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2/8/2014 Semiconductor Diodes Peter Mathys ECEN 1400 Semiconductors • Modern diodes and transistors are made from semi-conductive materials (conductivity in range of 103 to 10-8 S per cm). • Typical semi-conductive materials are silicon (Si), germanium (Ge), gallium arsenide (GaAs), and silicon carbide (SiC). • Silicon, atomic number 14, is the eighth most common element by mass in the universe. 1 2/8/2014 Periodic Table of Elements Semiconductors Silicon • To use silicon in semiconductor devices, it must be grown in crystals of very high purity. • The crystals are then cut into wafers which are doped to alter their conductive states. Atomic configuration of silicon (Si), atomic number 14, four valence electrons 2 2/8/2014 Pure (intrinsic) Silicon • Pure silicon has no free electrons. Applying a voltage produces almost no electron flow. All electrons are locked up in covalent bonds between neighboring atoms. Doping Silicon • Doping is a process where a controlled amount of impurities is added to change the conductivity of selected areas of the Si wafer. Impurity with additional valence electron Impurity with “missing” valence electron 3 2/8/2014 n-Type Silicon • Addition of atoms with 5 valence electrons. Additional electron is not bonded and can be easily moved around. p-Type Silicon • Addition of atoms with 3 valence electrons. Missing electron acts like positive charge that can be easily moved. 4 2/8/2014 Diodes • Symbol: • Forward Biased: Anode is more positive than cathode => Positive charge flows A -> C • Reverse Biased: Cathode is more positive than anode => No charge flows through diode. pn-Junction Diodes • pn junction diodes are formed by sandwiching ntype and p-type silicon together. • For “one-way gate”, make charge carriers (free electrons and holes) in n, p regions interact such that current flows only in one direction. 5 2/8/2014 Forward Biased • Electrons and holes are forced toward pn junction by electric field supplied by battery. Electrons and holes combine and current can flow. Reverse Biased • Holes are attracted to – and electrons are attracted to + of battery. This creates a zone (called depletion region) at the pn junction which is free of charge carriers (holes and electrons) and no current can flow. 6 2/8/2014 Shockley Equation for iD Real pn Junction Diodes • A real pn junction has resistance, from the semiconductor to metal contacts at both ends, and in the semiconductor material itself. • SPICE model example: .model 1N4148 D(Is=2.52n Rs=.568 N=1.752) 7 2/8/2014 i-v Characteristic Rs·iD Diode Applications • Half wave rectifier: 8 2/8/2014 Diode Applications • Full wave rectifier: Simple Diode Model • A simple model for a pn junction diode in forward biased mode is shown below. • For silicon vF is approximately 0.7 V. 9 2/8/2014 Solar Cell • A solar cell is essentially a pn junction with a thin transparent n layer facing the light source and a large surface area. Equiv. Circuit, i-v Char. 10 2/8/2014 LED: Light Emitting Diode 11