Download MOS Transistors

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Memory
SRAM
organization
• organized as an array of
2M rows ´ 2N columns
• Each chip can be
organized
1-bit, 4, 8 or 16-bit wide
• Eg 64M-bit would be 64M
x 1 226 bit address
• Storage cells organized as
a square array
Figure 11.17 A 2M+N-bit memory chip organized as an array of 2M rows ´ 2N columns.
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SRAM cell
Built with CMOS
transistors
Used in Cache
Sense Amps are used to
detect “sense” bit lines that
have small signal values
Figure 11.23 A differential sense amplifier connected to the bit lines of a particular column. This arrangement can be
used directly for SRAMs (which utilize both the B and B lines). DRAMs can be turned into differential circuits by
using the “dummy cell” arrangement shown in Fig. 11.25.
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NMOS transistor structure
Source (S), Drain (D), Gate (G)
L channel length, W width of transistor
NMOS transistor cross-section
Figure 4.1 Physical structure of the enhancement-type NMOS transistor: (a) perspective view; (b)
cross-section. Typically L = 0.1 to 3 mm, W = 0.2 to 100 mm, and the thickness of the oxide layer (tox) is
in
the range of 2 to 50 nm.
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NMOS transistor
+ve Gate (G) voltage  n-channel
gate
source
NMOS Transistor
• N-Channel MOSFET
• Built on p-type substrate
• MOS devices are smaller
than BJTs
• MOS devices consume less
power than BJTs
drain
Figure 4.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is
induced at the top of the substrate beneath the gate.
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NMOS transistor iD – VDS
+ve VGS > Vt & small VDS ===> resistive device
iD
VDS
Figure 4.4 iD–vDS characteristics of MOSFET in Fig. 4.3 when the voltage applied between drain
and source, vDS, is kept small. The device operates as a linear resistor whose value is controlled by
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GS.
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NMOS transistor
+ve VGS > Vt & small VDS ===> acts like resistor
Channel induced
Figure 4.3 NMOS transistor with vGS > Vt and with a small vDS applied. The device acts as a resistance
whose value is determined by vGS. Specifically, the channel conductance is proportional to vGS – Vt’ and
thus iD is proportional to (vGS – Vt) vDS. Note that the depletion region is not shown (for simplicity).
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NMOS transistor
+ve VGS > Vt & increase VDS ===> resistance increases
Figure 4.5 Operation enhancement NMOS transistor as vDS is increased. The induced channel acquires
tapered shape, its resistance increases as vDS increased. Here vGS is kept constant at a value > Vt.
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NMOS transistor id – VDS characteristic
+ve VGS …. increase VDS
Figure 4.6 The drain current iD versus the drain-to-source voltage vDS for an enhancement-type
NMOS transistor operated with vGS > Vt.
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Threshold Voltage Vt
Value of VG for which channel is “inverted
 threshold voltage VT (or Vt )
Characteristics of VT
• Inverting substrate from p-type to n-type 
inversion layer in channel
• Controlled by inversion in channel
• Adjusted by implantation of dopants into the
channel
• Can be positive or negative
• Influenced by the body effect
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NMOS circuit symbols
(c) Is most common
Figure 4.10 (a) Circuit symbol for the n-channel enhancement-type MOSFET. (b) Modified circuit symbol with
an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n
channel). (c) Simplified circuit symbol to be used when the source is connected to the body or when the effect of
the body on device operation is unimportant.
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NMOS transistor, 3 regions
Circuit &
ID – VDS characteristic
For Different VGS
Cutoff VGS < Vt
Triode VGS > Vt, small VDS
Saturation; large VDS, flat ID
Figure 4.11 (a) n-channel enhancement-type MOSFET with vGS and vDS applied and with the normal
directions of current flow indicated. (b) iD–vDS characteristics for a device with k’n (W/L) = 1.0
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mA/V
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P-channel MOSFET PMOS
(c) most common symbol
(c) Circuit, with V & I
Figure 4.18 (a) Circuit symbol for the p-channel enhancement-type MOSFET. (b) Modified symbol with an
arrowhead on the source lead. (c) Simplified circuit symbol for the case where the source is connected to the body.
(d) The MOSFET with voltages applied and the directions of current flow indicated. Note that vGS and vDS are
negative
and iD flows out of the drain terminal.
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CMOS structure, today’s ICs
both NMOS & PMOS transistors
PMOS formed in an n-well
Figure 4.9 Cross-section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate ntype region, known as an n well. Another arrangement is also possible in which an n-type body is used and the n
device is formed in a p well. Not shown are the connections made to the p-type body and to the n well; the latter
functions as the body terminal for the p-channel device.
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Why CMOS
• Advantages
•
•
•
•
Virtually, no DC power consumed
No DC path between power and ground –
VDD to load OR load to Gnd
Excellent noise margins (VOL=0,
VOH=VDD)
Inverter has sharp transfer curve
• Drawbacks
•
•
•
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Requires more transistors
Process is more complicated
pMOS size larger to achieve electrical
symmetry
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Digital CMOS Inverter (4.10)
Figure 4.53 The CMOS inverter.
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Inverter Transfer Characteristic
VTC = voltage transfer characteristic
Voh
Ideal
inverter
Typical
inverter
Noise margins
NML NMH
Vol
Vil Vih
Voh = output high; Vol = output low;
Vil = max input interpreted as “0”; Vih = min input interpreted as “1”;
Figure 1.29 The VTC is approximated by three straight line segments. Note the four parameters of the VTC (VOH, VOL,
VIL, and VIH) and their use in determining the noise margins (NMH and NML).
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CMOS inverter implementation
The most common inverter
Figure 1.32 A more elaborate implementation of the logic inverter utilizing two complementary switches. This is the
basis of the CMOS inverter studied in Section 4.10.
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CMOS Inverter – Xfer Curve
Figure 4.56 The voltage transfer characteristic of the CMOS inverter.
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Other Inverter Implementations
All NMOS - Not very popular -- FYI
(a) pseudo-NMOS logic inverter.
(b) The enhancement-load
NMOS inverter.
© The depletion-load NMOS
inverter.
Figure 10.19 (a) The pseudo-NMOS logic inverter. (b) The enhancement-load NMOS inverter. (c) The depletion-load
NMOS inverter.
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