Image Processing Reference
In-Depth Information
2
Device Ele
ments and Circuits for Image Sensors
In this chapter, components of device elements commonly used in image sensors, silicon
as a material for sensor parts, and circuit components are described.
2.1 Device Element Components
The term
components of device elements
here means the parts that form active semiconductor
devices and are also components of large-scale integration (LSI) circuits. They are com-
monly used components, especially in image sensors. We begin with an explanation of the
band structure of materials.
2.1.1 Foundation of Silicon Device Physics
Conceptual diagrams of the energy band structure of materials are shown in Figure 2.1.
According to the band theory of solid-state physics, electrons in crystals are arranged in
energy bands named allowed bands, separated by the regions in energy space in which
no electrons are allowed to exist. The energy width of this forbidden region is called the
bandgap or energy gap. The states in allowed bands are occupied from the lowest to higher
ones by electrons. The highest energy level of the occupying electron is the concept of
Fermi level.*
Electric current means the movement of electrons (charged particles) in real space. In
the allowed bands of some materials, states are partly filled, as shown in Figure 2.1a. As
the upper part of the band is energetically in continuance and has some space, acceleration
of the electric field can raise the energy a little, and electrons can rise to the upper states.
Thus, the electron distribution can move as a whole in energy space. That is, the electric
current can flow. On the contrary, in materials whose bands are filled or empty, as shown
in Figure 2.1b, the acceleration described above cannot occur. Since the energetic width of
the bandgaps prevents electrons from rising to upper empty bands, even if they try to get a
little energy from the electric field, there is no possible state to transit to within the acceler-
ated energy region. Therefore, electric current cannot flow in this case. Thus, Figure 2.1a
shows a conductor such as metal, while Figure 2.1b shows an insulator or semiconductor
at absolute zero. Insulators and semiconductors are categorized in the same group in this
respect, although the bandgap of an insulator is very wide, while that of a semiconductor
is narrow.
A band in which all states are filled with electrons is the valence band and one in which
all states are unfilled is the conductive band.
†
*
This is a conceptual expression and it is expressed more accurately as the parameter in Fermi distribution
function.
†
The energy value of the vertical axis is significant in Figure 2.1, while the horizontal one has no meaning.
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