Image sensor

1. An image sensor comprising:

a pixel array comprising a plurality of pixel units;

a controller configured to drive the pixel array; and

an analog-digital conversion block configured to convert a sensing signal output from the pixel array into a digital signal, wherein each of the pixel units comprises:

a photodiode in a semiconductor substrate; and

a transfer transistor, a reset transistor, a drive transistor, and a select transistor on the semiconductor substrate spaced from one another, wherein

the transfer transistor comprises a first gate electrode and a first gate dielectric layer,

the reset transistor comprises a second gate electrode and a second gate dielectric layer,

the drive transistor comprises a third gate electrode and a third gate dielectric layer,

the select transistor comprises a fourth gate electrode and a fourth gate dielectric layer, and each of the first, second, third, and fourth gate dielectric layers has a thickness, the thickness of the third gate dielectric layer is less than the thickness the first gate dielectric layer, the second gate dielectric layer or the fourth gate dielectric layer, and the thickness of the second gate dielectric layer is less than the thickness of the first gate dielectric layer and the thickness of the fourth gate dielectric layer.

2. The image sensor according to claim 1 , wherein the thickness of the third gate dielectric layer is less than the thickness of the first gate dielectric layer, the thickness of the second gate dielectric layer, and the thickness of the fourth gate dielectric layer.

3. The image sensor according to claim 1 , wherein the thickness of the third gate dielectric layer is equal to a thickness of the first gate dielectric layer, the second gate dielectric layer, or the fourth gate dielectric layer.

4. The image sensor according to claim 1 , wherein the thickness of one of the first gate dielectric layer, the second gate dielectric layer, and the fourth gate dielectric layer is less than the thicknesses of the others of the first gate dielectric layer, the second gate dielectric layer, and the fourth gate dielectric layer.

5. The image sensor according to claim 1 , wherein the thickness of the first gate dielectric layer and the thickness of the fourth gate dielectric layer are identical to each other.

6. The image sensor according to claim 1 , wherein the thickness of the first gate dielectric layer is identical to the thickness of the third gate dielectric layer.

7. The image sensor according to claim 1 , wherein (i) the first gate dielectric layer is between the semiconductor substrate and the first gate electrode, (ii) the second gate dielectric layer is between the semiconductor substrate and the second gate electrode, (iii) the third gate dielectric layer is between the semiconductor substrate and the third gate electrode, and (iv) the fourth gate dielectric layer is between the semiconductor substrate and the fourth gate electrode.

8. The image sensor according to claim 1 , wherein the first, second, third and fourth gate dielectric layers are on the semiconductor substrate spaced from one other.

9. The image sensor according to claim 1 , wherein:

the thickness of the third gate dielectric layer is greater than or equal to 1 nm and less than 6 nm, and

each of the thicknesses of the first gate dielectric layer, the second gate dielectric layer, and the fourth gate dielectric layer is greater than or equal to 6 nm and less than or equal to 10 nm.

10. An image sensor comprising:

a sensing area comprising a pixel array including a plurality of pixel units;

a digital area configured to control the sensing area; and

an analog area configured to convert a signal from the sensing area into a digital signal, wherein

each of the pixel units comprises a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a select transistor;

each of the transfer transistor, the reset transistor, the drive transistor, and the select transistor includes a gate dielectric layer having a thickness, and the thickness of at least one of the gate dielectric layers of the transfer transistor, the reset transistor, the drive transistor, and the select transistor is different from the thickness of at least one gate dielectric layer of the others of the transfer transistor, the reset transistor, the drive transistor, and the select transistor;

the digital area comprises at least one first transistor; and

the analog area comprises at least one second transistor;

the first and second transistors each have a gate dielectric layer having a thickness; and

each of the thicknesses of the gate dielectric layers of the transfer transistor, the reset transistor, the drive transistor, and the select transistor is less than the thicknesses of the gate dielectric layers of the first and second transistors.

11. An image sensor comprising:

a pixel array comprising a plurality of pixel units;

a controller configured to drive the pixel array; and

an analog-digital conversion block configured to convert a sensing signal output from the pixel array into a digital signal, wherein each of the pixel units comprises:

a photodiode in a semiconductor substrate; and

a transfer transistor, a reset transistor, a drive transistor, and a select transistor on the semiconductor substrate spaced from one another, wherein

the transfer transistor comprises a first gate electrode and a first gate dielectric layer,

the reset transistor comprises a second electrode and a second gate dielectric layer,

the drive transistor comprises a third gate electrode and a third gate dielectric layer,

the select transistor comprises a fourth gate electrode and a fourth gate dielectric layer,

each of the first, second, third, and fourth gate dielectric layers has a thickness, and

the thickness of at least one of the first, second, third and fourth gate dielectric layers is different from thicknesses at least one of the other gate dielectric layers, and the thickness of the first gate dielectric layer is identical to the thickness of the third dielectric layer.