Unit –II

MOSFET MOSFET
A MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

is a semiconductor device.

B.tech-I st YR (Branches) A MOSFET is most commonly used in the field of power electronics.
Electronics Engg.(FEC
A semiconductor is made of manufactured material that acts neither
201/101)
like a insulator nor a conductor.

Types
by
Er. R. S. Pathak,
Electrical & Electronics Engineering Dept.
Eshan College Of engineering

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Schematic structure of
BJT Vs MOSFET
MOSFET
MOSFET BJT

is controlled by the input gate is controlled by the input

Output Current
voltage. base current.

Cost More Expensive Lower Cost

Easily damaged by ESD

ESD Risk ESD is rarely a problem
Electrostatic Discharge.
Switching Speed
Faster than Bipolar Slower than MOSFETs.
and Frequency
Better frequency response Inferior frequency response.
Response

When bipolar transistors heat

up, the gain increases and so
the current through them
When MOSFETS heat up, the
increases too. This in turn
current flowing through them
causes further heating and
Thermal Runaway decreases. They are less likely
yet more gain and current.
to be destroyed by
This can cause catastrophic
overheating.
failure called thermal
runaway.

Very high current gain which

Lower current gain and it is
Gain is nearly constant for varying
not constant.
drain currents.

The MOSFET – Depletion

JFET Vs MOSFET MOSFET
• JFETs can only be operated in the depletion mode whereas Depletion Mode With a negative gate voltage, the negative charges on the gate repel
conduction electrons from the channel, leaving positive ions in their place. Thereby, the n
MOSFETs can be operated in either depletion or in channel is depleted of some of its electrons, thus decreasing the channel conductivity. The
enhancement mode. greater the negative voltage on the gate, the greater the depletion of n-channel electrons. At
sufficiently negative gate-to-source voltage, VGS(off), the channel is totally depleted and
drain current is zero.

In a JFET, if the gate is forward biased, excess- carrier injunction

occurs and the gate current is substantial. Thus channel
conductance is enhanced to some degree due to excess carriers
but the device is never operated with gate forward
biased because gate current is undesirable. Enhancement Mode With a
• MOSFETs have input impedance much higher than that of positive gate voltage, more
JFETs. This is due to negligibly small leakage current. conduction electrons are attracted
• JFETs have characteristic curves more flat than those of into the channel, thus increasing
MOSFETs indicating a higher drain resistance. (enhancing) the channel
• When JFET is operated with a reverse bias on the junction, conductivity.
the gate current IG is larger than it would be in a D-MOSFET schematic symbols.
comparable MOSFET.
• For the above reasons, and also because MOSFETs are
somewhat easier to manufacture,
Source
they are more widely used than are the JFET

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Depletion Mode
CONSTRUCTION OF N CHANNEL
MOSFET Construction DEPLETION TYPE MOSFET
• Two highly doped N regions are diffused into a lightly doped p
type substrate that may have an additional terminal connection
called SS
• The two highly doped n regions represent source and drain
connected via an n- channel. The N-channel is formed by diffusion
between the source and drain. The type of impurity for the channel
is the same as for the source and drain.
• Metal is deposited through the holes to provide drain and source
terminals, and on the surface area between drain and source, a
metal plate is deposited. This layer constitutes the gate. The n-
channel is connected to the Gate (G) via a thin insulating layer of
SiO2.
• The thin layer of SiO2 dielectric is grown over the entire surface and
holes are cut through the SiO2 (silicon-dioxide) layer to make
contact with the N-type blocks (Source and Drain).

OPERATION OF N CHANNEL
DEPLETION MOSFET
This SiO2 layer acts as a
A D-MOSFET may be biased to dielectric that sets up
operate in two modes: the opposing electric fields within
Depletion mode or the the dielectric when exposed to
an externally applied field.
Enhancement mode “There is no direct electrical
• When VGS =0 and drain is made positive with respect to connection between the gate
source current(in the form of free electrons) can flow terminal & the channel of a
between source and drain, even with zero gate potential MOSFET” “ It is the insulating
and the MOSFET is said to be operating in Enhancement layer of SiO2 in the MOSFET
mode. In this mode of operation gate attracts the charge
carriers from the P-substrate to the N-channel and concentration that accounts
thus reduces the channel resistance which increases the for very desirable high input
impedance of the device”.
drain-current.
• When VGS = negative with respect to the substrate, the
gate repels some of the negative charge carriers out of
the N-channel ,and attracts holes from the p type
substrate .This initiates recombination of holes and
electrons. This creates a depletion region in the channel
and, therefore, reduces the number of free electrons in
the n channel, increases the channel resistance and
reduces the drain current. The more negative the gate,
the less the drain current. In this mode of operation the
device is referred to as a depletion-mode MOSFET. Here
too much negative gate voltage can pinch-off the
channel.
• The more positive the gate is made, more number of
electrons from p substrate due to reverse leakage current
and collisions between accelerating particles the more
drain current flows.

