Environmental Engineering Reference
In-Depth Information
Specifically, their simple realization and low cost, the possibility of having a
complementary technology with the same characteristics for both
complementary devices, their small geometry and, consequently, the
feasibility of integrating a large number of devices in a small area, their
infinite input resistance at the gate terminal and the faculty of building
digital cells with no static dissipation, all motivate the great success of MOS
transistors in modern technologies.
A simplified cross section of an n-channel MOS (n-MOS) transistor is
shown in Fig. 1.5. It is built on a lightly doped p type substrate (p-) that
separates two heavily doped n type regions (n+) called source and drain. A
dielectric of silicon oxide and a polysilicon gate are grown over the
separation region. The region below the oxide is the transistor channel and
its length, that is the length that separates the source and the drain, is the
channel length, denoted by
L.
In present MOS technologies the channel
length is typically between
and
In a p-channel MOS (p-
MOS) all the regions are complementary doped.
There is no physical difference between the source and the drain as the
device is symmetric, the notations source and drain only depend on the
voltage applied. In an n-MOS the source is the terminal at the lower
potential while, in a p-MOS, the source is the terminal at the higher
potential.
1.3.1
Basic Operation
To understand the basic operation of MOS transistors we shall analyze
the behavior of an n-MOS depending on the voltages applied at its terminals.
If source, drain and substrate are grounded, the device works as a
capacitor. Specifically, the gate and the substrate above the
interface
are two plates electrically insulated by the silicon oxide.
