A new collector structure for NPT-IGBT with low dose p- Si injection layer and high dose p+

E V (b) Type Ill
Fig2. Energy band diagram for type 11 and tyy 111 at oollrrt0r region.
Takahide Sugiyama, Hiroyuki Ueda and Masayasu Ishiko
Power Device Div., Toyota Central R&D Labs., Inc. Nagakute, Aichi 480-11-561-63-4724 Fax: +8 1-561-63-6042 E-mail: t-sugiyama@mosk.tytlabs.co.jp
increase. In this paper, we propose a new collector structure with a low dose p- Si injection layer and a high dose p+ Ge contact layer for a thin wafer NPT-IGBT. This provides low contact resistances without sacrificing turnoff losses. We also demonstrate, for the first time, the performance of a thin wafer 1.2kV 200A IGBT using this new collector structure. Although NPT IGBTs are used as the motif device in this study, this proposed structure can also be applied effectively to thin wafer FS IGBTs [6].
DEVICE STRUCTURE AND CONCEPT Fig. 1 shows a cross-sectional view of each collector structure in this study. A collector structure with a conventional p- injection layer, the conventional structure with a p+ Si contact layer and the conventional structure with a p+ Ge contact layer are assigned as type I, type I1 and type 111, respectively. The proposed structure (type 111) corresponds to a structure where the p+ Si contact layer in type II is replaced with a p+ Ge contact layer. Additionally, Fig2 schematically shows an ideal energy-band diagram for the type 11 and type 111 structures. In type 11, the hole-injection from collector to base increases with increasing the pf dose in the contact layer. Moreover, the carrier transparency to the collector electrode is degraded by an increase in the contact layer thickness and the existence
INTRODUCTION IGBTs are one of the key components for realizing high-efficiency inverter system. In the automotive industry, compact and high-performance motor-inverter systems are indispensable for advanced hybrid vehicles. We have already developed 600V-200A-class low-loss and highly rugged planer IGBTs in 1997 and trench IGBTs in 2001 for the first mass production level hybrid vehicles [1,2]. On the other hand, new design concepts to improve IGBTs characteristics, other than regarding local lifetime control and trench gate structure, have been proposed lately. Especially, thin wafer IGBTs with a low dose injection layer are one of the main issues for middle voltage IGBTs from point of view of the performance and economical reasons. However, collector contact resistances are high because of the low canier concentration in the collector layer, while the good performance to the turnoff losses is provided by low hole-injection and good electron transparency [3,4]. Recently, to improve the low collector contact resistances, a structure where a high dose P+ contact layer is added on the low dose injection layer has been reported [SI. However, this structure has the possibility of causing not only an increase in hole-injection from the collector but also a degradation of electron transparency. Consequently, the turnoff losses
ABSTRACT We have proposed a new collector structure for thin wafer IGBTs to improve coutact resistances without sacrificing turnoff losses. The proposed structure bas a low dose p Si injection layer and a high dose pC Ge contact. We also demonstrate, for the first time, the performance of 1.2kV 2OOA class thin wafer NPT IGBTs using this new collector structure. As a result, from simulations and measurements, we found that the high dose p+ G e layer acts to suppress of hole-injection and also provides low contact resistauces without consequently sacrificing turnoll losses.
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In the proposed device (type Ill), turnoff times and on-state voltages were independent of the carrier concentration in the p+ contact layer, as shown in Fig. 3. As expected, the band offset (AEv) plays a part in creating this independent state, that is, the p+ Ge layer acts to suppress hole-injection froni the contact. In this case, the contact resistances might decrease due to the high concentration at the collector surface, though contact resistances could not be evaluated in this simulation. Fig. 4 shows the carrier (hole) distribution of type I1 and type Ill at a current density of 250A/cm2. Indeed, the injected carrier concentration at the n- base region in type 111 was lower than that in type II, in spite of the high boron doping concentration in the contact layer, which was specified as the same profile as that of type 11. From these simulation results, it is expected that contact resistances can be lowered without sacrificing turnoff losses by using this proposed collector structure.