Electrical overstress, especially due to ESD (electrostatic discharge), has become a mounting concern to designers due to increased system densities and speed.
These performance gains are due to the giant strides made in integrated circuit complexity and the resulting smaller device sizes. Internal protection devices built into integrated circuits have been reduced in size to minimize their impact on speed and circuit area.
Unfortunately the price paid is increased ESD sensitivity. External protection is now required at system inputs and outputs to eliminate overstress damage and surface mount technology (SMT) is now placing severe size constraints on components in addition to providing protection.
Overstress protection falls into three categories: filters (R-C, R-L-C, etc.), crowbars such as gas
discharge tubes or thyristors, and low voltage clamps like zener diodes or varistors. All semiconductor transient voltage protraction devices such as thyristors, zener diodes, or varistors need large active areas to handle the high peak currents associated with transients.
Silicon transient supressors like thyristors and zener diodes can only be fabricated in a single plane with increased size to accommodate peak current requirements. When packaging is included the resulting size can become unmanageable for SMT assemblies.
Varistors are semiconducting ceramics and are not limited to single plane processing even though discs or pressed pills have dominated in the past. Continuing advances in ceramic technology make high performance, low voltage surface mountable varistors possible by using a multilayer structure instead of single plane devices. These gains are similar to those when the shift was made from disc capacitors to multilayers many years ago.
There are also significant improvements in the electrical performance in addition to size and usability
in SMT assemblies when multilayer varistor or Transguard transient voltage suppressors are used.
Modern varistors are ceramics composed of conductive zinc oxide grains doped with bismuth, cobalt, manganese, and other metal oxides.
The nonlinear current/ voltage characteristic is due to the conduction of Schottky barriers at each grain interface with maximum current density limited by bulk grain resistivity. Typical zinc oxide (ZnO) varistor materials have a macroscopic breakdown voltage per intergranular barrier of approximately 3.6V. From this fixed breakdown voltage per grain, Vn, the varistor voltage and dielectric (ceramic) thickness are found.
Click http://www.avx.com/docs/techinfo/CircuitProtection_EMIFiltering/mi_ti.pdf for full texts.