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Digital Electronics
Class Lecture
October 22, 2008
MOS Field-Effect Transistors (MOSFETs)
Today We Will Discuss Following Topics:
4.1 Device Structure and Physical Operation
4.2 Current-Voltage Characteristics
4.3 MOSFET Circuits at DC
2
Introduction
• Three Terminal Device – (as seen in BJT) the
Control Signal used on Two terminals can cause
Current to Change on Third from Zero to Hi
value (switch) – Similar concept for MOSFET
• Switch is Basis for Logic Inverter – basic
Element of Digital Ckts/Electronics
• MOSFET can be made Smaller than BJT, and
Manf. Process is simple and Require Lower P
• > 200 Mil MOSFETs on Single Chip
• Can be used as Amp and Dig Logic Inverter
Typically L = 0.1 to 3 mm, W = 0.2 to 100 mm, and
thickness of the oxide layer (tox) = 2 to 50 nm.
4.1.2 No Gate Voltage Operation
• Two Back-to-Back Diodes in Series Between
Drain and Source
• Prevent Current Flow between Drain and
Source when vDS is Applied
• The path between Drain and Source has very
High Resistance in the Order of 1012 Ω
4.1.3 Creating a Channel for Current Flow
Positive Voltage to gate Repels (Pushes) Holes Down in
Substrate and Attracts Electron. When sufficient number
of electrons gather under Gate an n-region is created
thereby an n channel is INDUCED under the gate. Now if
vDS is applied current will flow in n-region (n-Channel)
Note …
• Because of n-channel This MOSFET is called NMOS
Transistor
• n-Channel (or Inversion Layer) created by inverting the ptype sub to n-type with application of Gate Voltage (Field)
• The value of vGS at which conducting channel is formed is
called Vt (Positive for n)
• Vt is controlled during fabrication process and typically
range from 0.5 to 1 V
• Gate and Channel form parallel plate capacitor (oxide as
dielectric) creating the E filed in Vertical direction
• E field controls the Charge in Channel and thus determines
the conductivity of channel
4.1.4 Applying a Small vDS
When a Small vDS (50 mV) is Applied along with vGS > Vt
NMOS acts as a resistor whose value is determined by vGS
Specifically, the channel conductance is proportional to
Excess Gate Voltage (vGS – Vt), and thus iD is proportional to
(vGS – Vt) vDS. (Depletion region not shown)
The iD–vDS characteristics of the MOSFET When
voltage applied between drain and source, vDS, is kept small,
device operates as a linear resistor whose value is controlled
by vGS. The increase of vGS above Vt Enhances the Channel,
Hence Enhancement-mode or Enhancement-type MOSFET
4.1.5 Operation as vDS is increased. vGS is kept
constant at a value > Vt vDS appears as voltage drop across
the channel L (voltage increases from 0 to vDS from S to D).
Thus voltage between gate and points along channel
decreases from vGS to vGS – vDS. Thus induced channel
acquires a tapered shape, and its resistance increases as vDS is
increased. And iD - vDS curve no longer straight line but
bends – Next Figure
When vDS increases to value so that voltage between gate and
drain side of channel reaches Vt – vGD = Vt or vGS – vDS = Vt
or vDS = vGS – Vt the channel depth at drain end is almost zero
(or channel is pinched off) Increasing vDS beyond this has no
effect on channel shape and iD stays constant
Eventually, as vDS reaches vGS – Vt’ the channel is pinched
off at the drain end. Increasing vDS above vGS – Vt has no
effect on the channel’s shape and current saturates and
MOSFET has entered Saturation Region. That is, vDSsat
= vGS – Vt
Note That … For every value of vGS > Vt there is
corresponding vDSsat. Device in Saturation if vDS ≥ vDSsat
4.1.6 Derivation of the iD–vDS Relationship
How Boolean Logic Works
http://computer.howstuffworks.com/boolean.htm
Have you ever wondered how a computer can
do something like balance a check book, or
play chess, or spell-check a document? These
are things that, just a few decades ago, only
humans could do. Now computers do them
with apparent ease. How can a "chip" made up
of silicon and wires do something that seems
like it requires human thought?
If you want to understand the answer to this
question down at the very core, the first thing
you need to understand is something called
Boolean logic.
Boolean logic, originally developed by George
Boole in the mid 1800s, allows quite a few
unexpected things to be mapped into bits and
bytes. The great thing about Boolean logic is that,
once you get the hang of things, Boolean logic (or
at least the parts you need in order to understand
the operations of computers) is outrageously
simple. In this edition of HowStuffWorks we will
first discuss simple logic "gates," and then see
how to combine them into something useful
Important Links
• 1.http://jas.eng.buffalo.edu/education/pn/c
v/index.html#
• 2.http://jas.eng.buffalo.edu/education/semi
con/diffusion/diffusion.html
• 3.http://jas.eng.buffalo.edu/education/mos/
mosfet/mos_0.html