Vlsi design notes(1st unit) according to vtu syllabus.(BE)
Introduction to MOS technology
Speed- power performance
Comparison of bipolar and CMOS
Integrated circuit (IC):- an IC is a combination of interconnection circuit
elements on a continuous substrate.
Substrate:- substrate is a supporting material upon which an IC is fabricated.
Wafer:- a wafer is a basic physical unit used in processing. It usually
contains large number of identical IC’s.
Transistor was invented in 1947.
The very first IC emerged at the beginning of 1960.
Generation of IC’s
Year generation number of transistors typical products
1961 SSI 10-100 logic gates, flip-flop
1966 MSI 100-1000 counters, mux, adders
1971 LSI 1000-20,000 8bit μps, RAM, ROM
1980 VLSI 20,000-106(1 million) 16-32 bit μps, DRAM
1990 ULSI 1-10 million DSPs, smart sensor
2000 GSI >10million ASIC
2 Moore’s first law:
1961 1964 1967 1970 1973 1994 1997
It states that the number of transistors per chip doubles every 18months
3 Speed/power performance of available technologies:
In order to improve the through put rate it will be necessary to improve the
technology both in term of scaling and processing, and through the
incorporation of other enhancements such as BICMOS to CMOS.
Above 100million instruction/sec, one must look to other technology like
gallium Arsenide (GaAs) based technology.
GaAs in combination with silicon will provide designer some exciting
4 Basic MOS transistor symbol
DE-MOSFET conducts when Vgs=0, Vgs=+ve and Vgs=-ve as a channel
exist between source(S) and drain (D).
E-MOSFET does not conduct when Vgs=0.
For E-MOSFET For DE-MOSFET
There are 2 types of nMOS
1. Enhancement mode transistor (E-MOSFET)
2. Depletion mode transistor(DE-MOSFET)
In depletion mode channel is established between S and D during
manufacturing process itself by implanting suitable impurities. Therefore
DE-MOSFET conducts when Vgs=0.
In E-MOSFET there is no channel between S and D. therefore device is
not conducting when Vgs=0.
a. nMOS Enhancement mode transistor
b. nMOS Depletion mode transistor
c. pMOS Enhancement mode transistor
d. pMOS Depletion mode transistor
Enhancement mode transistor action:
In nMOS enhancement mode transistor, there exist no physical layer
between source(S) and drain (D).
A minimum voltage level of threshold voltage (Vt) must be provided
between source(S) and gate (G) to establish channel between S and D.
Enhancement mode device operates under 3 set of conditions.
1. When Vgs > Vt and drain to source voltage Vds=0. There is a channel
between S and D, but no current is flowing in channel. Transistor is in
2. When Vgs>Vt and Vds<Vgs-Vt, current flows from D to S. it operates
in non-saturation region.
As Vds increase current flow increases the current increases the
voltage drop increases in the channel. As a results between gate and
channel varies with distance along the channel and is maximum at
source end (as Vds is +ve)
3. When Vds>Vgs-Vt, the transistor operates ib saturates region
Here depletion region increases as voltage drops in maximum at drain
end and cause pinch off at channel. Now constant current flows from
D to S as channel exhibits high resistance.
For enhancement mode device Vt=0.2Vdd
If VDD=5v then Vt=1v.
Depletion mode transistor action:
Here channel is established between S and D during the construction of
transistor so when Vgs=0, the current flows between S and D.
1. When Vgs=0 and Vds=0, there exists a channel between S and D but not
current flows from D to S.
2. When Vgs>=0 and Vds<Vgs-Vt, current will flow from D toS.as the
current flows, there will voltage drop.
3. When Vgs>=0 and Vds>Vgs-Vt, the voltage drop in channel near drain is
maximum and equal to pinch off voltage. The transistor here operates in
saturation region, constant flow from D to S.
Vtd is typically <-0.8Vdd.
5 nMOS fabrication
Processing is carried out on a thin wafer cut from a single crustal of
silicon of high purity into which the required p-impurities are introducer.
The wafer is typically 75-150mm in diameter and 0.4mm thick. The
substrate is doped with boron to give impurity concentration of 1015
, giving the resistivity of 25Ωcm to 2Ωcm.
