The iBreath device has a special semiconductor sensor chip inside of it that detects and measures alcohol content. Japanese engineers designed the sensor to react in the presence of ethanol gas.
The semiconductor substrate -- in other words, the foundation of the semiconductor -- has a very thin membrane of tin dioxide (SnO2). Connected to this membrane is a small metal heater. The heater is one of the elements in the iBreath device that requires power. The purpose of the heater is to warm the membrane to the optimal operating temperature. It turns out that ethanol sensors require heat to detect the presence of alcohol with accuracy. They also require isolation -- if the sensor loses heat during operation, it can give a false reading.
When ethanol gas comes into contact with the tin dioxide membrane, the membrane absorbs the ethanol molecules. The tin dioxide reacts with the alcohol molecules and its electrical resistance changes. The semiconductor measures the difference in resistance and generates an estimate of the user's BAC. Through multiple trials in the lab, engineers determined the relationship between the difference in the electrical resistance of tin dioxide and the density of ethanol within a gas. The iBreath's LED screen displays the estimate.
In addition to the breathalyzer function, the iBreath comes with an FM transmitter. When you plug the iBreath into an iPhone or iPod, it draws power from the device and can also accept data from the songs you've stored on it. The transmitter converts the data signal into a low-power radio frequency. If you tune a nearby radio to the same frequency, you'll be able to hear the music from your phone or MP3 player without tethering the device with a cable.