Buried Layer is a heavily doped region formed at the interface between the substrate and an epitaxial layer — used in bipolar and BiCMOS processes to provide a low-resistance collector contact and reduce collector series resistance.

What Is a Buried Layer?

• Formation: Implant/diffuse a high-dose dopant (Sb or As for N+, Boron for P+) into the substrate before growing the epitaxial layer.
• Result: After epi growth, the doped layer is "buried" beneath the surface.
• Connection: Reached from the surface via a "sinker" diffusion (deep, heavily doped plug).

Why It Matters

• Bipolar Performance: Reduces collector resistance ($R_C$) -> higher $f_T$ (transition frequency).
• Latchup Reduction: In CMOS, a buried N+ layer acts similarly to Deep N-Well for substrate isolation.
• Analog/RF: Essential for high-performance bipolar transistors in SiGe BiCMOS processes.

Buried Layer is the hidden highway — a conductive path buried beneath the silicon surface to reduce resistance and improve transistor performance.