Nonvolatile semiconductor memory device and method of manufacturing the same

1. A nonvolatile semiconductor memory device, comprising:

a plurality of memory strings each of which has a plurality of electrically rewritable memory cells connected in series; and

select transistors one of which is connected to each of ends of each of said memory strings,

each of said memory strings comprising:

a first semiconductor layer having a pair of columnar portions extending in a perpendicular direction with respect to a substrate, and a joining portion formed so as to join lower ends of said pair of columnar portions;

a charge storage layer formed to surround a side surface of said columnar portions; and

a first conductive layer formed to surround said charge storage layer and the side surface of said columnar portions,

and

each of said select transistors comprising:

a second semiconductor layer extending upwardly from an upper surface of said columnar portions;

a gate insulating layer formed to surround a side surface of said second semiconductor layer; and

a second conductive layer formed to surround said gate insulating layer and the side surface of said second semiconductor layer,

said first conductive layer functioning as a control electrode of said memory cells,

said second conductive layer functioning as a control electrode of said select transistors, and

an effective impurity concentration of said second semiconductor layer being less than or equal to an effective impurity concentration of said first semiconductor layer.

2. The nonvolatile semiconductor memory device according to claim 1 ,

wherein the effective impurity concentration of said first semiconductor layer is 1×10 19 cm −3 or more, and the effective impurity concentration of said second semiconductor layer is 3×10 17 cm −3 or less.

3. The nonvolatile semiconductor memory device according to claim 1 ,

wherein said first semiconductor layer and said second semiconductor layer include a hollow therein.

4. The nonvolatile semiconductor memory device according to claim 3 , further comprising:

a first insulating layer formed in said hollow of said first semiconductor layer; and

a second insulating layer formed in said hollow of said second semiconductor layer.

5. The nonvolatile semiconductor memory device according to claim 4 ,

wherein said first semiconductor layer and said second semiconductor layer are formed continuously in an integrated manner and configured as a first conductive type, and

wherein said second insulating layer has an effective impurity concentration of a second conductive type that is higher than said first insulating layer.

6. The nonvolatile semiconductor memory device according to claim 5 ,

wherein said first semiconductor layer and said second semiconductor layer are constituted by polysilicon doped with phosphorus.

7. The nonvolatile semiconductor memory device according to claim 1 ,

wherein said first conductive layer is constituted by polysilicon, and said second conductive layer is constituted by polysilicon doped with boron.

8. The nonvolatile semiconductor memory device according to claim 4 ,

wherein said first insulating layer and said second insulating layer are constituted by silicon oxide.

9. The nonvolatile semiconductor memory device according to claim 5 ,

wherein said first insulating layer and said second insulating layer are constituted by silicon oxide doped with boron.

10. The nonvolatile semiconductor memory device according to claim 1 , further comprising a third conductive layer formed so as to cover a lower portion of said joining portion.

11. The nonvolatile semiconductor memory device according to claim 1 ,

wherein said first conductive layer is formed in a plurality, ends of said first conductive layer being shaped into a stepped shape.