Lecture 4 - Carrier Properties and Doping

Lecture 4 - Carrier Properties and Doping - Carrier...

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1 Carrier Modeling Objective : 1. Understand the electronic properties of semiconductor materials. Questions to be answered : What do we mean by carrier “effective mass”? How do we manipulate the number of carriers in a semiconductor using donors and acceptor atoms? How does temperature affect donor and acceptors? EE360 – Lecture 4 Review The Energy Band Model shows us that: – There are two important ranges (bands) of allowed energies in for an electron in a semiconductor – These bands are separated by a range of forbidden energies called the band gap – semiconductors have an almost full valence band and an almost empty conduction band – charge carriers (electrons in the conduction band and holes in the valence band) allow a current to flow (charge to be transported) – for semiconductors at room temperature there is sufficient thermal energy to excite a few electrons from the valence to the EE360 – Lecture 4 conduction band – We draw energy-band diagrams to show energy of carriers in the valence and conduction bands as a function of position E V x E E C
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2 Carrier Effective Mass We know electrons have mass, m 0 = 9.11x10 -31 kg. We know how the electron will accelerate in response to an applied electric field, E : d m q F v = = However, when we place an electron in a crystal it is attracted by the atomic nuclei and repelled by the other electrons. Thus, on the microscopic level of the crystal, we have to model an electron’s motion using quantum mechanics. Fortunately, the quantum mechanical analysis simplifies to a result th t i l t id ti l t th l i l b h i t f th dt 0 From Pierret, p. 33 EE360 – Lecture 4 that is almost identical to the classical behavior, except for the substitution of an effective mass, m n * dt d m q F n v * = = From Pierret, p. 33 Carrier Effective Mass • Effective mass is an incredibly powerful concept: – replaces complicated quantum mechanical model with a “simple” semi-classical model – very accurate, as long as the correct effective mass is used • Important things to know about effective mass: – we also define an effective mass for holes, m p * – effective mass can change depending on direction in the crystal – we use different values for the effective mass depending on what physical property we are calculating (i.e. density of states effective mass, conductivity effective mass, etc.)
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This note was uploaded on 04/20/2008 for the course EE 360 taught by Professor Hastings during the Spring '08 term at Kentucky.

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Lecture 4 - Carrier Properties and Doping - Carrier...

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