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| SEC. 2-1 CONDUCTORS A neutral copper atom has only one electron in its outer orbit. Since this single electron can be easily dislodged from its atom, it is called a free electron. Copper is a good conductor because the slightest voltage causes free electrons to flow from one atom to the next. SEC. 2-2 SEMICONDUCTORS Silicon is the most widely used semiconductor material. An isolated silicon atom has four electrons in its outer or valence orbit. The number of electrons in the valence orbit is the key to conductivity. Conductors have one valence electron, semiconductors have four valence electrons, and insulators have eight valence electrons. SEC. 2-3 SILICON CRYSTALS Each silicon atom in a crystal has its four valence electrons plus four more electrons that are shared by the neighboring atoms. At room temperature, a pure silicon crystal has only a few thermally produced free electrons and holes. The amount of time between the creation and recombination of a free electron and a hole is called the lifetime. SEC. 2-4 INTRINSIC SEMICONDUCTORS An intrinsic semiconductor is a pure semiconductor. When an external voltage is applied to the intrinsic semiconductor, the free electrons flow toward the positive battery terminal and the holes flow toward the negative battery terminal. SEC. 2-5 TWO TYPES OF FLOW Two types of carrier flow exist in an intrinsic semiconductor. First, there is the flow of free electrons through larger orbits (conduction band). Second, there is the flow of holes through smaller orbits (valence band). SEC. 2-6 DOPING A SEMICONDUCTOR Doping increases the conductivity of a semiconductor. A doped semiconductor is called an extrinsic semiconductor. When an intrinsic semiconductor is doped with pentavalent (donor) atoms, it has more free electrons than holes. When an intrinsic semiconductor is doped with trivalent (acceptor) atoms, it has more holes than free electrons. SEC. 2-7 TWO TYPES OF EXTRINSIC SEMICONDUCTORS In an n-type semiconductor the free electrons are the majority carriers, and the holes are the minority carriers. In a p-type semiconductor the holes are the majority carriers, and the free electrons are the minority carriers. SEC. 2-8 THE UNBIASED DIODE An unbiased diode has a depletion layer at the pn junction. The ions in this depletion layer produce a barrier potential. At room temperature, this barrier potential is approximately 0.7 V for a silicon diode and 0.3 V for a germanium diode. SEC. 2-9 FORWARD BIAS When an external voltage opposes the barrier potential, the diode is forwardbiased. If the applied voltage is greater than the barrier potential, the current is large. In other words, current flows easily in a forward-biased diode. SEC. 2-10 REVERSE BIAS When an external voltage aids the barrier potential, the diode is reverse-biased. The width of the depletion layer increases when the reverse voltage increases. The current is approximately zero. SEC. 2-11 BREAKDOWN Too much reverse voltage will produce either avalanche or zener effect. Then, the large breakdown current destroys the diode. In general, diodes are never operated in the breakdown region. The only exception is the zener diode, a specialpurpose diode discussed in a later chapter. SEC. 2-12 ENERGY LEVELS The larger the orbit, the higher the energy level of an electron. If an outside force raises an electron to a higher energy level, the electron will emit energy when it falls back to its original orbit. SEC. 2-13 THE ENERGY HILL The barrier potential of a diode looks like an energy hill. Electrons attempting to cross the junction need to have enough energy to climb this hill. An external voltage source that forward-biases the diode gives electrons the energy required to pass through the depletion layer. SEC. 2-14 BARRIER POTENTIAL AND TEMPERATURE When the junction temperature increases, the depletion layer becomes narrower and the barrier potential decreases. It will decrease approximately 2 mV for each degree Celsius increase. SEC. 2-15 REVERSE-BIASED DIODE There are three components of reverse current in a diode. First, there is the transient current that occurs when the reverse voltage changes. Second, there is the minority-carrier current, also called the saturation current because it is independent of the reverse voltage. Third, there is the surface-leakage current. It increases when the reverse voltage increases. |