UNIT 1
MOS TRANSISTOR PRINCIPLES
MOS logic families (NMOS and CMOS), Ideal
and Non Ideal IV Characteristics, and CMOS
devices. MOS(FET) Transistor
Characteristics under Static and Dynamic
Conditions, Technology Scaling, power
consumption
INTRODUCTION
• Transistor was invented by William B Schockley, Walter H Brattain and
John Bardeen in 1947 at Bell Laboratories.
• Jack Kilby made the first IC in July 1958 with germanium material in
Texas Instruments.
• MOS transistor was invented by Mohamed M Atalia and Dawon
Kohng in 1959.
• Robert Noyce invented the first Monolithic IC by using silicon in the
year 1959.
TYPES OF ICs
APPLICATIONS OF IC
• Voice and data communication networks
• Digital signal processing
• Computers
• Commercial Electronics
• Automobiles
• Medicine etc
The MOSFET – Metal Oxide Semiconductor FET
• Also known as Insulated Gate Field Effect Transistor or IGFET.
• “Metal Oxide” Gate electrode which is electrically insulated
from the main semiconductor n-channel or p-channel by a
very thin layer of insulating material usually silicon dioxide.
Characteristics of MOSFET
• Bilaterally Symmetric device
• Unipolar device
• High Input Impedance
• Voltage Controlled
• Self Isolated
MOS Transistor Types and Construction :
• MOSFETs are three terminal devices with a Gate, Drain and Source
and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are
available.
• n-Channel MOS : Majority carriers are electrons.
• p-Channel MOS : Majority carriers are holes.
• Positive/negative voltage applied to the gate (with respect to
substrate) enhances the number of electrons/holes in the channel
and increases conductivity between source and drain.
• Vt defines the voltage at which a MOS transistor begins to conduct.
For voltages less than V t (threshold voltage), the channel is cut off.
MOSFETs are available in two basic forms:
1. Depletion Type – the transistor requires the Gate-Source voltage,
( VGS ) to switch the device “OFF”. The depletion mode MOSFET is
equivalent to a “Normally Closed” switch.
2. Enhancement Type – the transistor requires a Gate-Source
voltage, ( VGS ) to switch the device “ON”. The enhancement
mode MOSFET is equivalent to a “Normally Open” switch.
• The construction of the Metal Oxide Semiconductor FET is very different to
that of the Junction FET.
• Both the Depletion and Enhancement type MOSFETs use an electrical field
produced by a gate voltage to alter the flow of charge carriers, electrons for
n-channel or holes for P-channel, through the semiconductive drain-source
channel.
• The gate electrode is placed on top of a very thin insulating layer and there
are a pair of small n-type regions just under the drain and source electrodes.
• The gate of a junction field effect transistor, JFET must be biased in such a
way as to reverse-bias the pn-junction.
• With a insulated gate MOSFET device no such limitations apply so it is
possible to bias the gate of a MOSFET in either polarity, positive (+ve) or
negative (-ve).
• This makes the MOSFET device especially valuable as electronic switches or
to make logic gates because with no bias they are normally non-conducting
and this high gate input resistance means that very little or no control current
is needed as MOSFETs are voltage controlled devices.
• Both the p-channel and the n channel MOSFETs are available in two basic
forms, the Enhancement type and the Depletion type.
Enhancement-mode MOSFET:
• The more common Enhancement-mode MOSFET or eMOSFET, is the reverse
of the depletion-mode type.
• Here the conducting channel is lightly doped or even undoped making it non-
conductive.
• This results in the device being normally “OFF” (non-conducting) when the gate
bias voltage, VGS, equals zero.
• The circuit symbol above for an enhancement MOS transistor uses a broken
channel line to signify a normally open non-conducting channel.
