Square-gate source-follower for CMOS image sensor pixel
Techniques are described for implementing a square-gate source-follower (SGSF) transistor for integration with complementary metal-oxide semiconductor (CMOS) image sensor (CIS) pixels. The SGSF transistor can have an active layer with active regions, including a drain region separated from each of two source regions to form parallel current channels. A square-gate structure layer includes main-gate regions, each disposed above a corresponding one of the current channels, and a side-gate region to couple the main-gate regions. At a particular physical width (W) and current channel length (L), the parallel current channels can act similarly to a conventional linear source-follower having dimensions of 2W and the same L. SGSF implementations can provide a number of features, including higher frame rate, lower power consumption, and lower noise, as compared to those of a conventional source-follower transistor of comparable W and L dimensions.
1. A source-follower transistor comprising:
an active layer comprising a drain-doped region separated from a first source-doped region by a first current channel, and separated from a second source-doped region by a second current channel, each of the drain-doped region, the first source-doped region, and the second source-doped region physically and electrically coupled with a respective contact;
a square-gate layer comprising:
a first main-gate region disposed above the first current channel to a first side of the drain-doped region;
a second main-gate region disposed above the second current channel to a second side of the drain-doped region opposite the first side of the drain-doped region; and
a side-gate region disposed to a third side of the drain-doped region to electrically couple the first main-gate region to the second main-gate region; and
a gate contact physically and electrically coupled with the first main-gate region, and electrically coupled with the second main-gate region via the side-gate region.
2. The source-follower transistor of claim 1 , further comprising:
an edge-isolation region running along a first edge of the source-follower transistor corresponding to the third side of the drain region, such that the side-gate region at least partially overlaps the edge-isolation region.
3. The source-follower transistor of claim 2 , wherein the edge-isolation region is a shallow trench isolation (STI) region.
4. The source-follower transistor of claim 1 , wherein the first current channel and the second current channel each has a same nominal channel length.
5. The source-follower transistor of claim 1 , wherein:
the source-follower transistor has a width of W; and
each of the first current channel and the second current channel has a nominal channel length (L); and
0.6 W≤L≤W.
6. The source-follower transistor of claim 1 , wherein:
the active layer is a substrate structure;
each of the drain-doped region, the first source-doped region, and the second source-doped region is a respective n-doped region of the substrate structure;
each of the first and second current channels corresponds to respective p-doped region of the substrate structure;
the square-gate layer is a polysilicon structure; and
the first and second main-gate regions of the polysilicon structure are isolated from the first and second current channels by a thin silicon dioxide layer.
7. A semiconductor image sensor comprising:
a pixel having:
a photodiode; and
a square-gate source-follower (SGSF) transistor comprising:
an active layer comprising a drain-doped region separated from a first source-doped region by a first current channel, and separated from a second source-doped region by a second current channel, each of the drain-doped region, the first source-doped region, and the second source-doped region physically and electrically coupled with a respective contact;
a square-gate layer comprising a first main-gate region disposed above the first current channel to a first side of the drain-doped region, a second main-gate region disposed above the second current channel to a second side of the drain-doped region opposite the first side of the drain-doped region, and a side-gate region disposed to a third side of the drain-doped region to electrically couple the first main-gate region to the second main-gate region; and
a gate contact physically and electrically coupled with the first main-gate region, and electrically coupled with the second main-gate region via the side-gate region,
wherein the square-gate layer is coupled with the photodiode.
8. The semiconductor image sensor of claim 7 , further comprising:
a select transistor to couple the pixel with a bus,
wherein the first and second source-doped regions are coupled with the select transistor.
9. The semiconductor image sensor of claim 7 , further comprising:
an array of instances of the pixel.
10. The semiconductor image sensor of claim 7 , wherein each pixel comprises a plurality of photodiodes, all coupled with the square-gate layer.
11. The semiconductor image sensor of claim 7 , wherein the SGSF transistor further comprises:
an edge-isolation region running along a first edge of the SGSF transistor that corresponds to the third side of the drain-doped region, such that the side-gate region at least partially overlaps the edge-isolation region.
12. The semiconductor image sensor of claim 7 , wherein the first current channel and the second current channel each has a same nominal channel length.