Lecture%2018

Lecture%2018 - EEE 352: Lecture 18 Doping and Dopants The...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 EEE 352: Lecture 18 Properties of Extrinsic Semiconductors * Electron and hole density * Fermi level variation As B Donors and Acceptors The principle of doping involves REPLACING a small fraction of the silicon atoms with atoms with a DIFFERENT number of valence electrons, giving an EXCESS number of holes or electrons: Doping and Dopants ARSENIC HAS ONE MORE VALENCE ELECTRON THAN SILICON AND SO WHEN WE REPLACE A SILICON ATOM WITH AN ARSENIC ONE WE OBTAIN AN EXTRA ELECTRON THAT CAN CONDUCT ELECTRICITY, AS IT CAN BE FREED FROM THE ATOM EASILY. BORON HAS ONE LESS VALENCE ELECTRON THAN SILICON AND SO WHEN WE REPLACE A SILICON ATOM WITH AN BORON ONE WE OBTAIN AN VACANT BOND THAT CAN CONDUCT ELECTRICITY. THIS LEAVES A HOLE IN THE VALENCE BAND, AND SINCE THE BOND MOVES RELATIVELY EASILY, SO DOES THE HOLE. As B In INTRINSIC semiconductors the electron and hole densities are IDENTICAL since the semiconductor is UNDOPED In EXTRINSIC semiconductors, additional charge carriers are provided by DOPANTS: * The electron and hole densities will NO LONGER be the same * Charge NEUTRALITY must still hold for the crystal, however, which allows us to compute the electron and hole densities Electron and Hole Density n: NUMBER OF FREE ELECTRONS p: NUMBER OF FREE HOLES N a : NUMBER OF IONIZED ACCEPTORS N d + : NUMBER OF IONIZED DONORS + + = + d a N p N n quires Neutrality e Ch : Re arg = T k E E N n B F c c exp = T k E E N p B v F v exp Electron and Hole Density ± To increase the number of electrons, we will push the Fermi energy toward the conduction band. ± This will decrease the number of holes at the same time, by moving the Fermi energy away from the valence band. ± This will still result in the previous relationship still being valid: = = T k E E N N n np B v c v c i exp 2
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 This energy is very much SMALLER than the band gap energy of silicon (1.1 eV) * The effect of doping is therefore to creating an IMPURITY LEVEL near the conduction band This reduction in effective band gap allows MORE electrons to be excited * Into the conduction band at FINITE temperatures Doping and Dopants E c E d E v DONOR LEVEL As It is important to note that the number of ionized donors will VARY with temperature: * Recall that dopant electrons and holes must be IONIZED to conduct! * The electron and hole densities will therefore ALSO vary with temperature To compute the number of ionized donors recall that ionization requires, we need to know how many electrons have moved from the donor level ( E d ) to the conduction band ( E c ) Electron and Hole Density E c E d E v DONOR LEVEL [] ) ( 1 : d d d E f N N Donors Ionized of Number = + N d : TOTAL NUMBER OF DONORS f(E d ): PROBABILITY OF FINDING THE ELECTRON IN THE DONOR LEVEL 1-f(E d ): PROBABILITY THAT THE ELECTRON HAS LEFT THE DONOR LEVEL (AND GONE TO THE CONDUCTION BAND.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 10

Lecture%2018 - EEE 352: Lecture 18 Doping and Dopants The...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online