Apparatus with photodiode region in multiple epitaxial layers

1. An apparatus, comprising:

a substrate;

a first epitaxial layer on the substrate;

a second epitaxial layer on the first epitaxial layer; and

a photodiode region formed in the second epitaxial layer and extended to a predetermined depth into the first epitaxial layer.

2. The apparatus of claim 1 , wherein:

the first epitaxial layer comprises a doped epitaxial layer; and

the second epitaxial layer comprises an undoped epitaxial layer.

3. The apparatus of claim 1 , wherein:

the first epitaxial layer comprises a doped SiGe epitaxial layer; and

the second epitaxial layer comprises an undoped Si epitaxial layer.

4. The apparatus of claim 1 , wherein the first epitaxial layer comprises a doped SiGe epitaxial layer having a Ge concentration level that increases in a direction moving away from the substrate.

5. The apparatus of claim 1 , wherein the first epitaxial layer comprises:

a first SiGe epitaxial layer on the substrate;

a second SiGe epitaxial layer on the first SiGe epitaxial layer; and

a third SiGe epitaxial layer on the second SiGe epitaxial layer.

6. The apparatus of claim 5 , wherein:

the first SiGe epitaxial layer has a Ge concentration level that increases in a direction moving away from the substrate;

the second SiGe epitaxial layer has a uniform Ge concentration level; and

the third SiGe epitaxial layer has a uniform Ge concentration level.

7. The apparatus of claim 5 , wherein:

the second SiGe epitaxial layer is doped with impurities of a first conductivity type;

the third SiGe epitaxial layer is doped with impurities of the first conductivity type; and

the second SiGe epitaxial layer has a concentration level of first conductivity-type impurities that is greater than a concentration level of first conductivity-type impurities in the third SiGe epitaxial layer.

8. The apparatus of claim 1 , wherein:

the photodiode region comprises a first impurity region doped with impurities having a first conductivity type and a second impurity region doped with impurities having a second conductivity type;

the first impurity region is extended from the second epitaxial layer to the predetermined depth into the first epitaxial layer; and

the second impurity region is formed solely in the second epitaxial layer.

9. An apparatus, comprising:

a substrate;

a first epitaxial layer on the substrate;

a second epitaxial layer on the first epitaxial layer; and

a photodiode region extending from a top surface of the second epitaxial layer to a predetermined depth in the first epitaxial layer.

10. The apparatus of claim 9 , wherein:

the first epitaxial layer comprises a doped epitaxial layer; and

the second epitaxial layer comprises an undoped epitaxial layer.

11. The apparatus of claim 9 , wherein:

the first epitaxial layer comprises a doped SiGe epitaxial layer; and

the second epitaxial layer comprises an undoped Si epitaxial layer.

12. The apparatus of claim 9 , wherein the first epitaxial layer comprises a doped SiGe epitaxial layer having a Ge concentration level that increases in a direction moving away from the substrate.

13. The apparatus of claim 9 , wherein the first epitaxial layer comprises:

a first SiGe epitaxial layer on the substrate;

a second SiGe epitaxial layer on the first SiGe epitaxial layer; and

a third SiGe epitaxial layer on the second SiGe epitaxial layer.

14. The apparatus of claim 13 , wherein:

the first SiGe epitaxial layer has a Ge concentration level that increases in a direction moving away from the substrate;

the second SiGe epitaxial layer has a uniform Ge concentration level; and

the third SiGe epitaxial layer has a uniform Ge concentration level.

15. The apparatus of claim 13 , wherein:

the second SiGe epitaxial layer is doped with impurities of a first conductivity type;

the third SiGe epitaxial layer is doped with impurities of the first conductivity type; and

the second SiGe epitaxial layer has a concentration level of first conductivity-type impurities that is greater than a concentration level of first conductivity-type impurities in the third SiGe epitaxial layer.

16. The apparatus of claim 9 , wherein:

the photodiode region comprises a first impurity region doped with impurities having a first conductivity type and a second impurity region doped with impurities having a second conductivity type;

the first impurity region is extended from the second epitaxial layer to the predetermined depth into the first epitaxial layer; and

the second impurity region is formed solely in the second epitaxial layer.

17. An apparatus, comprising:

a first epitaxial layer;

a second epitaxial layer on the first epitaxial layer;

a photodiode extending from a top surface of the second epitaxial layer to a predetermined depth in the first epitaxial layer;

a gate oxide on the top surface of the second epitaxial layer; and

a gate electrode on the gate oxide, wherein the gate electrode is configured to control a transfer of charge generated by the photodiode.

18. The apparatus of claim 17 , wherein:

the first epitaxial layer comprises a doped SiGe epitaxial layer; and

the second epitaxial layer comprises an undoped Si epitaxial layer.

19. The apparatus of claim 17 , wherein the first epitaxial layer comprises:

a first SiGe epitaxial layer;

a second SiGe epitaxial layer on the first SiGe epitaxial layer; and

a third SiGe epitaxial layer on the second SiGe epitaxial layer.

20. The apparatus of claim 19 , wherein:

the first SiGe epitaxial layer has a Ge concentration level that increases in a direction moving toward the second SiGe epitaxial layer;

the second SiGe epitaxial layer has a uniform Ge concentration level; and

the third SiGe epitaxial layer has a uniform Ge concentration level.