Transistor
The transistor is the fundamental building block of modern electronic devices, and is ubiquitous in modern electronic systems. Julius Edgar Lilienfeld patented a field-effect transistor in 1926 but it was not possible to actually construct a working device at that time. The first practically implemented device was a point-contact transistor invented in 1947 by American physicists John Bardeen, Walter Brattain, and William Shockley. The transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things. The transistor is on the list of IEEE milestones in electronics, and Bardeen, Brattain, and Shockley shared the 1956 Nobel Prize in Physics for their achievement.
Oleg Losev
Although he was never able to complete a formal education and never held a research position, Losev conducted some of the earliest research into semiconductors, publishing 43 papers and receiving 16 "author's certificates" (the Soviet version of patents) for his discoveries. He observed light emission from carborundum point-contact junctions, the first light-emitting diode (LED), did the first research on them, proposed the first correct theory of how they worked, and used them in practical applications such as electroluminescence. He explored negative resistance in semiconductor junctions, and was first to use them practically for amplification, building the first solid-state amplifiers, electronic oscillators, and superheterodyne radio receivers, 25 years before the invention of the transistor. However his achievements were overlooked, and languished unknown for half a century before being recognized in the late 20th and early 21st century.