Diffusion transistor
（重定向自Planar transistor）
![Comparison of the mesa (left) and planar (Hoerni, right) technologies. Dimensions are shown schematically.](/Images/godic/202502/03/Сравнение_планарного_с_меза0536.PNG")
![Simplified cross section of a planar npn bipolar junction transistor](/Images/godic/202502/03/Npn_bjt_cross_section.svg0536.png")
A diffusion transistor is a bipolar junction transistor (BJT) formed by diffusing dopants into a semiconductor substrate. The diffusion process was developed later than the alloy junction and grown junction processes for making BJTs.
Bell Labs developed the first prototype diffusion transistors in 1954.