BiCMOS

BiCMOS

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BiCMOS is an evolved semiconductor technology that integrates two formerly separate semiconductor technologies - those of the analog bipolar junction transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 and the digital CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 transistor - in a single integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

 device.

Bipolar junction transistors offer high speed, high gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

, and low output resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

, which are excellent properties for high-frequency analog amplifiers, whereas CMOS technology offers high input resistance and is excellent for constructing simple, low-power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 logic gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

s. For as long as the two types of transistors have existed in production, designers of circuits utilizing discrete components have realized the advantages of integrating the two technologies; however, lacking implementation in integrated circuits, the application of this free-form design was restricted to fairly simple circuits. Discrete circuits of hundreds or thousands of transistors quickly expand to occupy hundreds or thousands of square centimeters of circuit board area, and for very high-speed circuits such as those used in modern digital computers, the distance between transistors (and the minimum capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 of the connections between them) also makes the desired speeds grossly unattainable, so that if these designs cannot be built as integrated circuits, then they simply cannot be built.

In the 1990s, modern integrated circuit fabrication technologies began to make BiCMOS a reality. This technology rapidly found application in amplifiers and analog power management circuits, and has some advantages in digital logic. BiCMOS circuits use the characteristics of each type of transistor most appropriately. Generally this means that high current circuits use metal–oxide–semiconductor field-effect transistor (MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

s) for efficient control, and portions of specialized very high performance circuits use bipolar devices. Examples of this include radio frequency (RF) oscillators, bandgap-based references and low-noise circuits. The Pentium, Pentium Pro
Pentium Pro
The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel introduced in November 1, 1995 . It introduced the P6 microarchitecture and was originally intended to replace the original Pentium in a full range of applications...

, and SuperSPARC
SuperSPARC
The SuperSPARC is a microprocessor that implements the SPARC V8 instruction set architecture developed by Sun Microsystems. 33 and 40 MHz versions were introduced in 1992. The SuperSPARC contained 3.1 million transistors. It was fabricated by Texas Instruments at Miho, Japan in a 0.8 micrometre...

 microprocessors also used BiCMOS.

Disadvantages


BiCMOS as a fabrication process is not currently as commercially viable for some applications, such as microprocessors, as with exclusively BJT or CMOS fabrication. Unfortunately, many of the advantages of CMOS fabrication, for example, do not transfer directly to BiCMOS fabrication. An inherent difficulty arises from the fact that optimizing both the BJT and MOS components of the process is impossible without adding many extra fabrication steps and consequently increasing the process cost. Finally, in the area of high performance logic, BiCMOS may never offer the (relatively) low power consumption of CMOS alone, due to the potential for higher standby leakage current. An exclusive mix of BiCMOS and CMOS has appeal if the performance attributes of each type of gate can be optimized. But since CMOS is already ideal for pure digital logic, this is only a serious issue when it is desirable to put logic circuits together on the same chip with other circuits that are not strictly logic: either for the purpose of a mixed-signal application, or simply to reduce the chip count in an electronic product by combining two chips into one, in order to reduce cost, size, and/or weight.