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Open AccessCommentary

A Fundamental Reason for the Need of Two Different Semiconductor Technologies for Complementary Thin-Film Transistor Operations

Department of Electronics, Pusan National University, Pusan 46241, Korea
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Author to whom correspondence should be addressed.
Crystals 2019, 9(11), 603; https://doi.org/10.3390/cryst9110603
Received: 8 November 2019 / Revised: 14 November 2019 / Accepted: 15 November 2019 / Published: 17 November 2019
(This article belongs to the Special Issue Advances in Thin Film Materials and Devices)
In this short commentary, we discuss a fundamental reason why two different semiconductor technologies are needed for complementary thin-film transistor (TFT) operations. It is mainly related to an energy-level matching between the band edge of the semiconductor and the work-function energy of the metal, which is used for the source and drain electrodes. The reference energy level is determined by the energy range of work-functions of typical metals for the source and drain electrodes. With the exception of silicon, both the conduction band edge (EC) and valence band edge (EV) of a single organic or inorganic material are unlikely to match the metal work-function energy whose range is typically from −4 to −6 eV. For example, typical inorganic materials, e.g., Zn–O, have the EC of around −4.5 eV (i.e., electron affinity), so the conduction band edge is within the range of the metal work-function energy, suggesting its suitability for n-channel TFTs. On the other hand, p-type inorganic materials, such as Cu–O, have an EV of around −5.5 eV, so the valence band edge is aligned with metal work-function energy, thus the usage for p-channel TFTs. In the case of p-type and n-type organic materials, their highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) should be aligned with metal work-function energy. For example, p-type organic material, e.g., pentacene, has a HOMO level around −5 eV, which is within the range of the metal work-function energy, implying usage for p-channel TFTs. However, its LUMO level is around −3 eV, not being aligned with the metals’ work-function energy. So it is hard to use pentacene for n-channel TFTs. Along with this, n-type organic materials (e.g., C60) should have HOMO levels within the typical metals’ work-function energy for the usage of n-channel TFT. To support this, we provide a qualitative and comparative study on electronic material properties, such as the electron affinity and band-gap of representative organic and inorganic materials, and the work-function energy of typical metals. View Full-Text
Keywords: thin-film transistors (TFTs); organic semiconductors; inorganic semiconductors; electron affinity; band-gap; metal work-function energy; complementary TFT operation thin-film transistors (TFTs); organic semiconductors; inorganic semiconductors; electron affinity; band-gap; metal work-function energy; complementary TFT operation
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Jang, J.; Lee, S. A Fundamental Reason for the Need of Two Different Semiconductor Technologies for Complementary Thin-Film Transistor Operations. Crystals 2019, 9, 603.

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