High voltage field effect device and method
View Patent ↗Methods and apparatus are provided for a MOSFET ( 50, 99, 199 ) exhibiting increased source-drain breakdown voltage (BVdss). Source (S) ( 70 ) and drain (D) ( 76 ) are spaced apart by a channel ( 90 ) underlying a gate ( 84 ) and one or more carrier drift spaces ( 92, 92 ′) serially located between the channel ( 90 ) and the source ( 70, 70 ′) or drain ( 76, 76 ′). A buried region ( 96, 96 ′) of the same conductivity type as the drift space ( 92, 92 ′) and the source ( 70, 70 ′) or drain ( 76, 76 ′) is provided below the drift space ( 92, 92 ′), separated therefrom in depth by a narrow gap ( 94, 94 ′) and ohmically coupled to the source ( 70, 70 ′) or drain ( 76, 76 ′). Current flow ( 110 ) through the drift space produces a potential difference (Vt) across this gap ( 94, 94 ′). As the S-D voltage (Vo) and current ( 109 , Io) increase, this difference (Vt) induces high field conduction between the drift space ( 92, 92 ′) and the buried region ( 96, 96 ′) and diverts part ( 112 , It) of the S-D current ( 109 , Io) through the buried region ( 96, 96 ′) and away from the near surface portions of the drift space ( 92, 92 ′) where breakdown generally occurs. Thus, BVdss is increased.
1. A method for forming a MOSFET comprising:
providing a substrate of a first conductivity type;
then in any order:
forming a region of a second conductivity type opposite the first conductivity type overlying at least a portion of the substrate;
providing spaced-apart source and drain regions of the first conductivity type and having a channel region of opposite conductivity type therebetween located in the region of the second conductivity type;
forming a drift space of the same conductivity type as the source or drain region coupled to the source or drain region and extending toward the channel region;
forming a buried region of the same conductivity type as the drift space, coupled at one end to the source or drain region and underlying the drift space and separated in depth therefrom by a gap sufficiently narrow to permit significant high field conduction current to flow to or from the drift space to the buried region in response to source-drain current flow through the drift space; and
wherein the source, drain and channel regions and the drift space extend to a first surface of the region of the second conductivity type.
2. The method of claim 1 wherein the forming steps comprise:
ion implanting the region of the second conductivity type;
ion implanting the drift space; and
ion implanting the buried region.
3. The method of claim 1 wherein the steps of forming the drift space and forming the buried region comprise:
forming the drift space adjacent the drain region; and
forming the buried region adjacent the drain region.
4. The method of claim 1 wherein the steps of forming the drift space and forming the buried region comprise:
forming the drift space adjacent the source region; and
forming the buried region adjacent the source region.
5. The method of claim 1 wherein the steps of forming the drift space and forming the buried regions comprise:
forming drift spaces adjacent both the source and drain regions; and
forming buried regions adjacent both the source and drain region.
6. The method of claim 1 further comprising:
forming a further buried region of the same conductivity type as the source or drain region ohmically coupling the source or drain region respectively to the buried region or regions underlying the source or drain region.