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The n-type Metal-Oxide-Semiconductor
Field-Effect-Transistor (MOSFET) consists of a source and
a drain, two highly conducting n-type semiconductor regions
which are isolated from the p-type substrate by reversed-biased
p-n diodes. A metal (or poly-crystalline) gate covers the
region between source and drain, but is separated from the
semiconductor by the gate oxide. The basic structure of
an n-type MOSFET and the corresponding circuit symbol are
shown in figure 7.1.1.
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Fig.7.1.1
Crosssection and circuit symbol of an n-type Metal-Oxide-Semiconductor-Field-Effect-Transistor
(MOSFET) |
| As can be seen on the figure the source and drain
regions are identical 1. It is the applied voltages which determine
which n-type region provides the electrons and becomes the source,
while the other n-type region collects the electrons and becomes
the drain. The voltages applied to the drain and gate electrode
as well as to the substrate by means of a back contact are refered
to the source potential, as also indicated on the figure. |
| A top view of the same MOSFET is
shown in Fig. 7.1.2, where the gate length, L, and gate width,
W, are identified. Note that the gate length does not equal
the physical dimension of the gate, but rather the distance
between the source and drain regions underneath the gate. The
overlap between the gate and the source and drain region is
required to ensure that the inversion layer forms a continuous
conducting path between the source and drain region. Typically
this overlap is made as small as possible in order to minimize
its parasitic capacitance. |
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| Fig.7.1.2 Top
view of an n-type Metal-Oxide-Semiconductor- Field-Effect-Transistor
(MOSFET) |
| The flow of electrons from the
source to the drain is controlled by the voltage applied to
the gate. A positive voltage applied to the gate, attracts electrons
to the interface between the gate dielectric and the semiconductor.
These electrons form a conducting channel between the source
and the drain, called the inversion layer. No gate current is
required to maintain the inversion layer at the interface since
the gate oxide blocks any carrier flow. The net result is that
the current between drain and source is controlled by the voltage
which is applied to the gate. |
| The typical current versus voltage
(I-V) characteristics of a MOSFET are shown in the figure below.
Implemented is the quadratic model for the MOSFET. |
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| Fig.7.1.3
I-V characteristics of an n-type MOSFET with VG = 5 V (top curve),
4 V, 3 V and 2 V (bottom curve) |
NOTE: We will primarily discuss the n-type
or n-channel MOSFET. This type of MOSFET is fabricated on
a p-type semiconductor substrate. The complementary MOSFET
is the p-type or p-channel MOSFET. It contains p-type source
and drain regions in an n-type substrate. The inversion
layer is formed when holes are attracted to the interface
by a negative gate voltage. While the holes still flow from
source to drain, they result in a negative drain current.
CMOS circuits require both n-type and p-type devices.
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A conceptually similar structure was first
proposed and patented by Lilienfeld and Heil2 in 1930, but
was not successfully demonstrated until 1960. The main technological
problem was the control and reduction of the surface states
at the interface between the oxide and the semiconductor.
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| Initially it was only possible to
deplete an existing n-type channel by applying a negative voltage
to the gate. Such devices have a conducting channel between
source and drain even when no gate voltage is applied and are
called "depletion-mode" devices.
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| A reduction of the surface states
enabled the fabrication of devices which do not have a conducting
channel unless a positive voltage is applied. Such devices are
refered to as "enhancement-mode" devices. The electrons
at the oxide-semiconductor interface are concentrated in a thin
(~10 nm thick) "inversion" layer. By now, most MOSFETs
are "enhancement-mode" devices. |
While a minimum requirement for amplification
of electrical signals is power gain, one finds that a device
with both voltage and current gain is a highly desirable
circuit element. The MOSFET provides current and voltage
gain yielding an output current into an external load which
exceeds the input current and an output voltage across that
external load which exceeds the input voltage.
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| The current gain
capability of a Field-Effect-Transistor (FET) is easily explained
by the fact that no gate current is required to maintain the
inversion layer and the resulting current between drain and
source. The device has therefore an infinite current gain in
DC. The current gain is inversely proportional to the signal
frequency, reaching unity current gain at the transit frequency.
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| The voltage gain
of the MOSFET is caused by the fact that the current saturates
at higher drain-source voltages, so that a small drain current
variation can cause a large drain voltage variation. |
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