Electronic Design

Bipolar Process Puts The Squeeze On Die Size, Noise

A new fab process for 36-V bipolar chips aligns analog densities more closely with Moore's Law for logic.

In industrial-control applications, many sensors and actuators must operate at tens of volts, while control-logic voltages have continued to shrink in step with successive generations of digital process geometries. For engineers who design industrial control systems, the voltage divide has typically meant the need to add ever more external signal conditioning and biasing circuits.

Late last year, though, Analog Devices released the first products in its iCMOS 36-V, trench-isolated mixed-signal process technology. Now comes iPolar, ADI's all-analog trench-isolation process technology.

The new bipolar process technology enables fabrication of smaller, higher-performance devices that can still run from 18-V power rails. This allows direct interfacing with industrial sensors and actuators that use higher voltages.

The iPolar process also lets the company fabricate denser chips, reducing package size and power consumption. In many cases, the footprint reduction means that industrial equipment designers can migrate from SOIC-packaged ICs to much smaller TSOT-23 packages. Smaller packaging is important, as system designers strive to increase the number of control channels handled by a piece of equipment.

CATCHING UP WITH MOORE
While logic-process geometries have continued to shrink according to Moore's Law, analog-IC manufacturers have been stuck building bipolar devices for the industrial market on process geometries as large as 2.5 mm. Because these dielectrically isolated (DI) processes require silicon-consuming diffusion areas to separate the transistors, they generally yield no more than 100 active bipolar devices per die, compared to the millions of devices on large logic ICs. Thanks to trench isolation, iPolar yields devices with a 75% smaller footprint than DI bipolar processes (see the figure).

PERFORMANCE BENCHMARKS
High transistor density and improved transistor design enable more precise matching of individual transistors. Also, iPolar optimizes 1/f noise performance by avoiding polysilicon emitter contacts. Cutoff frequencies are 1.1 GHz for npn transistors and 750 MHz for pnps.

The formation of the emitter/base junction in these transistors eliminates the burst noise and reduces offset voltage. Even with relatively large input voltage swings of industrial sensors, noise performance is important because the resolution of converters used in these systems has been increasing, from 8 bits a few years ago to 12 and 14 bits today.

Not everything in iPolar is brand new. ADI transferred its XFET (eXtra implanted FET), low-noise, low-current, linear temperature-coefficient voltage-reference technology to the process. Existing trimmable thin-film and high-performance poly-nitride-metal capacitor technologies also migrated.

Electronic Design reported on the first three iPolar products in the June 9 issue (see ED Online 10460). These include a low-noise, precision rail-to-rail amplifier with less than 3-nV/√Hz noise in single, dual, and quad versions. Its applications are wide-ranging, from phase-locked loops and other precision filter circuits, position and pressure sensors, and medical instrumentation to strain-gauge amplifiers and precision power-supply controls.

The second IC is a TSOT-packaged OP07-style op amp for temperature, pressure, current, and voltage sensing in building controls, medical systems, and automatic test equipment. The third chip is a quad precision amplifier with 2-nV/√Hz noise performance for electronic and analytical instruments, multipole dc-coupled filters, multichannel data acquisition and controls, and multichannel buffering and reference driving.

ADI's roadmap for the rest of the year includes a line driver/receiver pair, a voltage reference, an amplifier with integrated voltage reference, and a high-speed amplifier.

Analog Devices
www.adi.com

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