Conversation so far:
what kind of gas is in a tv tube
The cathode-ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen and is used to display images.
are cathode-ray tubes still used today
Since the late 2000s, cathode-ray tubes have been largely superseded by newer flat panel display technologies such as LCD, plasma display, and OLED displays.
what kind of flat panel display is most common
Most 2010s-era flat-panel displays use LCD and/or LED technologies.

Question: what kind do smartphones use
Self-contained rewrite: what kind of display do smartphones use