Injection Length in Staggered Organic Thin Film Transistors: Assessment and Implications for Device Downscaling

Abstract

of organic semiconductors show carrier mobility in excess of 5 cm 2 V −1 s −1 when employed as active materials in long (>10 μm) channel organic thin fi lm transistors (OTFT). [2][3][4][5][6][7][8][9][10][11][12] Therefore by scaling the channel length down to 10-1 μm, in principle OTFTs should be able to operate at relatively high (1-10 MHz) frequencies at reasonable (<10 V) applied voltages. [ 13 ] In practice, apart from some reports, [13][14][15][16][17][18][19][20][21][22] this is often not easy to achieve because of injection issues: [ 23,24 ] contact resistances tend to become dominant over the channel resistance in short channel transistors reducing the expected improvement of device performances. [ 25 ] This situation is severely limiting the range of applications for OTFTs. [ 26 ] Indeed contact resistances arise from contact/semiconductor interface properties hence they are not expected to scale with the transistor channel length. To address this issue not only suitable modifi cations of the contact/semiconductor interface aimed at enhancing the contact injecting properties have to be implemented, [ 27 ] but also the device topology has to be considered. Staggered OTFTs, where the contacts and the transistor channel do not lie in the same plane, are usually characterized by lower contact resistances than coplanar ones: [ 28 ] while in these latter the injection area is limited by the accumulated channel depth (few nanometers), in the former the overlap between gate and source/drain contacts (many micrometers or even tens of micrometers) can be exploited, as shown in Figure 1 . The gate/contact overlapping length has to be suitably engineered: on the one hand it should be large enough to accommodate the injected current and to minimize contact resistances; but on the other hand it should not be too large because overlapping areas not effectively involved in injection solely result in additional parasitic capacitances that deteriorate the device speed. [ 13,29 ] It is therefore important to quantify the injection length, which is the length over which sizeable carrier injection occurs. [ 30,31 ] Despite the large number of studies on contact effects in OTFTs, [ 24,32 ] few of them discuss the effect of the gate to contact overlap: Xu et al. found a relationship between the optimal contact length and the semiconductor thickness in staggered OTFT. [ 33 ] Park et al. found that threshold voltage, fi eld effect mobility and contact resistances are affected by the contact length in staggered OTFT; [ 34 ] Wang et al. simulated the effect of contact length scaling. [ 35 ] Ante et al. studied the effect of contact In staggered thin fi lm transistors, the injection length is the fraction of the gate to contact overlap that is effectively involved in current injection. Its assessment is important to properly downscale device dimensions. In fact, in order to increase transistor operation speed, the whole device footprint should be downscaled, which means both the gate to contact overlap and the channel length, as ...

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

“…Depending on the relative values of R C and R ch , the effective injection area can become smaller, with the regions of the electrodes closer to the channel injecting more charges, a phenomena referred to as current crowding . In addition, as L ov is decreased, the contact resistance increases . Ante et al found that R C increased by an order of magnitude when L ov is smaller than the carrier transfer length (the length near the contact through which 63% of charge is transferred), as compared to when L ov is much larger than the carrier transfer length .…”

“…To achieve high operational speeds, the length of the overlap between the gate and drain/source contacts L ov , shown in Figures c and c, needs to be minimized to reduce the capacitance of the contact. While this would increase device speed, in staggered geometries Natali et al found that there is a minimum contact length to allow for enough area such that there is sufficient current to operate the device . Assuming that the mobility is isotropic, in order to achieve R C ≤ 10% of R tot they suggested that L ov should be 1.5 times larger than the injection length (or transfer length), and that L should be 18 times larger …”

Precise Extraction of Charge Carrier Mobility for Organic Transistors

“…The former component originates from charge injection at the metal‐semiconductor interface and is mainly governed by the Schottky barrier (see Figure a) . The latter component arises from the access transport through the OSC bulk with relatively low charge concentrations and perhaps with an inferior charge transport profile (e.g., caused by contact defects) . Together, these resistances alter the device characteristics, making mobility extraction unreliable or even impossible.…”

Black phosphorus-based one-dimensional photonic crystals and microcavities

“…dual--gate or encapsulated top--gate field--effect transistor devices) [28,29]. Moreover, the typical deposition techniques used for fabricating multilayer PhCs such as Pulse Laser Deposition and Atomic Layer Deposition enable the deposition of high density oxides on top of soft material such as organic and inorganic thin--films [30]. Finally, the realization of the BP--based photonic systems is also compatible with the deposition of BP by means of liquid exfoliation assisted by sonication [31].…”