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Introduction to MOS Transistors
Section 6.1-6.4
Outline
• Similarity Between BJT & MOS• Introductory Device Physics• Small Signal Model
BJT & MOS Transistor
[Chapter 4,5]
[Chapter 6,7]
Analogous Devices
• Terminals– “C”↔”D”– “E” ↔”S”– “B” ↔”G”
• Analogous Devices– NPN ↔NMOS– PNP ↔PMOS
Similarity in the Small Signal Equivalent Circuit
Terminal Resistance
NPN to NMOS
Introductory Device Physics
A Crude Metal Oxide Semiconductor (MOS) Device
P-Type Silicon is slightly conductive.
Positive charge attractnegative chargesto the interface between insulator and silicon.
A conductive path is createdIf the density of electrons is sufficiently high.Q=CV.
V2 causes movement of negative charges,thus current.
V1 can control the resistivity of the channel.The gate
draws no current!
NMOS in 3D
n+ diffusion allowselectrons movethrough silicon.
(provide electrons) (drain electrons)
W
Typical Dimensions of MOSFETs
These diode mustbe reversed biased.tox is made really thin
to increase C, therefore, create a strong control of Q by V.
A Closer Look at the Channel Formulation
Need to tie substrate to GNDto avoid current through PN diode.
Positive charges repel the holescreating a depletion region, a region free of holes.
Free electrons appear at VG=VTH.
VTH=300mV to 500 mV(OFF) (ON)
Change Drain Voltage
Resistance is determined by VG.
Change Gate Voltage
A higher VG leads to a lower channel resistance, therefore a larger slope.
Length Dependence
The resistance of a conductor is proportional to the length.
fixed VD
fixed VG
Dependence on Oxide Thickness
Q=CVC is inversely proportional to 1/tox.
Lower Q implies higher channel resistance.
fixed VD fixed VG
Width Dependence
The resistance of a conductor is inversely proportional to the crosssection area.
A larger device also has a larger capacitance!
Channel Pinch Off• Q=CV– V=VG-VOXIDE-Silicon
• VOXIDE-Silicon can change along the channel! Low VOXIDE-Silicon implies less Q.
VG-VD is sufficiently largeto produce a channel
VG-VD is NOT sufficiently largeto produce a channel
No channel
Electronsare sweptby E to drain.
Drain can no longer affect the drain current!
Regions
No channel
(No Dependence on VDS)
Assumption:
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