A silicon dioxide (Sio2) layer of typically 1µm thick is grown all over the
surface of the wafer, this protective layer act as barrier to dopants during
Photo resist is deposited on the wafer to get even distribution required
The photo resist layer is exposed to UV light by means of required mask.
The exposed areas UV are polymerized (hardened) and unexposed areas
These areas (hardened areas) are etched away along underlying Sio2 so
that the wafer surface is exposed in the window defined by mask.
The photo resist is removed and thin layer of Sio2 is grown over the entire
surface. Then polysilicon is deposition on top of this to form the gate
structure. The polysilicon layer consists of heavily doped polysilicon
deposition by chemical vapour deposition (CVD).
Thin oxide is removed to expose areas into which n-type of impurities are
to be diffused to form S and D. diffusion is achieved by heating the wafer
to high temperature and passing a gas containing the desire n- impurity
over the surface. Polysilicon and thin oxide act as masks during diffusion
called as “self aligning”.
Sio2 is grown over all and is masked with photoresist and etched to
exposed selected areas of gate, drain and source areas where connections
are to be made.
Metal of 1µm thickness is deposited overall the chip’s surface. this metal
layer is then masked and etched to form the required interconnected
6 CMOS fabrication:
CMOS can be fabricated by P-well, n-well and twin tub process.
6.1 P-well process
On n-type substrate, p-well is diffused by suitable masking. The depth of
p-well is 4-5 µm.
p-well doping concentration will affect the threshold voltage and
breakdown voltage of n-transistor, hence p-well diffusion must be out
To achieve low threshold voltage, we need either deep-well diffusion or
high well resistivity. But limitation is that chip area becomes large due to
Within the parent n-substrate, the p-well act as a substrate for the n
device. As there are two substrate, these devices are to be electrically
Due to two substrate, it require two substrate connections VDD and Vss
that are connection by means of n+
In all other respects-masking, patterning and diffusion process are similar to
that of nMOS fabrication.
Processing steps are:
Mask 1: p-well diffusion in n-substrate.
Mask 2: defines thin oxide region, namely those areas where the thick oxide
is stripped and thin oxide is grown to accommodate p- and n- transistor and
Mask 3: polysilicon layer is deposited on thin oxide and then patterned.
Mask 4: P+
mask is used to define the area where p-diffusion is to take place.
Mask 5: p+
mask is used for n-diffusion.
Mask 6: contact cut are defined.
Mask 7: metal layer pattern is defined.
Mask 8: passivation (over glass) layer is applied.
Fig:-CMOS p-well inverter showing VDD and VSS substrate connection.
6.2 n-well process:
n-well is superior to p-well because of the low substrate biase effects on
transistor threshold and lower capacitance associated with source and drain
Fig: cross sectional view of n-well CMOS inverter.
N-well is diffusion into the p-substrate.
Within the parental p-substrate, n-well act as a substrate to diffuse p-
Two substrates two separate substrate connection i.e VDD and Vss for n+
Main steps in n-well process:
Fig: main steps in typical n-well process.
Note: refer figure 1.13, 1.14, 1.15 and1.16 from textbook.
7 BiCMOS technology
The load driving capabilities of MOS transistor is less, due to the limited
current sourcing and current sinking abilities of both p and n transistors.
Bipolar transistor provide higher gain, better noise and high frequency
characteristics than MOS transistors.
Bipolar can be combined with the CMOS technology to construct higher
speed devices i.e (BiCMOS devices).
Formation of n-well region
Define nMOS and pMOS active areas
Field and gate oxidations (thin oxide)
Form and pattern polysilicon
Deposit and pattern metallization
Over glass with cuts and bonding pads
Application of BiCMOS in subsystems like ALU,ROM, barrel shifter etc
is not always a way of improving speed because most gate in such
structures do not have to drive large capacitive loads, so here BiCMOS
arrangements give no speed advantages.
BiCMOS devices speed up VLSI circuits
Example: Pentium, Pentium pro, super SPARC, RF oscillator.
7.1 Comparison between bipolar and CMOS technology
CMOS technology bipolar technology
Low static power dissipation high static power dissipation
High input impedance low input impedance
High packing density low packing density
Bidirectional capability unidirectional capability
Low output driver current high output driver current
Low trans conductance high trans conductance
High delay sensitivity to load low delay sensitivity to load
Scalable threshold voltage