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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