EEE 329: VLSI I
MOS TRANSISTOR THEORY
LECTURE 2
AGENDA
• MOSFET
• TYPES of MOSFET
• MODE of Operation
• Region of Operation
• The MOSFET (metal oxide semiconductor field effect transistor) transistor is
a semiconductor device that is widely used for switching purposes and for
the amplification of electronic signals in electronic devices.
• A MOSFET is either a core or integrated circuit where it is designed and
fabricated in a single chip because the device is available in very small
sizes.
WHAT IS • A MOSFET is a four-terminal device having source(s), gate (g), drain (d)
and body (b) terminals.
MOSFET? • In general, the body of the MOSFET is in connection with the source
terminal thus forming a three-terminal device such as a field-effect
transistor.
TYPES OF MOSFET
MOSFET
NMOS PMOS
Enhancement Type
Depletion Type
ENHANCEMENT TYPE NMOS
DEPLETION TYPE NMOS
ENHANCEMENT TYPE PMOS
MODE OF OPERATION (ACCUMULATION)
A negative voltage is applied to the gate, so there is negative charge on the gate. The mobile
positively charged holes are attracted to the region beneath the gate. This is called the accumulation
mode.
MODE OF OPERATION (DEPLETION)
A small positive voltage is applied to the gate, resulting in some positive charge on the gate. The holes in
the body are repelled from the region directly beneath the gate, resulting in a depletion region forming
below the gate.
MODE OF OPERATION (INVERSION)
A higher positive potential exceeding a critical threshold voltage Vt is applied, attracting more positive
charge to the gate. The holes are repelled further and some free electrons in the body are attracted to the
region beneath the gate. This conductive layer of electrons in the p-type body is called the inversion layer.
REGION OF OPERATION (CUTOFF)
The gate-to-source voltage Vgs is less than the threshold voltage. The source and drain have free electrons.
The body has free holes but no free electrons. Suppose the source is grounded. The junctions between the
body and the source or drain are zero-biased or reverse-biased, so little or no current flows. We say the
transistor is OFF, and this mode of operation is called cutoff.
REGION OF OPERATION (LINEAR)
REGION OF OPERATION (LINEAR)
• The gate voltage is greater than the threshold voltage. Now an inversion region of electrons
(majority carriers) called the channel connects the source and drain, creating a conductive
path and turning the transistor ON. The number of carriers and the conductivity increases with
the gate voltage. The potential difference between drain and source is Vds = Vgs -Vgd . If
Vds = 0 (i.e., Vgs = Vgd), there is no electric field tending to push current from drain to
source.
• When a small positive potential Vds is applied to the drain, current Ids flows through the
channel from drain to source. This mode of operation is termed linear, resistive, triode,
nonsaturated, or unsaturated; the current increases with both the drain voltage and gate
voltage.
REGION OF OPERATION (SATURATION)
If Vds becomes sufficiently large that Vgd < Vt , the channel is no longer inverted near the drain and
becomes pinched off (Figure 2.3(d)). However, conduction is still brought about by the drift of electrons under
the influence of the positive drain voltage. As electrons reach the end of the channel, they are injected into
the depletion region near the drain and accelerated toward the drain. Above this drain voltage the current
Ids is controlled only by the gate voltage and ceases to be influenced by the drain. This mode is called
saturation.
CONTACT DETAILS
Dr. Md. Hasan Maruf
Associate Professor, EEE, GUB
Email: maruf@eee.green.edu.bd
