How did semiconductors improve thermoelectric generator efficiency after 1940?

Before 1940, the development of practical thermoelectric generators was limited by the reliance on metallic conductors, which restricted conversion efficiency to approximately 0.5 percent. The introduction of semiconductor materials significantly improved this performance, enabling the development of generators with 4 percent efficiency by 1940.1

Semiconductors improved efficiency by allowing for a much larger Seebeck effect compared to metals, particularly through the use of p-n junctions created from p-type and n-type materials. To optimize these devices, engineers utilize a figure of merit defined by the square of the Seebeck coefficient ($\alpha$) divided by the product of electrical resistivity ($\rho$) and thermal conductivity ($\kappa$). By balancing these material properties and optimizing leg geometry, designers can minimize ohmic heating and thermal conduction losses, which are critical factors in maximizing electrical power output. Further advancements in semiconductor materials and electrical contacts between 1955 and 1965 subsequently expanded the practical applications of these generators.23