Enhancement type Metal Oxide Semiconductor Field Effect Transistor, Basic Operation, Drain Characteristics, Transfer Characteristics, Regions of Operation

1. Electronic Devices & Circuits
1
Dr. S. M. Gulhane, PREC, Loni

2. Field Effect Transistor
E-MOSFET:
Enhancement Mode - Metal Oxide Semiconductor
Field Effect Transistor
Dr. S. M. Gulhane
2

3. n-type Enhancement MOSFET Construction
E-MOSFET
3
➢ A p-type substrate is used as the
foundation on which the device is
constructed.
➢ The Drain (D) and Source (S)
terminals are connected through
metallic contacts to n-doped
regions.
➢ The absence of channel between
two n-doped regions is the primary
difference between depletion and
enhancement type MOSFET
➢The SiO2 layer isolate the gate
from the region between the source
and drain.
Channel
is absent

4. Basic Operation:
E-MOSFET
4
➢ when VGS=0V and VDS=0V, the
absence of channel between drain and
source will result in zero current.
➢ when VGS=0V and VDS is some positive
voltage, there exist two reverse biased
junction between the n-doped regions
and p-substrate and absence of channel
between source and drain which cause
zero current to flow.
➢ Thus, when VGS=0 and voltage VDS is
applied across drain to source terminals,
the absence of channel will result in zero
drain current as against the depletion
type MOSFET where ID=IDSS when VGS=0.

5. Basic Operation:
E-MOSFET
5
➢ When VGS and VDS are set to some
positive voltages, then the positive potential
get established at drain and the gate with
respect to the source
➢ The positive potential at gate will repel the
holes in p-type substrate along the edge of
the SiO2 layer however the electrons which is
minority carrier in p-substrate will be
attracted to the positive gate and get
accumulated in the region near the surface of
the SiO2 layer. The insulating SiO2 layer
prevent the electrons being absorbed by
positive gate.
➢ As VGS increases, the concentration of
electrons increases such that a channel is
induced between drain and source which
allow flow of electrons from drain to source
hence the flow of drain current.
+
+

6. Basic Operation:
E-MOSFET
6
➢ The level of VGS that starts flow of
current is called threshold voltage VGS(th)
or V . For N-type MOSFET it is referred to
VTN
➢ Since the channel is not in existent with
VGS=0V and enhance by the application of
positive VGS, this type of MOSFET is called
an enhancement mode MOSFET.
➢ Both depletion and enhancement type
MOSFETs have enhancement region.
➢ The depletion type MOSFETs can operate
in both depletion and enhancement regions
whereas enhancement type MOSFET can
only operate in enhancement regions.
+
+

7. Basic Operation:
E-MOSFET
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Figure a. when VGS < VTN , Figure b. when VGS => VTN ,

8. Basic Operation: Applying a small VDS
E-MOSFET
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➢ when VGS > VTN , application of a small
VDS causes a current iD to flow through an
induced channel which increases with
increase in VDS.
➢ An E-MOSFET with a small VDS is applied
acts as a resistance whose value is
determined by VGS.
▪ when VGS < VTN, iD =0, R= infinity
▪ when VGS > VTN, a channel is induced
causing flow of electrons, hence flow of iD,
making R finite
▪ as VGS increases, free electrons
increases, drain current increases and R
decreases.

9. Basic Operation:
E-MOSFET
9
➢ when VGS > VTN, a channel is induced and
application of positive VDS cause drain current
to flow.
➢ Now if you keep VGS fixed and VDS is
increased, the drain terminal becomes more
positive than gate. The charge carriers get
attracted towards drain rather getting
accumulated near SiO2 surface and hence the
charge density in the channel towards drain
decreases. Therefore, increase in VDS will
narrow down the induced channel towards
drain but the high potential at drain attract
increased number of charge carriers to flow
through narrow channel.
➢ with further increase in VDS, the drain
current will eventually reach to a saturation
level that occurs due to pinch-off process
depicted by the narrower channel..
n
n

10. Basic Operation:
E-MOSFET
10
➢ Increase in VGS will cause the pinch-off
to occur at higher value of VDS than the
earlier.
➢Thus higher the VGS, higher is the current
flow and higher is the value of VDS that
cause pinch-off or saturation condition.
➢ The saturation value of VDS is given by
➢ If VGS < VT, the drain current ID = 0 and
the MOSFET is said to be in cutoff region
➢ for VGS > VT and VDS <= VGS-VT, the
MOSFET operate in non-saturation or
triode region
➢for VGS >VT and VDS >= VGS-VT, the
MOSFET operate in saturation or pinch-off
region

11. Drain Characteristics:
E-MOSFET
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➢ In non-saturation or triode region.
VGS > VTN and VDS < VDS(sat)
➢ when VGS > VTN and VDS >
VDS(sat) =>saturation region
where Cox is the oxide capacitance
per unit area.
The capacitance is given by
Cox = Єox/tox
where tox is the oxide thickness and
Єox is the oxide permittivity.
Conduction
parameter
Cut-off region VGS < VTN

12. Drain Characteristics curves:
E-MOSFET
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➢ when VGS > VTN and for a
small values of VDS, a
complete channel from drain
to source is induced
➢ E-MOSFET acts as a resistor
whose value is determined by
VGS
➢ when VGS > VTN and VDS is
larger value but VDS < VDS(sat)
➢the induced inversion charge
density near the drain
decreases and hence the
incremental conductance of
the channel at the drain
decreases,

13. Drain Characteristics :
E-MOSFET
13
➢ when VGS > VTN and VDS
= VDS(sat), the induced
inversion charge density at
the drain terminal is zero
hence the incremental
channel conductance at the
drain is zero
➢ Pinch-off point is at drain
➢ when VGS > VTN and VDS
> VDS(sat), the point in the
channel at which the
inversion charge is zero
moves toward the source
terminal
➢ The MOSFET operate in
pinch-off/saturation region

14. Transfer Characteristics: VGS-iD characteristics
E-MOSFET
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15. p-channel Enhancement-type MOSFET :
E-MOSFET
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➢ The p-channel Enhancement-type MOSFET is similar to the n-channel
except that the voltage polarities and current directions are reversed.

16. 16
Symbol of MOSFET :
Depletion-mode Enhancement-mode
E-MOSFET
p
➢ The dashed line between the source and drain in enhancement-type
MOSFET reflects that no channel is physically constructed between source
to drain . Channel get induced when VGS > VT.

17. 17
Symbol of MOSFET :
n-channel p-channel
E-MOSFET

18. E-MOSFET
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Non-ideal Current–Voltage Characteristics of MOSFET:
There are five non-ideal effects in the current–voltage characteristics of MOSFET
transistors
I. Finite output resistance in the saturation region,
II. Body effect,
III. Sub-threshold conduction,
IV. Breakdown effects, and
V. Temperature effects