sentences:  A subsurface Zener diode, also called 'buried Zener', is a device similar to the surface Zener, but with the avalanche region located deeper in the structure, typically several micrometers below the oxide.The hot carriers then lose energy by collisions with the semiconductor lattice before reaching the oxide layer and cannot be trapped there.The Zener walkout phenomenon therefore does not occur here, and the buried Zeners have voltage constant over their entire lifetime.