Subsurface Zener References

Subsurface or "buried" Zener references are more stable and accurate than the simple Zener-reference. They consist of diodes with the correct value of reverse-breakdown voltage, formed below the surface level of the integrated-circuit chip; then covered by a protective diffusion to keep the breakdown below the surface. At the surface of a silicon chip there are more impurities, mechanical stresses, and crystal-lattice dislocations than within the chip. Since these contribute to noise and long-term instability, the buried breakdown diode is less noisy and much more stable than surface Zeners. This is the preferred on-chip reference source for accurate IC devices. However, its breakdown voltage is normally about 5V or more and it must draw several hundred microamperes for optimum operation. Subsurface or buried-zener references is not suitable for references that must run from low voltage and have low power consumption. For such applications, the 'bandgap” reference is preferred.