Set of supplementary notes.

ucture ose, Gate discussed 1 of very completes low The resistivity metal
the structure.
area material, of the gate at forms the gate a capacitor is increased, with the
potential negative (see charge Figure “induced” 5). Positive incharges at the metal side
) depletion is from one region sideto spread insulating The mostly substrate, layers into the and which n-type the functions semiconductor the as Gate semiconductor 2 channel. of Figure The 1, increases ismetalof until the metal-oxide the region capacitor beneath induce the a corresponding negative
o most which cases an n-type the gates area of relatively internally is the top low connected plate; resistivity the material substrate oxide to maximize material effectively gain. and becomes For channel the are an charge n-type at the semiconductor region, side. As the positive charge
tetrode is then device diffusedcan be the same realized bottom purpose, by plate. not Gate making 1 is of very and low current resistivity can material, flow between at drain the gate and is source increased, through the the negative charge “induced” in
l ct connection. metallization allowing For the structure depletion region of Figure to spread “induced” 4, consider mostly channel. into a the positive n-type In other gatethe words, semiconductor drain current increases flow until is the region beneath the
potential channel. In
(see most Figure cases 5). the Positive gates are
“enhanced” charges internally
by at connected the gate metal potential. sideoxide effectively becomes an n-type semiconductor region,
Thus drain current flow can
2 of Figure 1, is of together. the metal-oxide A tetrode capacitor device can induce be realized
be modulated a corresponding by not making
by the gate negativeand current can flow between drain and source through the
voltage; i.e. the channel resistance
this internal connection.
“induced” channel. In other words, drain current flow is
ize gain. For the
“enhanced” by the gate potential. Thus drain current flow can
sistivity material, at the gate is increased, the negative may be changed charge “induced” to a p-channel in
be modulated device by by the reversing gate voltage; the i.e. the channel resistance
tly into the n-type the semiconductor
material types.
increases until the region beneath theis directly related to the gate voltage. The n-channel structure
rnally connected oxide effectively becomes an n-type semiconductor region, may be changed
to a p-channel device by reversing the
d by not making and current can flow between drain and source through the
material types.
“induced” channel. In other words, drain current flow is
“enhanced” by the gate potential. Thus drain current flow can
be modulated by the gate voltage; i.e. the channel resistance
is directly related to the gate voltage. The n-channel structure
may be changed to a p-channel device by reversing the
material types.
D-EFFECT TRANSISTORS (MOSFET)
MOS FIELD-EFFECT TRANSISTORS (MOSFET)
al-oxide-semiconductor (MOSFET) operates with
fferent control mechanism The than metal-oxide-semiconductor the JFET. Figure (MOSFET) operates with
he development.
The a substrate slightly different may be control highmechanism than the JFET. Figure
-type material, as for the 4 2N4351. shows the This development. time two The substrate may be high
resistivity p-type material, as for the 2N4351. This time two
w-resistivity n-type regions (source and drain) are
the
separate low-resistivity n-type regions
substrate as shown in Figure 4b. Next, the
Figure (source 3. and Junction drain) are
FET with Single-Ended Geometry
diffused into the substrate as shown in Figure 4b. Next, the
Figure 3. Junction FET with Single-Ended Geometry
he structure is covered with an insulating oxide
a
(MOSFET)
surface of the structure is covered with an insulating oxide
nitride layer. The oxide layer serves as a
bulklayer semiconductor
and a nitride layer.
crystal
The oxide
allows
layer serves
comparatively
as a
oating for the FET surface and to insulate the
simple manufacture of a JFET.
T) operates with protective coating for the FET surface and to insulate the
m the gate. However the oxide is subject to
the JFET. Figure channel from the gate. However the oxide is subject to
on e may by sodium be high ions which contamination are found by in sodium varyingions which are found in varying
n all environments. Such quantities contamination all environments. results Such contamination results
1. This time two
ce instability and drain) and are changes in in long device term characteristics.
instability and changes in device characteristics.
ure e is 4b. impervious Next, theto sodium Silicon Figure ions nitride and 3. Junction thus is impervious is used FET to with sodium Single-Ended
ions and thus
Geometry
is used
to shield the oxide layer from contamination.
e insulating oxide layer oxide from contamination. Holes are cut
Holes are cut
into the oxide and nitride layers allowing metallic contact to
de er and serves nitride as layers a allowing metallic contact to
the source and drain. Then, the gate metal area is overlaid
d and to drain. insulate Then, the the gate on metal the insulation, area is overlaid covering the entire channel region
and,
de
lation,
is subject
covering
to
the entire simultaneously, channel region metal and, contacts to the drain and
source are
found
sly, metal
in varying
contacts to the made drain as shown and source in Figure are4d. The contact to the metal
area
own
mination
in Figure
results
4d. The contact covering to the the channel metal area is the gate terminal. Note that there
e
channel
characteristics.
is the gate terminal. is no physical Note penetration that thereof the metal through the oxide and
and
al penetration
thus is used
of the metal nitride through into the substrate. oxide
andSince the drain and source are Figure 4. Development of Enhancement-Mode
the substrate. Since the isolated drain by and the source substrate,
areany drain-to-source Figure current 4. Development in the of Enhancement-Mode N-Channel MOSFET
n. Holes are cut
the substrate, any drain-to-source current in the
N-Channel MOSFET
etallic contact to
l area is overlaid
very high intrinsic source-drain resistance.
nnel region and,
and source are
o the metal area
Note that there
gh the oxide and
and source are establishes a conducting channel between source and drain.
ce current in the
nc...
Freescale Semiconductor, I
charge
8.4.
at the semiconductor side.
FIELD EFFECT TRANSISTOR is directly As the related positive to the charge gate voltage. The n-channel structure
117
Figure 8.16: Fabrication of a JFET: doping an annular p-type region into an otherwise n-type
Figure 8.17: Manufacture of an enhancement-mode n-channel MOSFET: a) p−doped substrate,
b) n−doped source and drain contacts. The depletion zones around these contacts produces
c) Insulating oxide layer (silicon nitride guards
against sodium diffusing in). d) Metallic contacts to source and drain are made through holes
which are etched into the insulating layer, the metallic gate electrode is insulated from the
substrate. Applying a positive voltage to the gate pulls electrons into the depletion zones and
Figure 4. Development of Enhancement-Mode
N-Channel MOSFET
2 For More Information MOTOROLA On This SEMICONDUCTOR Product, APPLICATION INFORMATION
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Pinch-off: At finite drain-source current, the potential in the channel changes along the
channel – it drops from drain to source. This causes the width of the depletion zone to change
along the channel. The depletion zone is be widest at the drain end, because there the potential
of the n-type channel is highest, and so the voltage between a positively charged gate and the
channel is lowest there. If we increase the drain-source voltage while keeping the gate potential
ore Information MOTOROLA On This SEMICONDUCTOR Product, APPLICATION INFORMATION
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constant, then the depletion zone will widen near the drain electrode, until it eventually covers
most of the width of the semiconductor at that point, pinching off the conducting channel.
Plotting the I − V characteristic, drain-source current I D as a function of drain-source voltage
V DS , at constant gate-source voltage V GS (Fig. 8.15) therefore shows saturation of I D at high
V DS .
Amplifier: The saturation of the drain-source current at a level depending on V GS is the
basis for operating the JFET as an amplifier: we can control the current through the device
by changing the gate voltage, without having to worry about keeping the drain-source voltage
exactly constant. Basically, the JFET acts like a voltage-controlled current source.