Direct evaluation of low-field mobility and access resistance in pentacene field-effect transistors

Abstract

Organic field-effect transistors (OFETs) suffer from limitations such as low mobility of charge carriers and high access resistance. Direct and accurate evaluation of these quantities becomes crucial for understanding the OFETs properties. We introduce the Y function method (YFM) to pentacene OFETs. This method allows us to evaluate the low-field mobility without the access or contact resistance influence. The low-field mobility is shown to be more appropriate than the currently applied field-effect mobility for the OFETs’ performance evaluation. Its unique advantage is to directly suppress the contact resistance influence in individual transistors, although such contact resistance is a constant as compared to the widely accepted variable one with respect to the gate voltage. After a comparison in detail with the transmission-line method, the YFM proved to be a fast and precise alternative method for the contact resistance evaluation. At the same time, how the contact resistance affects the effective mobility and the field-effect mobility in organic transistors is also addressed.

Modified transmission-line method for contact resistance extraction in organic field-effect transistors

“…4,[11][12][13] As indicated in Fig. 1͑d͒ by solid curve, it satisfies well the empirical dependence R sd 14 where R sd0 , V 0 and ␤ are fitting parameters. R sd0 represents the intrinsic contact resistance level at V G − V 0 =1 V, V 0 denotes the gate voltage at which the channel begins to conduct below threshold and ␤ characterizes the gate voltage dependence of the contact resistance.…”

Contact engineering in organic field-effect transistors

“…2a) [27]. If the transconductance ðg m ¼ ð@I d =@V g ÞÞ starts to decrease from a certain V g (usually for large values of V g and small channel lengths L), then the contact effect starts to dominate the m FET attenuation and R c as well as a contact-free transistor mobility can be extracted from the Y-function ðY ¼ ðI d = ffiffiffiffiffiffi ffi g m p ÞÞ [27]. Because the Y-function method assumes a constant R c with V g , one should turn to TLM to extract R c at arbitrary values of V g (especially at large values of V g ).…”

“…The Y-function method requires only one transfer scan (I d -V g ) of an individual device in linear regime with applying a small sourcedrain voltage (V d ( V g ) (Fig. 2a) [27]. If the transconductance ðg m ¼ ð@I d =@V g ÞÞ starts to decrease from a certain V g (usually for large values of V g and small channel lengths L), then the contact effect starts to dominate the m FET attenuation and R c as well as a contact-free transistor mobility can be extracted from the Y-function ðY ¼ ðI d = ffiffiffiffiffiffi ffi g m p ÞÞ [27].…”

Origin of low-frequency noise in pentacene field-effect transistors

“…the S Id =I 2 D upswing at high I D is bigger for the BC device than for the TC one, which has a lower contact resistance than its BC counterpart (cf. The contact resistances in TC and BC devices have been extracted using the transmission-line method (TLM) [21,22] and was found to vary with V G following an empirical expression R sd (V G ) = R sd0 (V G À V 0 ) Àb , where R sd0 , V 0 and b are fitting parameters [21]. A calculation involving a contact resistance contribution in Eq.…”