Introduction to MOS Transistors

1
Introduction to MOS Transistors

• Section 6.1-6.4

2
Outline

• Similarity Between BJT MOS
• Introductory Device Physics
• Small Signal Model

3
BJT MOS Transistor
Chapter 4,5
Chapter 6,7
4
Analogous Devices

• Terminals
• C?D
• E ?S
• B ?G
• Analogous Devices
• NPN ?NMOS
• PNP ?PMOS

5
Similarity in the Small Signal Equivalent Circuit
6
 

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7
 
 
 
 
8
Terminal Resistance
9
 
 
10
NPN to NMOS
 
 
 
 
11
Introductory Device Physics
12
A Crude Metal Oxide Semiconductor (MOS) Device
V2 causes movement of negative charges, thus
current. V1 can control the resistivity of the
channel.
Positive charge attract negative charges to the
interface between insulator and silicon. A
conductive path is created If the density of
electrons is sufficiently high. QCV.
P-Type Silicon is slightly conductive.
The gate draws no current!
13
NMOS in 3D
(provide electrons)
(drain electrons)
W
n diffusion allows electrons move through
silicon.
14
Typical Dimensions of MOSFETs
These diode must be reversed biased.
tox is made really thin to increase C, therefore,
create a strong control of Q by V.
15
A Closer Look at the Channel Formulation
Need to tie substrate to GND to avoid current
through PN diode.
VTH300mV to 500 mV
(OFF)
(ON)
Free electrons appear at VGVTH.
Positive charges repel the holes creating a
depletion region, a region free of holes.
16
Change Drain Voltage
Resistance is determined by VG.
17
Change Gate Voltage
A higher VG leads to a lower channel resistance,
therefore a larger slope.
18
Length Dependence
fixed VD
fixed VG
The resistance of a conductor is proportional to
the length.
19
Dependence on Oxide Thickness
fixed VD
fixed VG
QCV C is inversely proportional to 1/tox. Lower
Q implies higher channel resistance.
20
Width Dependence
The resistance of a conductor is inversely
proportional to the cross section area.
A larger device also has a larger capacitance!
21
Channel Pinch Off

• QCV
• VVG-VOXIDE-Silicon
• VOXIDE-Silicon can change along the channel! Low
  VOXIDE-Silicon implies less Q.

22
VG-VD is sufficiently large to produce a channel
VG-VD is NOT sufficiently large to produce a
channel
No channel
Electrons are swept by E to drain.
Drain can no longer affect the drain current!
23
Regions
(No Dependence on VDS)
No channel
Assumption