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Lecture%2017 - 1 EEE 352 Lecture 17 Extrinsic...

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Unformatted text preview: 1 EEE 352: Lecture 17 Extrinsic Semiconductors * Extrinsic semiconductors—those with dopants to control the properties ⇒ Silicon * Doping and dopants ⇒ Donors ⇒ Acceptors ⇒ Donor and acceptor levels WHY? ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − = T k E E N n B F c C exp If we push the Fermi level closer to the conduction band, the density of electrons increases. ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − = T k E E N p B v F V exp If we push the Fermi level closer to the valence band, the density of electrons increases. ¾ Devices work on the control of the densities—hence control of the Fermi energy. ¾ We need to know only 2 things to make all devices: (1) control of the density/Fermi level, and (2) the relationship between charge and voltage on a capacitor! Remember this picture from our “Atoms to Transistor” set of videos (lecture 1). What is happening here? n-type material capacitor n-type material has more electrons than holes. Capacitor: + Positive charge accumulates on this plate Negative charge accumulates on this plate Negative charge here means more free electrons, just like in n-type material. 2 n-type material The negative charge of the capacitor creates a conducting path between the two n-type materials. This is controlled by the voltage on the capacitor. This is a transistor! ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − = T k E E N n B F c C exp If we push the Fermi level closer to the conduction band, the density of electrons increases. So, how do we make n-type material? If we can increase the number of free electrons, we will pull the Fermi level close to the conduction band. We do this with dopants, called “ donors ,” (because they donate an electron to the conduction band). O.K., but how is this achieved? • The usefulness of semiconductors arises when we DELIBERATELY introduce Impurities (known as DOPANTS) to IMPROVE their electrical properties * Such DOPED materials are referred to as EXTRINSIC semiconductors • To discuss the key aspects of doping we consider the behavior exhibited by SILICON. In its undoped state this is a TETRAHEDRALLY bonded covalent crystal Extrinsic Semiconductors EACH SILICON ATOM BONDS TO FOUR NEAREST NEIGHBORS Each bond is composed of two electrons, one from each atom at the ends of the bond. • 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....
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Lecture%2017 - 1 EEE 352 Lecture 17 Extrinsic...

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