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

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

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).

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