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==Replacement==
Selenium rectifiers had a shorter lifespan than desired. During catastrophic failure they produced significant quantities of malodorous and highly toxic [[hydrogen selenide]]<ref>{{Cite journal |date=1950-08-22 |title=Constitution and mechanism of the selenium rectifier photocell |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1950.0112 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |language=en |volume=202 |issue=1071 |pages=449–466 |doi=10.1098/rspa.1950.0112 |issn=0080-4630}}</ref><ref>{{Cite web |last= |date= |year=1978 |title=Occupational Health Guideline for Hydrogen Selenide |url=https://www.cdc.gov/niosh/docs/81-123/pdfs/0336.pdf |access-date=2022-10-01 |website=cdc.gov}}</ref><ref>{{Cite web |last=PubChem |title=Hydrogen selenide |url=https://pubchem.ncbi.nlm.nih.gov/compound/533 |access-date=2022-10-01 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> that let the repair technician know what the problem was. By far the most common failure mode was a progressive increase in forward resistance, increasing forward [[voltage drop]] and reducing the rectifier's efficiency. During the 1960s they began to be superseded by [[silicon rectifiers]], which exhibited lower forward voltage drop, lower cost, and higher reliability.<ref>[http://pubs.usgs.gov/of/2003/of03-018/of03-018.pdf Selenium, U.S. Department of the Interior].</ref>Selenium diode computer logic
In 1961 IBM started developing a low-speed computer logic family<ref>[https://www.google.com/patents/US3218472 US Patent 3218472]: Transistor switch with noise rejection provided by variable capacitance feedback diode.</ref> that used selenium diodes with similar characteristics to silicon but cost less than one cent. The [[computer terminal|terminal]] development departments were begging for low cost and did not need speed. It was possible to punch 1/8-inch discs from a sheet of selenium diode. GE claimed that they could make reliable selenium diodes. A design was achieved for a DDTL circuit with two levels of [[diode logic]] feeding one [[Alloy-junction_transistor|alloy transistor]] and no series input resistor or speed-up capacitor. The family was called SMAL<ref name="ibm" /> or SMALL, for "selenium matrix alloy logic". The alloy transistor proved to be too fast for the selenium diode [[Diode#Reverse-recovery_effect|recovery]]. To solve this problem, a selenium diode was connected around the base–emitter to slow it down. The two-level logic was similar to the PLAs programmable logic array that would come on the market many years later. Nearly any static logic function that yielded one output could be achieved with one transistor and a handful of cheap diodes. Several years later the selenium diodes were found to be not reliable and were replaced by silicon diodes. The logic family was packaged on [[IBM Standard Modular System|SMS cards]].<ref name="ibm">{{cite book |title=The 1060 Data Communications System |publisher=IBM |page=2 |url=http://ed-thelen.org/comp-hist/IBM-ProdAnn/1060.pdf}}</ref>
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