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 Working drain characteristics

Case-1- VGS = 0 V and VDS > 0 the result is an

attraction for the positive potential at the drain
by the free electrons of the N-channel and a
current similar to that established through the
channel of the JFET.

Case-2- VGS = -ve and VDS > 0 In this case,

electrons move towards P-type substrate &
attracts holes from P- type substrate.
Depending on the magnitude of the negative bias
established by VGS, a level of recombination
between electrons and holes will occur that will
reduce the number of free electrons in the N-
channel available for conduction. The resulting
level of drain current is therefore reduced with
increasing negative bias for VGS.
Case-3- VGS = +ve and VDS > 0 For +ve value of
VGS , the positive gate will draw additional
electrons (free carriers ) from the p-type
substrate due to the reverse leakage current and 14
established new carriers through the collisions
resulting between accelerating particles.

drain characteristics
• For VGS exceeding zero the device operates in The transfer characteristics are similar
to the JFET
enhancement mode. In Depletion Mode operation:
When VGS = 0V, ID = IDSS
• These drain curves again display an ohmic region, When VGS < 0V, ID < IDSS
a constant-current source region and a cut-off When VGS > 0V, ID > IDSS
Enhancement Mode operation
region. In this mode, the transistor operates with
• For a specified drain-source voltage VDS, VGS (OFF) VGS > 0V, and ID increases above IDSS
Shockley’s equation,
is the gate-source voltage at which drain current the formula used to plot the Transfer Curve
reduces to a certain specified negligibly small still applies but VGS is positive:
value,
• For VGS between VGS (0FF) (-ve value)and zero, the  VGS 2
ID = IDSS 1 -
device operates in depletion-mode  VP 

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Drain & Transfer Characteristics of N- Drain & Transfer Characteristics of P-
Channel Depletion type MOSFT Channel Depletion type MOSFT

Depletion (D) Type

P-Channel D- MOSFET MOSFET

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2- Enhancement type
MOSFET
Working
Case-1- VGS= 0 V If VGS is set at 0 V and a
voltage applied between the drain and source of
N-Channel- the N-channel
the device, the absence of an N-channel will
enhancement type
result in a current (ID) effectively 0 A. Case- 2-
MOSFET consists of a
When both VGS > 0 V and VDS> 0 V when both
lightly doped P-type
VGS and V DS have been set at positive voltage
substrate into which two
highly doped N-regions are greater than 0V., establishing the drain and gate
diffused. The absence of a at a positive potential with respect to the source.
channel between the two As the positive potential is applied between gate
N-doped region. & source due to the presence of SiO2 layer which
acts as a dielectric, is attracts the charge carriers
(electrons) from substrate. As VGS increases in
magnitude the concentration of electrons near
the SiO2 surface increases until eventually the
induced N-type region can support a measurable
flow between drain & source,
resulting the formation of inversion layer, this
inversion layer. This inversion layer is formed
when a certain gate to source voltage (VGS) is
applied. Thus, the minimum value of gate voltage
at which inversion of semiconductor surface takes
place is known as threshold voltage (VT).

Drain and Transfer characteristics of N-

Transfer characteristics Channel Enhancement type MOSFET

Since the channel is no-existent with VGS= 0V

and “enhanced” by the application of a positive
gate to source voltage. This type of MOSFET is
called Enhancement type MOSFET. Drain and
Transfer characteristics of N-Channel
Enhancement type MOSFET. r levels of VGS > VT
the drain current is related to the applied gate
to source voltage by the following non-
relationship. ID = K (VGS- VT)2 Where VGS
=applied gate to source voltage VT = Threshold
Voltage ID = Drain Current K = Constant The
values of K can be determined from the
following equation. ID(on) & VGS (on) are the
values for each at a particular part on the
characteristics of the device.

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 E-MOSFET Transfer Characteristic p-Channel Enhancement Mode
Characteristics and Parameters –
MOSFETs
The E-MOSFET for all practical purposes does not conduct until VGS reaches the
threshold voltage (VGS(th)). ID when it is when conducting can be determined by the
The p-channel Enhancement mode MOSFET is similar
formulas below. The constant K must first be determined. ID(on) is a data sheet given to the n-channel except that the voltage polarities and
value.
current directions are reversed.
K = ID(on) /(VGS - VGS(th))2
ID = K(VGS - VGS(th))2

An n-channel device requires a

positive gate-to-source voltage, and
a p-channel device requires a
negative gate-to-source voltage.

E-MOSFET general transfer

characteristic curves.
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Symbols: Enhancement Type MOSFET Summary Table

JFET D-MOSFET E-MOSFET

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Important Points for FET Biasing