MEMS1082
Chapter 3-2 Field Effect Transistors MOSFET
Department of Mechanical Engineering
MOSFET
 Junction
Field Effect Transistor
 Insulated Gate Field Effect Transistor, in which
Gate input is electrically insulated from the main
current carrying channel and is therefore called
IGFET.
 The most common type of insulated gate FET or
IGFET as it is sometimes called, is the Metal Oxide
Semiconductor Field Effect Transistor or
MOSFET for short.
Department of Mechanical Engineering
MOSFET
MOSFET is one of the cornerstones of modern semiconductor technology. The general structure
is a lightly doped p-type substrate, into which two regions, the source and the drain, both of
heavily doped n-type semiconductor have been embedded. The symbol n+ is used to denote this
heavy doping.
The source and the drain are about 1 μm apart. Metallized contacts are made to both source and
drain, generally using aluminium. The rest of the substrate surface is covered with a thin oxide
film, typically about 0.05 μm thick. The gate electrode is laid on top of the insulating oxide layer,
and the body electrode in the above diagram provides a counter electrode to the Gate. The thin
oxide film contains silicon dioxide (SiO2), but it may well also contain silicon nitride (Si3N4) and
silicon oxynitride (Si2N2O).
The p-type doped substrate is only very lightly doped, and so it has a very high electrical
resistance, and current cannot pass between the source and drain if there is zero voltage on the
gate. Application of a positive potential to the gate electrode creates a strong electric field across
the p-type material even for relatively small voltages, as the device thickness is very small and
the field strength is given by the potential difference divided by the separation of the gate and
body electrodes.
When the gate electrode is positively charged, it will therefore repel the holes in the p-type
region. For high enough electrical fields, the resulting deformation of the energy bands will cause
the bands of the p-type region to curve up so much that electrons will begin to populate the
conduction band.
Department of Mechanical Engineering
MOSFET
This isolation of the
controlling gate makes the
input resistance of the
MOSFET extremely high in
the Mega-ohms region and
almost infinite.
The metal oxide semiconductor field effect
transistor (MOSFET)
Department of Mechanical Engineering
MOSFET
 As
the gate terminal is isolated from the main
current carrying channel "NO current flows into
the gate" and like the JFET, the MOSFET also
acts like a voltage controlled resistor. Also like the
JFET, this very high input resistance can easily
accumulate large static charges resulting in the
MOSFET becoming easily damaged unless
carefully handled or protected.
Department of Mechanical Engineering
MOSFET
Department of Mechanical Engineering
MOSFET
 The
P-channel and the N-channel MOSFET is
available in two basic forms,
 Enhancement type : Normally switched “off”
 Depletion type. Normally switched "ON"
without a gate bias voltage but requires a gate
to source voltage (Vgs) to switch the device
"OFF".
Department of Mechanical Engineering
Depletion-mode MOSFET and circuit Symbols

Depletion-mode MOSFET's are constructed similar to their JFET transistor
counterparts where the drain-source channel is inherently conductive with
electrons and holes already present within the N-type or P-type channel. This
doping of the channel produces a conducting path of low resistance between the
drain and source with zero gate bias.
Department of Mechanical Engineering
Enhancement-mode MOSFET

The Enhancement-mode MOSFET is the reverse of the depletionmode type. The conducting channel is lightly doped or even
undoped making it non-conductive. This results in the device
being normally "OFF" when the gate bias voltage is equal to zero.
Department of Mechanical Engineering
MOSFET
Measured Characteristic Curves for 2N7000
for VG = 2.5 V, 2.75 V, 3 V, ...., 4 V
Department of Mechanical Engineering
MOSFET Summary
MOSFET type
Vgs = +ve
Vgs = 0
Vgs = -ve
N-Channel Depletion
ON
ON
OFF
N-Channel Enhancement
ON
OFF
OFF
P-Channel Depletion
OFF
ON
ON
P-Channel Enhancement
OFF
OFF
ON
Department of Mechanical Engineering
Enhancement MOSFET
Department of Mechanical Engineering
Enhancement MOSFET as a Switch


Enhancement-mode MOSFET's make excellent
electronics switches due to their low "ON" resistance
and extremely high "OFF" resistance and extremely
high gate resistance.
Enhancement-mode MOSFET's are used in integrated
circuits to produce CMOS type logic gates and power
switching circuits as they can be driven by digital logic
levels.
Department of Mechanical Engineering
Enhancement MOSFET as a Switch

Example:
MOSFET Power
Switch
If the resistive load of the
lamp was to be replaced by
an inductive load such as a
coil or solenoid, a
"Flywheel" diode would be
required in parallel with the
load to protect the MOSFET
from any back-emf.
Department of Mechanical Engineering
Enhancement MOSFET as a Switch

Example:
MOSFET
Analog
Switch
Department of Mechanical Engineering
Enhancement MOSFET as a Switch

Example:
Circuit to
switch Power
Open-collector
circuit
npn transistor
Department of Mechanical Engineering
Enhancement MOSFET as a Switch

Example:
Simple Power
MOSFET Motor
Controller
Department of Mechanical Engineering