been a primary application of Chemical Vapor Deposition (CVD).
CVD is a process whereby a thin solid film is synthetized from the gaseous
phase by a chemical reaction .
The purpose of epitaxy is to grow a silicon layer of uniform thickness and
accurately controlled electrical properties and so to provide a perfect
substrate for the subsequent device processing. 
All the well-established epi processes in production today are based on
CVD and use a Si precursor gas diluted in H2 as a carrier gas.
Because they concern wafer fabrication or operations at the very front end
of technology, they are run at high temperature, in a domain where the Si
surface is easily/naturally cleaned, using silicon chlorides. The epi quality
is high, and high growth rates are achieved. In addition, these processes
are relatively simple in terms of chemistry (one gas used for deposition
and one for doping), kinetics (growth rate and doping are proportional to
gas flow), and structure (one individual blanket film deposited on a full-sheet
Today epi depositions are highly desirable in the course of device fabrication
to improve the electrical performances of advanced technologies. 
is done to improve the performance of bipolar devices. By growing a lightly
doped epi layer over a heavily-doped silicon substrate, a higher breakdown
voltage across the collector-substrate junction is achieved while maintaining
low collector resistance. Lower collector resistance allows a higher operating
speed with the same current.
The core of an epitaxial (epi) reactor is the reaction chamber, typically
made of quartz.
Inside the chamber a holder for the silicon substrates, typically made of
graphite coated with silicon carbide, is heated up to 900-1250°C while
gases flow inside. These gases contain a volatile silicon compound and some
dopant compounds carried by a chemical reducing or inert main gas flow.
Reactor geometry is generally classified into two shapes of wafer holders,
called susceptors: the disk (pancake or single-wafer type) and the pyramid-shaped
body (barrel type).
In the former, the wafer lies horizontally on a disk which has one (single)
or more receptacles, while in the latter a truncated pyramid accepts the
wafers into cavities located on the faces, close to vertical; depending
on the wafer size and bell-jar dimension, the susceptor may accept one or
more rows in the pyramid faces. 
• Horizontal Reactors
The horizontal reactor can be considered a landmark in the field of deposition
reactors: the gases flow in a horizontal duct whose heated lower side constitutes
the susceptor. Horizontal reactors are also commonly operated under cold-wall
conditions at atmospheric or reduced pressure. New-generation reactors are
equipped with a rotating susceptor to increase the radial uniformity of
the deposited film and with robotized wafer loading-unloading equipment.
• Barrel Reactors
Silicon Epitaxy performed in barrel reactors has the advantage of processing
a large number of wafer per batch.
In a barrel reactor, the wafers are held by a heated prismatic susceptor
contained in a quartz bell that is externally cooled. 
 M.L. Hitchmann, K.F. Jensen, "Chemical Vapor Deposition-Principles and Applications"
 V,-M. Airaksinen, "Silicon Epitaxy", Epitaxial Layer Characterization and Metrology
 D. Dutartre, "Silicon Epitaxy", Silicon Epitaxy: New Applications
 C.L. Paulnack, K.E. Benson, Epitaxy Courtesy of D.L. Rode, W.R. Wagner and N.E. Shumaker, Appl. Phys.,
Lett. 30, 75 (1977)
 V. Pozzetti, "Silicon Epitaxy", Epitaxial Growth Facilities, Equipment, and Supplies
 M. Masi, S. Kommu, "Silicon Epitaxy", Epitaxial Growth Modeling