SOI handles the power - Philips Semiconductors ' 60V A-BCD1 - Product Announcement

Electronics Times, May 10, 1999

Philips Semiconductors' SOI technology promises to have a radical impact on chip fabrication. Wladimir Punt, the company's product marketing manager for infotainment systems, explains how it works

Late last year, Philips Semiconductors announced its 60V A-BCD1 (advanced bipolar-CMOS-DMOS) silicon-on-insulator (SOI) process.

Emerging details are now offering a glimpse of some of the world's first ICs to handle rectified mains supply voltages on the same chip as low voltage analogue and digital circuitry.

These developments promise a radical impact on chip fabrication, dramatically increasing functional integration within ICs and improving the overall energy efficiency of the products in which they are employed.

Using developments in SOI technology, Philips has produced commercially available Class D audio amplifier chips with rated outputs up to 2 x 100W that are 95% efficient, highly compact and require no additional heatsinking.

The A-BCD1 SOI technology employed to fabricate these devices uses a 1.5 micro m layer of active silicon on top of a 1 micro m layer of buried oxide, allowing the integration of very low on-resistance power transistors.

The result is significantly lower power losses than are achievable using bulk silicon processes.

In addition, the isolation of components on the chip by the underlying oxide layer and by local oxidation of the thin silicon removes problems with parasitic capacitances and latch-up, improving performance and increasing ruggedness under adverse operating conditions.

The TDA8920, the first in a family of audio power amps built using the process, is also the first single-chip audio amplifier to handle 2 x 50W. Its efficiency is typically 95% compared with 50% for Class AB amplifiers.

This increase in efficiency means a substantial reduction in power dissipation, allowing the TDA8920 to be mounted directly on the PCBs of audio equipment, TVs and computers without additional heatsinking.

The cut in power dissipation also means that power supplies can be reduced in size, and the battery life of portable audio products extended.

Previous Class D audio amplifiers required a combination of discrete power transistors and a separate control IC. By combining bipolar, CMOS and DMOS circuitry in a single system-on-silicon device, Philips' A- BCD1 process allows a competitively priced solution to be implemented in one IC.

In addition to the 2 x 50W TDA8920, the family of Class D power amps will include 2 x 10W, 2 x 25W and 2 x 100W amplifiers.

In a separate development, Philips Semiconductors has also perfected an SOI process that allows high-voltage components to be integrated alongside low-voltage analogue and digital circuitry on commercial silicon chips.

The company's EZ-HV process withstand voltages as high as 650V, allowing it to directly handle rectified mains inputs.

By combining high-voltage switching transistors with low-voltage analogue and digital circuitry on the same chip, EZ-HV will make it possible to produce a new generation of intelligent single-chip power management solutions that save energy.

It will also enable size and cost reductions in a wide range of consumer products such as energy-efficient light bulbs and mobile phone power plugs.

The conventional way to handle high voltage on an IC is to use a thick layer of silicon overlaying an insulating material, in this case silicon dioxide. But this thick silicon layer, which is required to limit the electric field strength to a value below the threshold for avalanche breakdown, is expensive and difficult to dope effectively.

Rather than using this traditional thick silicon approach to handling high voltages on ICs, Philips took a fresh look at the fundamental physics of SOI devices at its Briarcliff research laboratories in New York, US.

A thick layer of silicon potentially allows charge carriers to be accelerated over distances that result in them acquiring high enough energy levels to cause avalanche breakdown. Philips' researchers concluded that a better solution would be to make the silicon layer so thin that charge carriers simply cannot move far enough to acquire these energy levels.

Instead of the 10 to 20 micro m layer used in conventional high-voltage SOI processes, the EZ-HV process uses a layer of silicon only 0.5 micro m thick (about 1000 atoms), which is considerably cheaper to produce.

Although with 650V applied across this layer, the vertical field strength is extremely high, there is still insufficient distance between the top and bottom surfaces of the silicon layer for charge carriers to be accelerated to avalanche breakdown energy levels.

The latch-up immunity of the A-BCD1 and EZ-HV processes is a direct consequence of the oxide isolation used between devices on the chip.

By eliminating the reverse biased junctions that are needed in a bulk silicon IC to isolate adjacent components, the EZ-HV process does not create thyristor-like pnpn junctions that can latch up in response to voltage spikes or leakage currents.

This makes both processes suitable for fabricating ICs for use in electrically noisy environments. For example, A-BCD1 Class D audio power amplifiers are ideal for in-car entertainment systems, where spikes from the vehicle's starter motor or alternator can be as high as 50V.