Fabrication of a cadmium sulfide thin film transistor using chemical bath deposition Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r207ts38p

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  • The digital revolution has brought information to every corner of our daily lives. Inexpensive and flexible integrated circuits are needed for this continuing revolution. Silicon technology, the current workhorse of microelectronic industry, is far from inexpensive and flexible. Researchers are taking several different routes to achieve this goal. Amorphous silicon is the current material of choice for low cost thin film transistors (TFT). They are relatively cheap compared to their single crystal silicon cousin. Organic (molecular crystals or polymeric semiconductors) electronics with the advantages of flexibility and compatibility with low cost plastic substrates are the other major candidate. Another promising alternative is to fabricate compound semiconductor devices by low temperature (plastic compatible) processing technology. In this work, a solution-based low temperature processing method known as Chemical Bath Deposition (CBD) was explored to fabricate thin film transistors. CBD is an aqueous analog of Chemical Vapor Deposition (CVD). The constituent ions are dissolved in a water solution, and the thin films are produced through a heterogeneous surface reaction at a temperature of 60° C. It is known that CBD is capable of producing an epitaxial layer (e.g., CdS) on a single crystal surface. This thesis reports a functional TFT using a CBD deposited CdS semiconductor layer. A 950 A thick CBD CdS film was deposited on thermally grown SiO₂ at the surface of a heavily antimony doped (n⁺) silicon wafer. Twenty and forty micron transistors were fabricated by conventional photolithography using Al deposited on CdS as source and drain contacts. Thus, the device is essentially an inverted gate thin film MOS transistor with n⁺ Si wafer serving as the gate contact. It has clearly demonstrated transistor action as shown from the I-V measurements. More importantly, the device characterization data and modeling shows that the CdS film is n-type with a doping concentration of 2x10¹⁶ cm⁻³ and an effective carrier mobility of 2 cm²/V s. This performance is comparable to most amorphous Si (0.1 to 1 cm²/V⋅s) and better than a typical organic thin film transistor (10⁻² cm²/V⋅s). The interface state distribution at the CdS-SiO₂ interface was also determined by C-V measurements.
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