Metal oxide varistors (MOV) are semiconductors that protect electronic components and systems from transient voltages. These clamping devices contain a matrix of zinc oxide grains sandwiched between two metal plates which serve as electrodes. The boundaries between grains form diode junctions which allow current to flow in only one direction. Applying low or moderate voltages causes reverse leakage through the diode junctions and a small flow of current. Higher voltages trigger the avalanche effect and cause the diode junctions to break down. As a rule, metal oxide varistors exhibit highly non-linear current-voltage characteristics. They have high resistance at low voltages and a low resistance at high voltages. Unlike transient voltage suppression (TVS) diodes, however, metal oxide varistors degrade as they absorb repeated transients.