American physicist John Bardeen is considered one of the fathers of modern electrical circuits thanks to his work on developing the electrical transistor. He later discovered the properties of superconductivity, which went on to allow the creation of CAT and MRI scanners. This work in both fields saw Bardeen awarded two separate Nobel prizes, though is name is known only to those working in science.
Fritz Haber has had a major role to play in modern warfare and mass production of food. His discovery of a way to synthesize ammonia was a vital process to the creation of modern fertilizers, allowing the huge growth in the amount of food grown. He also played a major role in the development of chlorine and mustard gas during World War I for the Germans, effectively kick starting the idea of chemical warfare.