Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
1998-11-03
2001-02-27
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S329000, C257S330000
Reexamination Certificate
active
06194741
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to MOSgated semiconductor devices and more specifically relates to such devices using a silicon carbide (SiC) substrate and with improved gate dielectric breakdown characteristics and increased channel mobility.
It is well known to use silicon carbide as a substrate material for power semiconductor devices. It is also known to use a trench structure for such devices (sometimes called UMOSFETs) in which an invertible channel is formed along the walls of such a trench.
It is known that silicon carbide UMOSFETs can have up to 200 times lower drift region resistance than monocrystaline silicon devices of the same rating. However, UMOSFETs fabricated on SiC have much lower breakdown voltages than is theoretically predicted. This is due to fact that the blocking performance of the SiC trench device is limited by gate dielectric breakdown usually at the trench corner. The highest blocking voltages observed to date on trench MOSFETs in SiC is 1400V (A. K. Agarwal et al., Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden). Gate breakdown at the trench corner was improved by using a thick deposited oxide at the trench bottom and corner. Alternate structures such as Double Implanted MOSFETs (DIMOSFETs, J. N. Shenoy et al., Dev. Res. Conf. 1996), accumulation mode FETs (ACCUFETs, P. M. Shenoy et al., Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden) and epitaxial channel FETs (EC-FETs, K. Hara, Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden) have also been reported.
Silicon carbide comes in many poly types, predominantly “4H” and “6H”, made by Cree Research Corp. The mobility of carriers in these 4H and 6H substrates is directionally dependent. Both the lateral and vertical bulk electron mobility of 4H material is relatively high in each direction (about 800 cm
2
/V.s in the lateral direction and about 1000 cm
2
/V.s in the vertical direction) whereas the 6H type has a lower lateral mobility (about 400 cm
2
/V.s) and a substantially lower (about 80 cm
2
/V.s) bulk electron mobility in the vertical direction. As a result 4H material is the substrate of choice for devices requiring vertical current flow.
It is also desirable to reduce premature breakdown of the device by breakdown of the gate oxide in the corner of the trench. It would further be desirable to create a depletion mode type MOSFET with a SiC substrate and a simplified manufacturing process.
BRIEF SUMMARY OF THE INVENTION
In order to improve the ability of the gate insulation, usually an oxide, which lines the trenches and which has relatively sharp corner edges, it has been found that a shallow junction formed under the bottom of the trench, usually a P type region in an N type silicon substrate, will form a depletion region extending from the bottom of the channel region to the edge of U-groove, thereby electrostatically shielding the gate insulation from high field stress due to source to drain potential under reverse bias. The gate insulation is also be shaped in cross-section to provide a thicker insulation layer at the edge of the groove, and at a given distance from the end of the vertical invertible channel. This increased distance can be formed by causing the interior top of the gate oxide within the U-groove to terminate on a concave radius to define a convex bottom for the polysilicon gate which fills the groove.
As a further feature of the invention, the device trench geometry can be modified to permit construction of a depletion mode MOSFET.
REFERENCES:
patent: 4116720 (1978-09-01), Vinson
patent: 5378914 (1995-01-01), Ohzu et al.
patent: 5770878 (1998-06-01), Beasom
patent: 5831288 (1998-11-01), Singh et al.
patent: 5915180 (1999-06-01), Hara et al.
patent: 6020600 (2000-02-01), Miyajima et al.
Kinzer Daniel M.
Sridevan Srikant
International Rectifier Corp.
Meier Stephen D.
Ostrolenk Faber Gerb & Soffen, LLP
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