Environmental Engineering Reference
In-Depth Information
Chapter 1
INTRODUCTION TO DEVICE MODELING
Gianluca Giustolisi
This chapter will deal with the operation and modeling of semiconductor
devices in order to give the reader a basis for understanding, in a simple and
efficient manner, the operation of the main
building
blocks of
microelectronics.
1.1 DOPED SILICON
A semiconductor is a crystal lattice structure with free electrons and/or
free holes or, which is the same, with negative and/or positive carriers. The
most common semiconductor is silicon which, having a valence of four,
allows its atoms to share four free electrons with neighboring atoms thus
forming the covalent bonds of the crystal lattice.
In intrinsic silicon, thermal agitation can endow a few electrons with
enough energy to escape their bonds. In the same way, they leave an equal
number of holes in the crystal lattice that can be viewed as free charges with
an opposite sign. At room temperature, we have carriers of each
type per This quantity is referred to as and is a function of
temperature as it doubles for every 11 °C increase in temperature [
]-[2].
This intrinsic quantity of free charges is not sufficient for the building of
microelectronic devices and must be increased by doping the intrinsic
silicon. This means adding negative or positive free charges to the pure
material. Several doping materials can be used to increase free charges.
Specifically, when doping pure silicon with a pentavalent material (that is,
doping with atoms of an element having a valence of five) we have almost
one extra free electron that can be used to conduct current for every one
atom of impurity. Likewise, doping the pure silicon with atoms having a
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