Transcript CH5
Chap. 5 Field-effect transistors (FET)
•Importance for LSI/VLSI
–Low fabrication cost
–Small size
–Low power consumption
•Applications
–Microprocessors
–Memories
–Power Devices
•Basic Properties
–Unipolar device
–Very high input impedance
–Capable of power gain
–3/4 terminal device, G, S, D, B
–Two possible device types: enhancement
mode; depletion mode
–Two possible channel types: n-channel; pchannel
5-1
MOSFET Structure
Physical structure of a n-channel device:
Typically L = 0.35 to 10 m, W = 2 to 500 m, and the
thickness of the oxide layer is in the range of 0.02 to 0.1
m.
Gate (G) insulated by thin layer of silicon dioxide
Source (S) and Drain (D) regions are heavily doped n+
regions in the substrate (B) (also called body)
5-2
MOSFETs
•MOS - metal oxide semiconductor structure (original
devices had metal gates, now they are silicon)
•NMOS - n-channel MOSFET
•PMOS - p-channel MOSFET
•CMOS - complementary MOS, both n-channel and pchannel devices are used in conjunction with each other
(most popular in IC’s)
•MESFET - metal semiconductor structure, used in highspeed GaAs devices
•JFET - junction FET, early type of FET
5-3
With VGS = 0 there are two pn junctions between drain and
source. Current cannot flow in either direction because one
or the other of the junctions would be reverse-biased.
However, if VGS > VT (threshold voltage), electrons are
attracted to the region below the gate, and an induced,
conducting n-channel forms between the drain and source.
NOTE:
iS = iD and iG = 0
A p-channel enhancement-type MOSFET is similar in
construction but has an n-type substrate with p+ regions for
the drain and source.
5-4
CMOS
Cross section of a CMOS integrated circuit. Note that the
PMOS transistor is formed in a separate n-type region,
known as an n well.
The two devices are isolated from each other by a thick
region of oxide.
5-5
Symbols
D
D
B
G
S
p Channel MOSFET
(enhancement-type)
-simplified symbol
shown below
S
n Channel MOSFET
(enhancement-type)
-simplified symbol
shown below
drain
gate
+
VDS
-
+
VGS
-
source
B
G
iG = 0
iD
drain
iS
source
gate
(Substrate is connected to source)
5-6
Output characteristics (n-channel)
+
VDS
-
An n-channel MOSFET with vGS and vDS applied and with
the normal directions of current flow indicated. The
characteristics for p-channel devices are exactly the same
except that voltage polarities and current directions are
inverted. (Operates in triode and cutoff regions as a switch.)
5-7
Input characteristics (n-channel)
+
VDS
-
ID = K(VGS-VT)2
5-8
Summary of MOSFET behavior
•VGS > VT (threshold voltage) for the device to be on
•VDS > VGS - VT for device to be in saturation region
•ID = K(VGS-VT)2
•Enhancement mode device, VT > 0
(we will be dealing with enhancement mode devices in
Chapter 13– MOS Digital Circuits)
•Depletion mode device, VT < 0 (conducts with VGS = 0)
5-9
Comparison of BJT and FET
BJT
•current controlled
FET
•voltage controlled
•VBE 0.7 V
•for device to be on
•VGS > VT
•for device to be on
•operates in saturation region
•operates in linear region
(amplifier);
(amplifier);
BE junction forward biased, VDS > VGS - VT
CB junction reversed biased
•IC = bIB
•ID = K(VGS-VT)2
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MOSFET aspect ratio
ID = K(VGS-VT)2
K = transconductance parameter
K = k' (W/L)
where k’ is the process transconductance
parameter
k' = n Cox, where n is the mobility of
electrons, and Cox is the capacitance of the oxide
(It’s value is determined by fabrication
technology.)
W/L is the aspect ratio, W is the width of the
gate, L is the length of the gate.
ID W/L
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