Khashayar Ghaffarzadeh

Publications (3)11.53 Total impact

• Article: Trap-limited and percolation conduction mechanisms in amorphous oxide semiconductor thin film transistors

  Sungsik Lee, Khashayar Ghaffarzadeh, Arokia Nathan, John Robertson, Sanghun Jeon, Changjung Kim, I-Hun Song, U-In Chung
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  ABSTRACT: The electron conduction mechanism in the above-threshold regime in amorphous oxide semiconductor thin film transistors is shown to be controlled by percolation and trap-limited conduction. The band tail state slope controls the field effect mobility, while the average spatial coherence length and potential fluctuation control percolation conduction. In these limits, the field effect mobility is found to follow a power law, from which a universal mobility versus carrier concentration dependence is extracted.
  Applied Physics Letters 06/2011; · 3.84 Impact Factor
• Article: Persistent photoconductivity in Hf–In–Zn–O thin film transistors

  Khashayar Ghaffarzadeh, Arokia Nathan, John Robertson, Sangwook Kim, Sanghun Jeon, Changjung Kim, U-In Chung, Je-Hun Lee
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  ABSTRACT: Passivated Hf–In–Zn–O (HIZO) thin film transistors suffer from a negative threshold voltage shift under visible light stress due to persistent photoconductivity (PPC). Ionization of oxygen vacancy sites is identified as the origin of the PPC following observations of its temperature- and wavelength-dependence. This is further corroborated by the photoluminescence spectrum of the HIZO. We also show that the gate voltage can control the decay of PPC in the dark, giving rise to a memory action.
  Applied Physics Letters 11/2010; · 3.84 Impact Factor
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  Article: Instability in threshold voltage and subthreshold behavior in Hf–In–Zn–O thin film transistors induced by bias-and light-stress

  Khashayar Ghaffarzadeh, Arokia Nathan, John Robertson, Sangwook Kim, Sanghun Jeon, Changjung Kim, U-In Chung, Je-Hun Lee
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  ABSTRACT: Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC). In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (ΔVT), while under light stress, VT consistently shifts negatively. In all cases, there was no significant change in field-effect mobility. Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days. We observe that the PPC becomes more pronounced at shorter wavelengths.
  Applied Physics Letters 09/2010; 97(11):113504-113504-3. · 3.84 Impact Factor