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Lecture – 19
Farzana R. Zaki
CSE 177/ EEE 177
1
MOSFET
• Construction & operation of Depletion
type MOSFET
•Plotting transfer characteristic curve
Farzana R. Zaki
CSE 177/ EEE 177
2
MOSFETs Basics
• MOSFETs (Metal Oxide Semiconductor Field
Effect Transistors) have been used in power
electronics applications since thee early 80's
due to their appreciable current carrying and offstate voltage blocking capability with low onstate voltage drop.
• They have managed to replace BJTs in many
applications due to their simpler gate drive
requirements and higher positive temperature
coefficient which allows devices to be paralleled
for higher current capabilities.
Farzana R. Zaki
CSE 177/ EEE 177
3
MOSFET Types
•N-channel enhancement type MOSFETS are the most
popular for use in power switching circuits and
applications.
Farzana R. Zaki
CSE 177/ EEE 177
4
Depletion Type MOSFET
• Depletion type MOSFET has characteristic similar to
those of a JFET between cut off and saturation at IDSS
but then has the added feature of characteristics that
extend into the region opposite polarity for VGS.
• It is also known as IGFET (Insulated Gate Field Effect
Transistor).
Farzana R. Zaki
CSE 177/ EEE 177
5
Basic Construction: n-channel depletion type MOSFET
• A slab of p-type material is formed from Si base and is
referred to as Substrate.
• Source (S) and Drain (D) terminals are connected
through metallic contacts to n-doped region linked by nchannel.
• The gate is also connected to a metal contact surface
but remains insulated from n-channel by a very thin SiO2
layer.
• SiO2 is a particular type of insulator referred to as a
dielectric that sets up opposing electric fields within
dielectric when exposed to an externally applied field.
• It is the insulating layer of SiO2 in the MOSFET
construction that for the very desirable high input
impedance of the device.
Farzana R. Zaki
CSE 177/ EEE 177
6
Farzana R. Zaki
CSE 177/ EEE 177
7
Basic Operation and Characteristics
When VGS=0V
• VGS=0V is done by direct
connection from one
terminal to other and VDS
is applied across the
drain-to-source terminal.
• The result is an attraction
of the positive potential at
the drain by the free
electrons of n-channel
and a current ID=IS=IDSS
will flow.
Farzana R. Zaki
CSE 177/ EEE 177
8
When VGS is negative:
n-channel
• When VGS is negative, the
SiO2
negative potential at the gate will
layer
tend to pressure electrons toward
p-type substrate (like charges
repel) and attracts holes from ptype substrate (opposite charges
attract) .
G• Depending on magnitude of –ve
bias established by VGS, a level of
recombination between electrons
and holes will occur that will
Metal
reduce the number of free
Contact
electrons in the n-channel
available for conduction. The
resulting level of drain current is
therefore reduced with increasing
negative bias for VGS.
Farzana R. Zaki
CSE 177/ EEE 177
e
+
e
+
e
+
e
+
e
+
e
p-material substrate
+
e
+
e
+
e
+
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Transfer Characteristic curve for n-channel
depletion type MOSFET
ID mA
ID mA
VGS = +1V
VGS
Farzana R. Zaki
CSE 177/ EEE 177
10
When VGS = +ve
• For positive values of VGS, the positive gate will
draw addition electrons (free carriers) from ptype substrate due to reverse leakage current
and establish new carriers through the collisions
resulting between accelerating particles.
• As VGS continues to increase in the positive
direction, ID will also increase at a rapid rate.
• Due to the rapid rate, the user must be aware of
maximum drain current rating since it could be
exceeded with a positive gate voltage.
Farzana R. Zaki
CSE 177/ EEE 177
11
• The application of the +ve VGS has “enhanced”
the level of free electrons in the channel
compared to that encountered with VGS=0V.
• For this reason, the region of +ve gate voltages
on drain or transfer characteristic is called
enhancement region, with the region between
cut-off and saturation level of IDSS referred to as
the depletion region.
• Shockley’s equation is applicable for depletion
type MOSFET.
I D  I DSS (1 
Farzana R. Zaki
CSE 177/ EEE 177
VGS
VP
)
2
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Example: Sketch the transfer characteristics for an nchannel depletion type MOSFET with IDSS=mA and VP=-V
Farzana R. Zaki
CSE 177/ EEE 177
13
P-channel depletion type MOSFET
• The construction of p-channel depletion type MOSFET is exactly the
reverse of that of n-channel.
• It has n-channel substrate and p-type channel.
• The terminals remains the same, but all the voltage polarities and
the current directions are reversed.
• Drain characteristic would exactly the same pattern but with VDS
having –ve , ID having +ve and VGS having opposite polarities.
Farzana R. Zaki
CSE 177/ EEE 177
14