High Temperature Mixed Signal

General Description

High temperature electronics operate in one of the most demanding electronic environment. Junction leakage doubles with every 10ºC increase in temperature. For bulk silicon devices, the leakage becomes significant above 175ºC, and intolerable above 225ºC. And while digital logic can withstand some leakage, most analog circuits cannot.

Leakage is a problem that cannot be overcome with clever design tricks. It requires a different process. And that process is SOI (Silicon On Insulator).

In this process, oxygen is implanted beneath the devices. And the field oxide extends down to the implanted oxide layer, providing external isolation. Then, the diffusions are deep enough so that they too touch the buried oxide. This eliminates the bottom and sidewall junctions. And without a junction, there can be no junction leakage.

There is a small junction between the source / drain diffusions and the transistor channel. So while the junctions have not been totally eliminated, that have been reduced in area by a factor of 1,000. And that gives the SOI process an additional 100ºC of operation.

As with all engineering choices, there are tradeoffs. On the positive side, stray capacitance has been eliminated, along with the voltage dependency of diffused resistors. And with most wells tied to the transistor sources, the body effect has been eliminated.

On the negative side, layout density is decreased, and overall layout is more difficult. There are no vertical bipolar devices, just lateral ones. We do not have an EEPROM cell, so trimming becomes more complicated. And the 1u technology limits the circuit size.

Applications

To start with, anything that can be done in bulk silicon can also be done in the SOI process.

In the down-hole environment, there is demand for instrumentation amps, ADCs, DACs, and the occasional op-amp. All of these are easily made, and because of the lack of junctions, will have better performance than achieved with bulk devices.

Many down-hole measurements are made on low frequency signals, allowing Sigma-Delta ADCs to be used with up to 24-bit accuracy.

And since the SOI devices are totally isolated from the substrate, the analog supplies can be +/- 2.5v volts, and still work with a digital section operating from a 5 volt supply.