Buried Oxide (BOX) is the thin silicon dioxide layer sandwiched between the device layer and the handle wafer in an SOI substrate — providing complete electrical isolation between active devices and the bulk substrate.

What Is BOX?

• Material: Thermal SiO₂ or implanted oxide (SIMOX).
• Thickness: 10-400 nm depending on application.
• FD-SOI: Ultra-thin BOX (~25 nm) for back-gate biasing.
• RF-SOI: Thick BOX (~400 nm) for capacitance reduction.
• Photonics SOI: ~2 $mu m$ BOX for waveguide cladding.
• Quality: Must be defect-free and have excellent thickness uniformity.

Why It Matters

• Isolation: Eliminates substrate leakage, latchup, and parasitic capacitance.
• Back-Gate: In FD-SOI, the BOX acts as the gate dielectric for body biasing from the back side.
• Thermal Bottleneck: SiO₂ has ~100x lower thermal conductivity than Si — BOX impedes heat dissipation (self-heating concern).

Buried Oxide is the insulating foundation of SOI — the glass floor that gives transistors their isolation advantage.