[Editor’s note: This report comes to us from R.K. Pandey, PhD, Professor at Ingram School of Engineering at Texas State University in San Marcos, Texas.]
by R. K. Pandey
A varistor is a simple two terminal device primarily based on ceramic substrates. It is an indispensable component of almost all electrical and electronic circuits because of its unique ability to offer protection to the circuit and its electronic components, such as a diode or a transistor, from abrupt surges in voltage or current caused by violent weather or by instrumentation failure at power stations.
The varistor—an abbreviation for “variable resistor with non-ohmic varying resistance”—was discovered in 1927 as a new type of rectifier based on copper oxide.1 However, a more modern, brief translation of varistor might be “voltage dependent resistor (VDR)”.
A varistor is connected in parallel to the circuit (or load), which it is supposed to protect. Normally it remains in a passive state until a surge appears, at which time it becomes active and absorbs excess voltage while protecting sensitive electronic components in the circuit. Once the surge subsides, the varistor returns to a passive state.
Ever since its discovery almost 90 years ago, the varistor has remained an indispensable electronic element because of its unique application. Modern varistors for power electronics and general power circuits are based exclusively on metal-oxide semiconductor ceramic substrates of highly modified zinc oxide (ZnO), with the exception of silicon carbide (SiC) substrates. However SiC varistors are far less widespread in use than ZnO varistors.
In spite of its practical importance and long productive life, varistors today remain the unsung hero of electronics, especially when compared to more glamorous silicon-based transistors and diodes. By now the origin of the word “transistor”—a combination of words “transconductance” (or transfer) and “varistor”—has been forgotten.
When the transistor, which is the first solid-state device, was discovered in Bell Labs in 1947, the lab launched a search for a generic name for the new device. Multiple suggestions followed, and, out of six proposals, John R. Pierce’s suggested name of “transistor” emerged as the finalist.2 Already in 1947–1948, scientists suspected or realized an inherent relationship between varistors and transistors.
The discovery of the transistor was recognized as a giant step forward for solid-state physics. Consequently, transistor discoverers Walter Brittain, John Bardeen, and William Shockley received the Nobel Prize in physics in 1956. Transistors are the king of microelectronics today, while varistors, which precede transistors by 20 years, remain the poor cousin.
Nonetheless, new horizons for varistors began to unfold at Texas State University around 2011–2012. With the help of just a few undergraduate electrical engineering students and collaboration with faculty colleague William A. Stapleton, I have begun searching for new applications for varistors.
While studying the modified current–voltage characteristics of varistors based on iron titanate ceramic substrates, my lab soon realized that varistors and transistors are indeed strongly coupled devices. Over all these years, the full potential of varistor devices has not been fully realized. This led to the discovery of so-called varistor–transistor hybrid (VTH) devices.
Three types of transistors embedded in a varistor were identified: (a) voltage-biased transistors (VBT), electric field effect transistors (E-FET), and magnetic field effect transistors (H-FET).3–7 These transistors have all the typical attributes of conventional transistors and encompass multiple applications. VBT transistors are particularly suitable for high-temperature electronics and space electronics, where radiation levels are so high that conventional transistors simply fail to operate.
Forging ahead, we stumbled into the discovery of a novel varistor—or, more precisely, a VDR-based magnetic sensor—with many applications, including exploration of new energy sources by well logging.8,9 Other devices that could be produced using the modified current–voltage characteristics of a varistor include signal amplifiers, low pass filters that include the range of human auditory systems, and audio amplifiers. We hope that by paving the pathway for expanded horizons of a varistors we have shown the way for new ceramic-based electronic technology to emerge.
1 L.O. Grondahl and P.H. Geiger, A new electronic rectifier, Trans. AIEE 46, 357-366, (1927).
2 Bell Telephone Laboratories, Technical Memorandum; May 28, 1948.
3 R. K. Pandey, William A. Stapleton, Ivan Sutanto, Amanda A.Scantlin and Sidney Lin, “Properties and applications of varistor-transistor hybrid devices”, Journal of Electronic Materials, 43(5), 1307-1316195, (2014). DOI 10.10071/s11664-014-3067-8.
4 R. K. Pandey, W. A. Stapleton, I. Sutanto, A. A.Scantlin and S. Lin, “Configurations, characteristics and applications of novel varistor-transistor hybrid devices using pseudobrookite oxide semiconductor ceramic substrates”, Ceramic Transactions, 249, 175-195, (2014), (in Processing and Properties of Ceramics and Composites Series, VI, American Ceramic Society).
5 R. K. Pandey, W.A. Stapleton and Ivan Sutanto, “Nature and characteristics of a voltage biased varistor and its embedded transistor”, IEEE J. Electron Device Soc. 3, 1-8, (2015). DOI 10.1109/JEDS 2015, 2409023.
6 R. K. Pandey, W. A. Stapleton, P. Padmini, J. Dou and R. Schad, “Magnetically tuned varistor-transistor hybrid device”, AIP Advances, 2, 042188/1-8, (2012).
7 US Patent Application Serial # 61/569379/International Application #PCT/US2012/055503 (pending; filed September 2012): “Varistor-transistor hybrid devices”, Inventors: R. K. Pandey, William A. Stapleton, Ivan Sutanto and Amanda A. Scantlin (pending).
8 R. K. Pandey, William A. Stapleton, Ivan Sutanto and M. Shamsuzzoha, “Voltage-biased magnetic sensors based on tuned varistors”, Journal of Electronic Materials, 44(4), 1100-1109, (2015). DOI 10.1007/s1664-015-3632-9.
9 R. K. Pandey, William A. Stapleton and Ivan Sutanto, “Voltage biased magnetic sensors based on IHC 45 VDRs”, Ceramic Transactions, Vol. 252, Processing and Properties of Advanced Ceramics and Composites VII (in press; to appear in print by September 2015).