ARTICLE ON BICMOS

1. TERM PAPER OF ECE-563, NOVEMBER, 2014
BICMOS
Himanshu Shekhar
ABSTRACT
This paper deals with BICMOS. First of all
the need of transistor is justified. After this it
is explained, the reason behind switching
from vacuum tube to BJT to FET to
BICMOS. Then simple working of BICMOS
inverter working, its advantages,
disadvantages and its application.
I. INTRODUCTION
The transistor is a three terminal, solid state
electronic device. In a three terminal device
we can control electric current or voltage
between two of the terminals by applying an
electric current or voltage to the third
terminal. This three terminal character of the
transistor is what allows us to make an
amplifier for electrical signals, like the one in
our radio. With the three-terminal transistor
we can also make an electric switch, which
can be controlled by another electrical
switch. Before transistor vacuum tube were
the device that were used for control
conduction. But Vacuum tube had to warm
up before they worked (and sometimes
overheated when they did), they were
unreliable and bulky and they used too much
energy.so we moved toward BJT, to reduce
power consumption, area and increase
execution speed and more reliable. But for
low power application and to reduce leakage
current in BJT, FET was developed and most
famous one is CMOS. From early 1980s BJT
and CMOS are combined together to make a
transistor that uses plus point of both to
nullify the negative points of both.
II. BICMOS
BICMOS is a combination of both bipolar
and CMOS that allows the designer to use
both devices on a single integrated circuit.
The development of BICMOS technology
began in the early 1980s. In general, bipolar
devices are attractive because of their high
speed, better gain, better driving capability,
and low wideband noise properties that allow
high-quality analog performance. CMOS is
particularly attractive for digital applications
because of its low power and high packing
density. Thus, the combination of both device
types would not only lead to the replacement
and improvement of existing integrated
circuit, but would also provide access to
design completely new circuits.
Let’s take an example to know the
importance of BICMOS.
Fig.1 Cascade inverter
As shown in Fig. 1 cascaded inverter is made
to drive a bigger load than just a single
inverter, and this has to do with speed. The
problem is that a CMOS gate can drive a
current proportionally to the width of its
channel: so doubling the channel width, we

2. will be able to charge a capacitor twice as
fast.
If we double the channel width, it also
double the input capacitance of the gate, so
the stage before will take twice the time to
drive the gate. So we need a gate which has
the minimum possible input capacitance,
while having as much as driving strength as
possible. This is obtained cascading several
inverters (the most elementary CMOS gate)
with increasing channel width, so that the
first has the required input capacitance and
the last has the required driving strength. In
comparison, bipolar junction transistors
(BJTs) have more current driving capability,
and hence, can overcome such speed
bottlenecks using less silicon area. However,
the power dissipation of bipolar logic gates is
typically one or two orders of magnitude
larger than that of comparable CMOS gates.
Therefore, such all-bipolar high speed VLSI
circuit are difficult to realize and require very
elaborate heat-sink arrangements.
An alternative solution to the problem of
driving large capacitive loads can be provide
by merging CMOS and bipolar devices
(BICMOS) on chip Taking advantage of the
low static power consumption of CMOS and
the high current driving capability of the
bipolar transistor during transients, the
BICMOS configuration
The BICMOS combination has significant
advantages to offer, such as improved
switching speed and less sensitivity with
respect to the load capacitance. BICMOS
logic circuits are not bipolar-intensive i.e.
most logic operations are performed by
conventional CMOS sub circuits, while the
bipolar transistors are used only when high
on-chip or off-chip drive capability is
required.
III. BASIC BICMOS CIRCUIT
In BICMOS inverter as shown in Fig. 2, the
complementary pMOS and nMOS transistors
MP and MN supply base currents to the
bipolar transistor and thus act as a trigger
device for bipolar output stage configuration.
Depending on the logic level of the input
voltage, either MN or MP can be turned on in
steady state, therefore assuring a fully
complementary push pull operation mode for
the two bipolar transistors. In this very
simplistic configuration, configuration, two
resistors are used to remove the base charge
of the bipolar transistors when they are when
they are in cut-off mode
Fig. 2 Simple BICMOS inverter circuit with
resistive base pull-down.
The superiority of the BICMOS gate lies in
the high current drive capability of the
bipolar output transistors, the zero static
power dissipation, and the high input
impedance provided by the MOSFET
configuration. To reduce the turn-off time of
the bipolar transistors during switching,
Two minimum-size nMOS transistors (MB1
and MB2) are usually added to provide the
necessary base Discharge path, instead of the
two resistors. As shown in Fig.3

