Transcript NEEP 541
NEEP 541 Ionization in Semiconductors Fall 2002 Jake Blanchard Outline Ionization in Semiconductors Transients Semiconductor fundamentals Band theory Doping Junctions Ionization Effects Radiation ionizes target through collisions with electrons As electrons slow, they remain free electrons In semiconductors, we think of positive charges (holes) and negative charges (electrons) as free particles The key question concerns the average densities of electrons and holes in the solid and the subsequent transport of these “particles” Transient Effects The primary manifestation of ionization is a transient increase in the electrical conductivity and transient currents across the semiconductor junctions In optical materials, ionization changes the absorption coefficient and luminescence (we’ll discuss this later) How a Semiconductor Works Si has four electrons in it’s outer shell These form bonds with four neighboring atoms A perfect Si crystal is an insulator because all these outer electrons are tied up with neighboring atoms By mixing in impurities, you can alter this behavior Doping Adding P or As makes N-type Si. These have 5 outer electrons, so in Si they have one free electron and thus permit conduction A small amount makes a big difference N-type Si is a good conductor Doping Adding B or Ga creates P-type Si These have 3 electrons in the outer shell, so they form “holes” A Si electron is left free P-type Si is also a good conductor P-N junctions Combine a layer of Ptype with a layer of Ntype Si The interface is a “junction” This forms a diode – current can only flow in one direction P-N junction When diode is working, both holes and electrons flow towards junction They combine at interface Net current results P-N Junction P-N Junction IV Curve Band Theory Fermi energy is the highest energy state that would be occupied at 0 K In solids, only certain energy levels can be occupied by electrons The allowed levels smear into bands, due to periodicity of the lattice In metals, the Fermi energy lies within an allowed energy band Hence, electrons close to Fermi level can scatter into it (by electric fields) fairly easily and it will conduct at 0 K Band Theory In semiconductors, the electrons with the highest energies exactly fill one energy band at 0 K The next higher band is empty Resistivity is infinite Filled band is the “valence band” Higher band is the “conduction band” The energy separation between the bands is the “band gap” The Fermi energy lies in this gap Band Gap EC EF EG EV Real Materials Previous comments are for perfect crystals Boundaries and defects disrupt periodicity This creates allowed energy levels in gap In single crystal Si, defects are isolated, so the electrons in these levels are bound Semiconductors Intrinsic semiconductors have finite probability (above 0 K) that some electrons will reach conduction band Extrinsic semiconductors have some energy levels in the gap, due to defects and impurities These levels can capture holes and electrons Doping Most practical semiconductors rely on impurities for their properties Impurities can produce energy levels with any charge at just about any location within the gap Donor defects give up electrons to the conduction band Acceptor defects capture an electron from the valence band