E.J. Lous

Organic n-type field-effect transistor

Abstract We have used tetracyanoquinodimethane (TCNQ) as the active semiconducting material in metal-insulator-semiconductor field-effect transistors (MISFETs). TCNQ behaves as an n-type semiconductor. Differential capacitance measurements on metal-insulator-semiconductor (MIS) devices confirm the n-type behaviour. A maximum field-effect mobility of 3 × 10 −5 cm 2 V −1 s −1 is observed. On exposure to air the on/off ratio of the FETs improves to in excess of 450, due to oxidative dedoping of the TCNQ and narrowing of the channel.

Metal-insulator-semiconductor Schottky-type diodes of doped thiophene oligomers

Abstract Schottky barrier diodes were made from films of the thiophene oligomer α,α′-coupled dodecathiophene substituted with four n-dodecyl side chains, T12d4(2,5,8,11), which were doped in solution with various amounts of mainly 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Eutectic GaIn alloy or evaporated In were applied as Schottky contacts. The diodes exhibited strong rectification up to 106 at +1 V/−1 V, diode quality factors between 1.2 and 2, and forward current densities up to a few A cm−2 at 1 V forward bias. Analysis of complex impedance spectra and capacitance-voltage measurements showed that the impedance of the diodes is due to the actual Schottky junction, which is similar for all doped diodes, and to a ‘non-conducting’ interface layer which is formed by chemical reaction upon application of the Schottky metal. Current transport at high forward bias is not bulk limited, but due to charge injection across this thin interfacial film and, hence, manifested as space charge limited current. It will be argued that the diodes can tentatively be described as metal-insulator-semiconductor (MIS) Schottky diodes with a graded dopant profile. This interpretation is also supported by a correlation found between forward and saturation current densities.

FORMATION OF A SCHOTTKY BARRIER BETWEEN EUTECTIC GA,IN AND THIOPHENE OLIGOMERS

The formation of a Schottky barrier between an eutectic (Ga,In) alloy and a highly doped thiophene oligomer is followed as a function of time using current density–voltage and capacitance–voltage measurements. Within 1 h, the diode characteristics change from almost nonrectifying, leaky behavior into a rectification ratio of 104 with a considerably reduced leakage current. Measurements and energy band diagram calculations show that the depletion width increases with time due to a decrease in the ionizable acceptor density of the semiconductor at the Schottky interface. This is probably caused by a chemical reaction between the in-diffusing metals and the doped oligomer.

Electrical characterization of gap states in a highly doped conducting oligomer

The electrical characteristics of organic semiconducting devices are strongly influenced by the presence of interface states, midgap states and trap states in the band gap of the semiconductor. Therefore, knowledge of the density of states (DOS) in the band gap of the organic semiconductor is important to characterize and improve future organic devices. In this paper we explore a method that is based on frequency-dependent capacitance-voltage measurements on a highly doped In-thiophene Schottky diode. The obtained DOS of the highly doped thiophene clearly contains structure, with a peak around 0.52 ± 0.06 eV above the valence band.