Cadmium sulfide is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. More here.
I’ve purchased a few heater-less vacuum tubes, that use CdS to provide photosensitive features, from an excellent Ebay seller, ven-tronics.
The tubes are as follows:

ORP 50 (12.7mm x 38mm | 6MOhms in dark and 300KOhms in bright light)
OAP 12 (2.38mm x 9.5mm)
ORP 60 (3.17mm x 12.7mm | 5MOhm in dark and 220KOhm in bright light)
ORP 63 (3.17mm x 25mm | 90KO in dark and 3KO lighted)
RCA 4403 (1MO dark and 360Ohms in bright light)
RCA 7536 ()
RCA 7846 ()
Pressler Cesium photocell ()