What is the difference between a semi-conductor and electrochemical (fuel cell) sensor?

Breathalyser sensor technology has advanced from purely semi-conductor to the use of electrochemical sensors as well in recent time. Quality electrochemical (fuel cell) sensors are more precise and hold their accuracy substantially longer than semi-conductor sensors. They are also more expensive however. That being said good quality semi-conductor sensors still have fairly high accuracy and will last at least 50 tests / 9 months before requiring calibration.