A light emitting device comprising: a light emitting structure including a second conductivity type semiconductor layer, an active layer over the second conductivity type semiconductor layer, and a first conductivity type semiconductor layer over the active layer; a dielectric layer in a cavity extending through the second conductivity type semiconductor layer, and the active layer, and extending partially into or exposing the first conductivity type semiconductor layer of the light emitting structure, the cavity having a cross section area A; and a second electrode layer under the dielectric layer, a first electrode over the first conductivity type semiconductor layer, wherein the first electrode spatially overlaps with a portion of the cavity, wherein the second electrode includes: a reflective layer under the dielectric layer; and a conductive layer under the reflective layer and disposed in contact with the reflective layer, wherein the reflective layer includes a first portion that is covered with the dielectric layer and is vertically overlapped with the first electrode, and a second portion that is directly connected to the second conductivity type semiconductor layer, characterized in that the dielectric layer completely fills the cavity from a given height of the cavity, the dielectric layer has a thickness d corresponding to this given height, a capacitor C is formed by the first conductivity type semiconductor layer, the dielectric layer and the reflective layer and having a capacitance C=εxA/d, wherein ε is permittivity of the dielectric layer, and the reflective layer completely fills the remaining part of the cavity, and the second portion of the reflective layer extends outside the cavity. a light emitting structure including a second conductivity type semiconductor layer, an active layer over the second conductivity type semiconductor layer, and a first conductivity type semiconductor layer over the active layer; a dielectric layer in a cavity extending through the second conductivity type semiconductor layer, and the active layer, and extending partially into or exposing the first conductivity type semiconductor layer of the light emitting structure, the cavity having a cross section area A; and a second electrode layer under the dielectric layer, a first electrode over the first conductivity type semiconductor layer, wherein the first electrode spatially overlaps with a portion of the cavity, wherein the second electrode includes: a reflective layer under the dielectric layer; and a conductive layer under the reflective layer and disposed in contact with the reflective layer, a reflective layer under the dielectric layer; and a conductive layer under the reflective layer and disposed in contact with the reflective layer, wherein the reflective layer includes a first portion that is covered with the dielectric layer and is vertically overlapped with the first electrode, and a second portion that is directly connected to the second conductivity type semiconductor layer, characterized in that the dielectric layer completely fills the cavity from a given height of the cavity, the dielectric layer has a thickness d corresponding to this given height, a capacitor C is formed by the first conductivity type semiconductor layer, the dielectric layer and the reflective layer and having a capacitance C=εxA/d, wherein ε is permittivity of the dielectric layer, and the reflective layer completely fills the remaining part of the cavity, and the second portion of the reflective layer extends outside the cavity. the dielectric layer completely fills the cavity from a given height of the cavity, the dielectric layer has a thickness d corresponding to this given height, a capacitor C is formed by the first conductivity type semiconductor layer, the dielectric layer and the reflective layer and having a capacitance C=εxA/d, wherein ε is permittivity of the dielectric layer, and the reflective layer completely fills the remaining part of the cavity, and the second portion of the reflective layer extends outside the cavity.