3. Fig. 3 Conventional BICMOS inverter
circuit with active base pull-down
V. BICMOS INVERTER
Consider first the output pull-up transient
response, which starts with the input voltage
abruptly falling from VOH to VOL at t = 0.
The initial condition of the output node
voltage is assumed to be VO, = VOH. The
inverter circuit during this switching event is
depicted in Fig.4, where the active
(conducting) devices are highlighted.
Fig. 4: BiCMOS inverter during transient
output pull-up event. The active devices in
thecircuit are highlighted (darker).
As the input voltage drops, the pMOS
transistor MP is turned on and starts
operating in the saturation region. The nMOS
transistors MN and MB 1 are turned off; thus,
the lower "pull-down" part of the inverter
circuit can be ignored except for the
corresponding parasitic capacitances of the
nMOS transistors and the bipolar transistor
Q2. The base pull-down transistor MB2 is
turned on, which effectively drains the excess
base minority carrier charge of Q2 and
assures that Q2 remains in cut-off mode. At
the same time, MP is supplying the base
current of Q1, which starts to charge up Cload
with its emitter current.
Now consider the output pull-down transient
response, which starts with the input voltage
abruptly rising from VOL to VOH at t = 0.
The initial condition of the output node
voltage is assumed to be Vout = VOL. The
inverter circuit during this switching event is
depicted in Fig.5, where the active
(conducting) devices are highlighted.
Fig. 5: BiCMOS inverter during a transient
output pull-down event.The active devices in
the circuit are highlighted (darker).

4. As the input voltage rises, the pMOS
transistor MP is turned off and the nMOS
Transistors MN and MB 1 are turned on. The
bipolar pull-up transistor Q1 immediately
ceases to conduct because its base current
drops to zero, and MB 1 starts to remove the
excess minority carrier base charge of Q1.
The nMOS transistor MN operates initially in
the saturation region and supplies the base
current of the bipolar pull-down transistor
Q2.
IV. USES OF BICMOS TECHNOLOGY
There have been two significant uses of
BICMOS technology.
One of the usages is in the design of the high-
performance microprocessor unit (MPU)
using the high driving capability of bipolar
junction transistor because bipolar junction
transistor has better transconductance.
Comparing the gate delay time and load
capacitance capability for same area design,
BICMOS has a lower gate delay time than the
CMOS at high load capacitive environment
as illustrated in Fig 6.
Fig. 6 CMOS vs BICMOS
And second one is in the mixed signal circuit
design, BICMOS design utilizes the excellent
analog performance of the double poly self-
aligned bipolar junction transistor
V. BICMOS APPLICATION
1. In some applications (in which there is
finite budget for power) the BICMOS speed
performance is better than that of bipolar.
2. This technology is well suited for the
intensive input/output applications.
3. The applications of BICMOS were initially
in RISC microprocessors rather than
traditional CISC microprocessors.
4. It can be used for sample and hold
applications as it provides high impedance
inputs.
5. This is also used in applications such as
adders, mixers, ADC and DAC
VI. CONCLUSION
The most significant drawback of the
BICMOS circuits lies in the increased
fabrication process complexity more than
that of CMOS. Apart from this it can be used
as an alternate of the previous bipolar, ECL
and CMOS in the market.
VII. REFERENCE
[1] http://www.nobelprize.org
[2] http://blog.oscarliang.net/bjt-vs-mosfet
[3] http:// www.elprocus.com
[4] Digital Integrated Circuits, 2/E Jan M.
Rabaey, University of California, Berkeley
Anantha Chandrakasan, Massachusetts
Institute of Technology, Cambridge Borivoje
Nikolic, University of California, Berkeley.
[5] CMOS Digital integrated Circuits Sung-
Mo-Kang & Yusuf Leblebici 3rd 2003 Tata
McGraw Hill