Vertical power transistor with termination area having doped trenches with variable pitches
Various improvements in vertical transistors, such as IGBTs, are disclosed. The improvements include forming periodic highly-doped p-type emitter dots in the top surface region of a growth substrate, followed by growing the various transistor layers, followed by grounding down the bottom surface of the substrate, followed by a wet etch of the bottom surface to expose the heavily doped p+ layer. A metal contact is then formed over the p+ layer. In another improvement, edge termination structures utilize p-dopants implanted in trenches to create deep p-regions for shaping the electric field, and shallow p-regions between the trenches for rapidly removing holes after turn-off. In another improvement, a dual buffer layer using an n-layer and distributed n+ regions improves breakdown voltage and saturation voltage. In another improvement, p-zones of different concentrations in a termination structure are formed by varying pitches of trenches. In another improvement, beveled saw streets increase breakdown voltage.
1. A vertical transistor structure comprising:
a cell array in a first semiconductor material of a first conductivity type;
a termination area surrounding the cell array, the termination area comprising:
concentric trenches of substantially equal width in the first semiconductor material surrounding the cell array, wherein spaces between the trenches increase with distance away from the cell array; and
first dopants of a second conductivity type implanted into the trenches and diffused to form merged zones of the first dopants, wherein a dopant concentration of the first dopants within the merged zones decreases laterally with distance from the cell array due to the varying spaces between the trenches;
a first electrode on a bottom surface of the transistor structure; and
a second electrode overlying at least portions of the cell array.
2. The structure of claim 1 further comprising a metal field plate electrically coupled to the merged zones and overlying at least a portion of the merged zones.
3. The structure of claim 2 wherein the metal field plate is also coupled to a reference voltage.
4. The structure of claim 2 wherein the metal field plate is floating.
5. The structure of claim 2 further comprising a plurality of separate metal field plates electrically coupled to the merged zones and overlying at least a portion of the merged zones.
6. The structure of claim 1 wherein the concentric trenches are formed in groups of trenches, wherein each group has a same pitch between trenches, and different groups have different pitches between trenches, wherein the pitch between trenches in the different groups increases with distance away from the cell array.
7. The structure of claim 1 wherein the varying dopant concentration in the termination area spreads an electric field in the first semiconductor material.
8. The structure of claim 1 wherein the first semiconductor material is segmented between the trenches to form segmented regions, wherein the segmented regions are floating.
9. The structure of claim 8 wherein the segmented regions assume a local potential of an abutting portion of the merged zones.
10. The structure of claim 8 further comprising a semiconductor layer of the second conductivity type overlying each of the segmented regions between the trenches.
11. The structure of claim 1 wherein the trenches are filled with a dielectric.
12. The structure of claim 1 wherein the cell array comprises vertical gates and a body region of the second conductivity type between the vertical gates, wherein the body region is configured for being inverted proximate to a vertical gate, when the vertical gate is electrically biased, for creating a conductive channel for turning on the transistor.
13. The structure of claim 1 wherein the transistor structure is formed as a die, and wherein the termination area is formed proximate to an edge of the die.
14. The structure of claim 1 further comprising a shield area between the cell array and the termination area, the shield area having a breakdown voltage that is less than a breakdown voltage of the cell array.
15. The structure of claim 14 wherein the shield area comprises one or more additional trenches, with the first dopants of the second conductivity type implanted through the additional trenches to form deep regions of the second conductivity type below the additional trenches.
16. The structure of claim 1 wherein the termination area increases a breakdown voltage of the transistor structure.