An imperfection in the structure of a crystal. When dislocations occur in the crystalline structure of a semiconductor they can have serious deleterious effects. They introduce unwanted energy levels in the forbidden band (traps), they can alter the etching properties of the material, and can seriously change the electrical properties of devices. For example the values of source-drain current and threshold voltage of field-effect transistors are strongly dependent on the dislocation density in the semiconducting substrate. The dislocation density in any particular crystal depends on the material used, the purity, and the method of production. Perfect or near perfect small crystals can be produced, but larger crystals are more difficult to produce without dislocations. Large virtually dislocation-free silicon crystals are now being produced, but the dislocation density in large gallium arsenide crystals is significant. Dislocation density can be determined by etching the crystal in a solvent that preferentially etches at dislocations, and then counting the etched pits. Dislocation density is therefore sometimes termed etch pit density.