Microelectronic device with improved vertical breakdown voltage

A microelectronic device comprising:

• a first field effect transistor comprising a first active layer as well as a first drain, a first source, a first gate surmounting the first active layer,
• a second field effect transistor comprising a second active layer as well as a second drain, a second source and a second gate surmounting the second active layer, the second source being electrically connected to the first drain,
• a first rear electrode, underlying the first active layer in a stack direction (Z) perpendicular to a transverse plane (XY) defined by a first direction (X) and a second direction (Y), the first rear electrode being electrically connected to the first source,
• a second rear electrode, underlying the second active layer in the stack direction (Z), the second rear electrode being separated from the first rear electrode, the second rear electrode being electrically connected to the second source,
• a stack,
• wherein the stack comprises:
• a continuous and GaN-based third active layer, underlying the first active layer, on the one hand, and underlying the second active layer, and
• an insulating layer extending, in the stack direction (Z), between the first rear electrode and the first active layer, and between the second rear electrode and the second active layer, insulating layer being continuous and with the basis of a first dielectric, the insulating layer having, in the stack direction (Z), a critical field Ec, the insulating layer having, in the stack direction (Z), a thickness called dielectric thickness e1500 of between 2*e1500,min and 10*e1500,min, with e1500,min=Vtarget/Ec, Vtarget being a target breakdown voltage of the insulating layer, the first dielectric having a heat conductivity λ1 greater than 1 W·m−1·K−1.