Transistor structure having a trench drain
A semiconductor device is formed having a trench adjacent to a current carrying region of the device. The trench is formed having a depth greater than the depth of a tub region of the device. Increasing the trench depth moves a region of higher field strength from the tub region to a region along the trench. The region along the trench does not have a junction and may withstand the higher field strength.
1. A method to achieve planar breakdown in a drain region of a transistor, the method comprising:
forming a tub region that includes a channel region of the transistor to a first depth where the tub region is an opposite conductivity type as the drain region;
forming a trench in the drain region of the transistor to a second depth;
forming a dielectric layer overlying the trench wherein the second depth is greater than the first depth to reduce a field strength in proximity to the tub region and wherein a voltage breakdown of the transistor is increased;
forming a pedestal disposed proximate to the trench;
forming a first conductive shield disposed in the trench approximately parallel to a sidewall of the trench; and
forming a second conductive shield at least partially disposed within the pedestal.
2. The method as recited in claim 1 further comprising moving a region of maximum or nearly maximum field strength to a corner region of the trench substantially below a major surface of the transistor where the dielectric layer does not breakdown under the maximum or nearly maximum field strength.
3. The method as recited in claim 1 further comprising:
implanting dopant of a same type as the drain region in proximity to a surface of the drain region and in proximity to a channel of the transistor to support current flow out of the channel region and into the drain region; and
implanting dopant through a sidewall of the trench to support current flow in proximity to the trench adjacent to the sidewall of the trench.
4. The method as recited in claim 1 further comprising a process of etching the trench greater than two times the depth of the tub region.
5. The method as recited in claim 1 further comprising a process of providing field line edge termination for the transistor.
6. A method to fabricate a semiconductor device, comprising:
forming a tub region including a channel region of a transistor, the tub region being disposed at a first depth wherein the tub region has a conductivity type opposite to a conductivity type of a drain region of the transistor;
forming a trench formed in the drain region of the transistor to a second depth;
forming a dielectric layer overlying the trench;
wherein the second depth is greater than the first depth to reduce a field strength proximate to the tub region to increase a voltage breakdown of the transistor; and
forming a second trench at least partially disposed below the trench that is formed in the drain region of the transistor.
7. A method as claimed in claim 6 , wherein a region of maximum or nearly maximum field strength is located at or near a corner region of the trench substantially below a surface of the transistor such that the dielectric layer does not breakdown under the maximum or nearly maximum field strength.
8. A method as claimed in claim 6 , further comprising:
implanting a first dopant of a same type as the drain region implanted in proximity to a surface of the drain region and in proximity to the channel region of the transistor to support current flow out of the channel region and into the drain region; and
implanting a second dopant implanted through a sidewall of the trench to support current flow in proximity to the trench adjacent to the sidewall of the trench.
9. A method as claimed in claim 6 , wherein the first depth of the trench is greater than two times the second depth of the tub region to result in a correspondingly increased breakdown voltage of the transistor.
10. A method as claimed in claim 6 , further comprising a forming a conductive shield disposed in the trench approximately parallel to a sidewall of the trench.
11. A method as claimed in claim 6 , further comprising:
forming a pedestal disposed proximate to the trench;
forming a first conductive shield disposed in the trench approximately parallel to a sidewall of the trench; and
forming a second conductive shield at least partially disposed within the pedestal.
12. A method as claimed in claim 6 , wherein the second depth of the trench extends to a region away from a current carrying region of the transistor.