Semiconductor device

A semiconductor device, comprising:

• a gate stack on a substrate, the gate stack including a gate insulating layer and a gate electrode on the gate insulating layer, the gate insulating layer including a first dielectric layer and a second dielectric layer on the first dielectric layer, wherein a dielectric constant of the second dielectric layer is greater than a dielectric constant of the first dielectric layer;
• a spacer structure on a side surface of the gate stack, the spacer structure including a first spacer and a second spacer on the first spacer, wherein a dielectric constant of the second spacer is less than a dielectric constant of the first spacer;
• an interlayer insulating layer covering the spacer structure;
• and
• a contact plug penetrating the interlayer insulating layer to connect the substrate,
• wherein the contact plug includes a first side surface adjacent to the second spacer and a second side surface opposite to the first side surface, and
• the first side surface has a sunken region recessed towards the second side surface.

A semiconductor device, comprising:

• a first dielectric layer on a substrate, the first dielectric layer having a side surface defining a concave portion recessed towards a center of the first dielectric layer;
• a second dielectric layer on the first dielectric layer, wherein a dielectric constant of the second dielectric layer is greater than a dielectric constant of the first dielectric layer;
• a gate electrode on the second dielectric layer;
• a gate capping pattern on the gate electrode;
• a first spacer on a side surface of the second dielectric layer and a side surface of the gate electrode;
• a second spacer on the first spacer;
• a third spacer between the first spacer and the second spacer and filling at least a portion of the concave portion;
• an interlayer insulating layer covering the third spacer and the gate capping pattern;
• and
• a contact plug penetrating the interlayer insulating layer to connect the substrate,
• wherein a dielectric constant of the second spacer is less than the dielectric constant of the first spacer and greater than a dielectric constant of the third spacer.