metal oxide semiconductor Field Effect Transistors.ppt

Metal oxide semiconductor field effect transistors

1. MOSFET V-I Characteristics
Vijaylakshmi.B
Lecturer, Dept of Instrumentation Tech
Basaveswar Engg. College
Bagalkot, Karnataka
IUCEE-VLSI Design, Infosys, Mysore

2. Types of Transistors

3. MOSFET (Types)
Four types:
n-channel enhancement mode
•Most common since it is cheapest to manufacture
p-channel enhancement mode
n-channel depletion mode
p-channel depletion mode
Depletion type
n-channel p-channel
Enhancement type
n-channel p-channel

4. MOSFET
FET = Field-Effect Transistor
A four terminal device (gate, source, drain,
bulk)
Symbols of
MOSFET

5. MOSFET characteristics
Basically low voltage device. High voltage
device are available up to 600V but with limited
current. Can be paralleled quite easily for higher
current capability.
Internal (dynamic) resistance between drain and
source during on state, R
DS(ON), , limits the
power handling capability of MOSFET. High
losses especially for high voltage device due to
R
DS(ON) .
Dominant in high frequency application
(>100kHz). Biggest application is in switched-
mode power supplies.

6. The transistor consists of three regions, labeled the
``source'', the ``gate'' and the ``drain''.
The area labeled as the gate region is actually a
``sandwich'' consisting of the underlying substrate
material, which is a single crystal of semiconductor
material (usually silicon); a thin insulating layer
(usually silicon dioxide); andan upper metal layer.
Electrical charge, or current, can flow from the
source to the drain depending on the charge applied
to the gate region.
The semiconductor material in the source and drain
region are ``doped'' with a different type of material
than in the region under the gate, so an NPN or PNP
type structure exists between the source and drain
region of a MOSFET.

7. •Most important device in digital design
•Very good as a switch
•Relatively few parasitics
•Rather low power consumption
•High integration density
•Simple manufacturing
•Economical for large complex circuits

8. n-Channel MOSFET

9. NMOS Structure
MOS (Metal-Oxide-Semiconductor) Nowadays gate is made of poly-silicon
?????? Channel length L and width W
??????

In most digital design, L is set at the minimum feature size
??????

W is selectable by the designer
?????? Bulk is connected to the Gnd in NMOS to prevent forward-biased PN junction
On state
Off state

10. n-MOSFET Characteristics
Plots V-I characteristics
of the device for various
Gate voltages (VGS)
At a constant value of VDS , we can
also see that IDS is a function of the
Gate voltage, VGS
The transistor begins to conduct
when the Gate voltage, VGS , reaches
the Threshold voltage: VT

11. PMOS Structure
PMOS transistor has a negative threshold voltage (Vtp) -0.3v~-1.2v
A pMOS turns on when Vgs<Vtp

12. The terminal characteristics of the device are given by
drain-to-source current Ids against drain-to-source
voltage Vds for different values of gate-to-source
voltage Vgs. All voltages are referenced with respect
to the source voltage, which is assumed to be at
ground potential.
P-MOSFET Characteristics

13. Switch models of MOSFETs
g
s
d
g = 0
s
d
g = 1
s
d
g
s
d
s
d
s
d
nMOS
pMOS
OFF
ON
ON
OFF

14. Thank You