Research Project Highlight

Tuning of Threshold Voltage in Organic Field-Effect Transistors

Figure 1. Schematic of individual OFET structure, top view of charged and uncharged regions, plot of surface potential vs position along x and y on an SEM-charged sample, and charge density vs beam energy (lower and left axis, ●, SEM) or Vgrid (upper and right axis, red △, corona).

Figure 1. Schematic of individual OFET structure, top view of charged and uncharged regions, plot of surface potential vs position along x and y on an SEM-charged sample, and charge density vs beam energy (lower and left axis, ●, SEM) or Vgrid (upper and right axis, red △, corona).

Organic field-effect transistors (OFETs) were made with the hydrophobic gate dielectric exposed to an electron beam before semiconductor deposition, shifting the threshold voltage toward positive gate bias for a p-channel semiconductor. A 1-µm Cytop film was irradiated in specified regions with defined doses of electron beams with different energies. Charge deposition and threshold voltage shifts were observed selectively in the exposed regions (Figure 1).

The charges/polarizations embedded in the dielectric by the irradiation have effective charge densities of ca 10-8 C/cm2. OFETs were completed using 5,5’-bis(4-hexylphenyl)-2,2’-bithiophene as the semiconductor, and showed corresponding shifts in Vth. Other OFETs were made where the gate dielectric was treated with a corona discharge. Both types of devices showed similar shifts in Vth and transfer characteristics. There is no change in mobility of the charge carriers after either charging process. The charges do not contribute to the gate capacitance, but induce changes in the onset of capacitance increase caused by accumulation of mobile channel charge during capacitance-voltage experiments in two-terminal metal-insulator-semiconductor-metal vertical capacitor configurations. Print-compatible processes are under development.

Related

We are developing methods of imparting static charge to the transistors during the device fabrication so that transistors with the same materials and dimensions can serve different functions in circuits.

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