Ex. 4. Look through the text and: 1) find different types of diodes and try to translate them ; 2) try to explain what a diode is; 3)recollect the type:of a diode you know; 4) look through text and say: what phenomenon is the base for diode operation; 5) say if the following sentence is true: "Diodes are made in order to meet the purpose for which they will be used"; 6) read the text and be ready to translate it. A diode is a p-n junction semiconductor that allows current to flow in only one direc- tion. A diode generally refers to a two-terminal solid-state semiconductor device that presents low impedance to current flow in one direction and high impedance to curzent flow in the op- posite direction.Most diodes are made from a host crystal of silicon (S i) with appropriate lm- purity elements introduced to modify , in a controlled manner, the electrical characteristics of the device. These diodes are the typical p-n junction (or bipolar) devices used in electronic circuits.Another type is the Schottky diode (unipolar), produced by placing a metal layer di- rectly onto the semiconductor. The metal semiconc luctor interface serves the same function as the p-n junction in the common diode structure.Other semiconductor materials such as galli- um-arsenide (GaAs)and silicon-carbide (SiC) are also in use for new and specialized appli- cations of diodes . Diodes are made , based on the purpose for which they are going to be used. For example, a varactor diode is used as a variable capacitor and a zener diode is operated in the reverse biased mode, hence their manufacturing process is also different. An LED is con- structed, in such a way that the holes and electrons on recombination , release energy in the form of light. Hence , they are manufactured from materials like gallium arsenide , gallium phosphide, etc instead of silicon, so as to have a higher potential barrier.