Structure and fabrication method of high voltage MOSFET with a vertical drift region
Embodiments of the present disclosure include a transistor with a vertical drift region and methods for forming the transistor. The transistor may include a well region of a first conductivity type, a gate region disposed above the well region, and a drift region of a second conductivity type, different from the first conductivity type. The drift region may have a lateral portion disposed above a portion of the well region and laterally adjacent to a semiconductor channel in the well region. The drift region may also have a vertical portion extending vertically from the lateral portion of the drift region.
1. A transistor comprising:
a well region of a first conductivity type;
a gate region disposed above the well region in a first direction;
a drain region or a source region of a second conductivity type, different from the first conductivity type; and
a drift region of the second conductivity type, comprising:
a lateral portion disposed above a portion of the well region in the first direction, and laterally adjacent to a semiconductor channel in the well region in a second direction perpendicular to the first direction; and
a vertical portion extending vertically from the lateral portion of the drift region to the drain region or to the source region in the first direction.
2. The transistor of claim 1 , wherein the vertical portion of the drift region comprises a graded doping profile which increases as a distance from the well region increases.
3. The transistor of claim 2 , wherein the graded doping profile is graded from a first doping concentration (C1) to a second doping concentration (C2), C2 being 1.1 to 100 times greater than C1.
4. The transistor of claim 1 , wherein:
the lateral portion of the drift region is disposed between the vertical portion and the semiconductor channel.
5. The transistor of claim 1 , wherein:
the gate region comprises:
a gate oxide layer disposed above the well region, and
a gate layer disposed above the gate oxide layer; and
a top of the vertical portion of the drift region is above a top of the gate layer.
6. The transistor of claim 1 , wherein:
the gate region comprises:
a gate oxide layer disposed above the well region, and
a gate layer disposed above the gate oxide layer; and
a top of the vertical portion of the drift region is above a top of the gate oxide layer.
7. The transistor of claim 1 , wherein a length of the vertical portion of the drift region is directly proportional to a breakdown voltage of the transistor.
8. The transistor of claim 1 , wherein a length of the vertical portion of the drift region is between 0.1 μm and 0.6 μm.
9. The transistor of claim 1 , wherein:
the vertical portion of the drift region has a first length that
extends in the first direction between a surface of the lateral portion and the drain region, or
extends in the first direction between the surface of the lateral portion and the source region,
the lateral portion of the drift region has a second length that extends in the second direction between an edge of the semiconductor channel and an edge of the vertical portion of the drift region, and
the sum of the first length and the second length are configured to achieve a breakdown voltage of the transistor of at least 30 V.
10. A three-dimensional NAND flash memory integrated circuit, comprising a plurality of transistors including the transistor of claim 1 .