SiC is about 20 times larger than for Si. As indicated by the following schematic plots, this will lead to a much lower R ON resistance for the same blocking voltage rating. Also for bipolar devices the improvement is due to the reduction of the layer in which it must be created the conductivity modulation, and to the reduction of the stored charges that must bew removed in the turn-off transient. electric field electric field 3

4 Specific ON resistance of unipolar devices in Si or SiC 4

5 SiC Products and R&D Schottky diodes $10m already Infineon/SiCED Dynex EcoTron GE Mitsubishi Rohm Semisouth Int. Rectifier Rockwell STMicroelectronics PiN diodes Soon (< 2 years) GE Rockwell SiC devices MESFET $3m already New Japan Radio JFET/SIT emerging Northrop Semisouth Infineon/SiCED Hitachi Intrinsic Toshiba Rockwell, Thyristors GE MOSFET Before 2009 Fairchild Mitsubishi Nippondenso Philips Rohm BJT 5

6 Advances in new technologies A technique to create a thick and low doped layer without sorting to the expensive and long epitaxial growth is the bonding of a wafer of low doping and given thickness to a second one, with larger doping. This latter will act as the substrate of the composite wafer, made by putting the two wafers polished in contact, with right crystalline orientation, at a high temperature and at high pressure. The interface will react by making the crystal lattice at the boundary. N N+ N N+ The same can be done for making an isolation layer made by a thin oxide buried inside a wafer; it can be done by wafer bonding after having grown a thin oxide layer on both faces of the two starting wafers. 6

7 New Technologies: Integrated Power Modules (IPM) 7

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