Klaus Dimmler

Radio frequency rectifiers based on organic thin-film transistors

One important technical hurdle that has to be overcome for using organic transistors in radio-frequency identification tags is for these devices to operate at rf frequencies (typically 13.56MHz) in the front end. It was long thought that organic transistors are too slow for this. In this letter we show that organic transistor based full-wave rectifier circuits utilizing pentacene, a p-channel organic semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the nonquasistatic state of the transistors.

Organic transistors: improved performance and fast response

This paper reviews the transport phenomena in pentacene transistors and presents a model of how fast rectifier circuits work. Nanoscale organic and polymer transistor characteristics are also discussed.

Organic complementary circuits using solution deposited active semiconductors

Byungwook Yoo a, Debarshi Basu a, Taeho Jung a, Daniel Fine a, Brooks A. Jones b, Antonio Facchetti b Michael R. Wasielewski b, Tobin J. Marks b, Klaus Dimmler c, and Ananth Dodabalapur a a Department ofElectrical and Computer Engineering, Microelectronics Research Center, The University ofTexas at Austin, 10100 Burnet Rd, Bldg 160, Austin, TX 78758, email: bwyoo(dmail. utexas. edu, ph: 1-512-232-1846 b Department of Chemistry, the Materials Research Center, and the Centerfor Nanofabrication & Molecular SelfAssembly, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208-3113 cOrganicID, 422 East Vermio Ave, Colorado Springs, CO 80903

Flexible Polymer Thin-Film Transistor Device Structures And Processes For 13.56 MHz RF Rectifier Circuits

Polymeric thin-film transistors (TFTs) have been proposed for several applications including displays, electronic paper, chemical sensors, and radio-frequency identification (RFID) tags. One important technical hurdle that has to be overcome for using organic transistors in RFID tags is for these devices to operate at RF frequencies (typically 13.56 MHz) in the front end. It was long thought that conjugated polymer transistors are too slow for this. In this presentation we will demonstrate that polymer transistor based full-wave rectifier circuits utilizing a polythiophene, a p-channel semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the non-quasi static (NQS) state of the transistors. Bottom gate and top gate structures are explored and a comparison is made between the observed electrical properties. These circuits are fabricated on PEN (polyethylenenapthalate) or PET (polyethylene-terepthalate) substrates using a spin on dielectric. Gate, source and drain contacts are defined photolithographically using evaporated gold as the metal. Field-effect mobilities in the range of 0.02 and 0.2 cm 2 /Vs that equal or exceed the highest reported among the ones employing similar geometries on plastic substrates are easily obtained in these systems. In order for NQS based rectification to take place the carrier velocity in a 2 micrometer channel length device needs to be more than 2×10 4 cm/s. This would correspond to a mobility of 0.1cm 2 /V-s at a field of 105 V/cm. The rectifiers were based on a 4-transistor full-wave design. A coil (transformer secondary) drives the AC inputs in differential mode. The lower diode connected transistors in a manner similar to the half-wave rectifier define the DC voltage level of the two AC input signals. The two upper transistors are connected as switches and are used to move current from the AC inputs to the positive DC output. The capacitor is used to hold the peak voltage level supplied by the coil through the switch-connected devices. The voltage rectification efficiency is relatively high being as much as 45% at 13.56 MHz. This is among the fastest polymer transistor circuit of any kind demonstrated to date.

Modeling the Organic Thin Film Transistors

Organic thin-film transistors (OTFTs) appear to have become strong contenders to silicon based MOSFET devices whenever low-cost and relatively low performance circuits are required in applications such as radio frequency identification (RFID) for large volume supply chains. In order to develop circuits based on OTFTs, circuit designers require circuit models that predict the operation of OTFT with a reasonable accuracy. Although, generally, OTFT operation is similar to ordinary silicon MOSFET devices, there are several characteristics that clearly differentiate them. One important difference between the operation of the OTFT and the silicon MOSFET (that is a direct consequence of the physical implementation of OTFT) is that the organic transistor is normally operated in the accumulation mode, while the silicon transistor regularly operates in the inversion mode. Due to the molecular nature of the semiconductor, the carrier mobility is orders of magnitude lower than for the silicon MOSFET. Variable carrier mobility law, low on/off ratio, and the Schottky barrier at the interface between the source/drain metal contact and the organic semiconductor are among other important effects that had to be considered for developing of an accurate circuit model of the organic transistor. The developed model has been used to simulate DC characteristics and also simple circuits such as logic gates, ring oscillators, rectifiers, etc. This paper presents the developed model as well as a comparison between the simulated data and the experimental data. The experimental circuits were fabricated on flexible plastic substrates and employed a solution-cast dielectric. Pentacene was the semiconductor of choice with carrier mobility in the range of 0.1 – 1.5 cm 2 /V.s.

Pentacene organic field-effect transistors for flexible electronics

This paper presents electrical characteristics for high-performance pentacene-based organic field-effect transistors (OFETs) manufactured on polymer substrates. The mobilities as high as 2.13 cm2/V-s are reported for devices with a bottom-contact configuration and solution cast dielectric layers. The influence of the dielectric choice on pentacene structure and carrier mobility as well as a method for the improvement of current injection is discussed.