Germanium, discovered in 1886 by Clemens Winkler, historically played an important role in what has come to be known as the “semiconductor age”. While today germanium is considered a valuable semiconductor, when it was originally discovered, it was believed to a weakly conducting metal without much use.1 Today, germanium’s excellent electronic properties are exploited, as there is a growing demand for faster computers, higher efficiency photovoltaics, and more sensitive detectors.
The worth of germanium did not become recognized until World War II when there was a need for high frequency rectifiers to achieve high resolution from radar receivers.2 Lark-Horovitz was the first to realize and tabulate the valuable properties of germanium, including its low melting point and relatively high stability.1 As a result, point contact rectifiers used for radar receivers during World War II were made of germanium. Extrapolating on the development of the germanium-based rectifier, J. Bardeen, W.H. Brattain, and W. Shockley invented the first germanium transistor. The invention of the transistor transformed the possibilities for electronic devices; electronic devices that previously relied on vacuum tubes could now be operated at a much higher frequencies with the use of transistors.2 Nearly every electronic device used today relies on transistors, including computers and televisions. A computer chip is composed of millions of transistors that switch between two binary states (0 and 1), allowing the computer to perform operations. Germanium played an essential role in the development of the first transistor and its usefulness as a semiconductor thus became recognized.
In the late 1950s, interest grew in using silicon instead of germanium because of its larger bandgap and the existence of stable silicon oxides.1 However, just as the use of germanium in transistors was becoming obsolete, a new application for germanium semiconductors was developed: gamma-ray and IR detectors.1 Interest in far IR detectors began in the 1980s for use in the first far IR space telescope.1 NASA was interested in far IR detectors because this region of the electromagenetic spectrum contains valuable information about star formation, interstellar dust, planet formation, and accretion disks around young stars.1 Today, there continues to be interest in IR detectors for applications in military surveillance, target detection, and target tracking.3 Because of germanium’s large absorption coefficient in the near IR, its high stability, and the recent progress in integrating germanium with silicon technology, germanium is a promising material for IR detection.3
Today, as current technology with silicon based electronic devices has reached the upper boundaries in terms of efficiency and speed, there has been a recent revival of interest in using germanium in transistors.1,4 The superior electron and hole mobility of germanium as well as its low melting point has spurred much research into replacing current silicon based devices with germanium.1,4 The electron and hole mobility of a material is the rate at which the electrons and holes can move through the material. There is a strong correlation between the electron and hole mobility of a material and the efficiency and speed of the device composed of that material.4 Rather than continue to decrease the size of transistors, which has historically been the method for increasing performance, researchers are now turning to new materials for use in transistors. Germanium’s superior electron and hole mobility offers a new means by which we can develop higher performance devices.4 In the recent years, considerable progress has been made toward replacing the silicon in transistors, photovoltaics, and IR detectors with germanium.
References
1) Haller, E. E. Mater. Sci. Semicond. Process. 2006, 9, 408-422.
2) Brinkman, W. F.; Haggan, D. E.; Troutman, W. W. Solid-State Circuits, IEEE 1997, 32, 1858-1865.
3) Zeng, L. H.; Wang, M. Z.; Hu, H.; Nie, B.; Yu, Y. Q.; Wu, C. Y.; Hu, J. G.; Xie, C.; Liang, F. X.; Luo, L. B. ACS App. Mater. Interfaces 2013, 5, 9362-9366.
4) Meyerson, B. S. Sci. Am. 1994, 207, 62-67.