有机薄膜晶体管电路

We have fabricated organic thin-fifilm transistors and integrated circuits using pentacene as the active material. Devices were fabricated on glass substrates using low-temperature ionbeam sputtered silicon dioxide as the gate dielectric and a doublelayer photoresist process to isolate devices. These transistors have carrier mobility near 0.5 cm²/V-s and on/off current ratio larger than 107. Using a level-shifting design that allows circuits to operate over a wide range of threshold voltages, we have fabricated ring oscillators with propagation delay below 75μs per stage, limited by the level-shifting circuitry. When driven directly, inverters without level shifting show submicrosecond rise and fall time constants.

THIN-FILM TRANSISTORS (TFT’s) using organic semiconductors as the active material have made impressive progress over the last ten years, and it appears increasingly likely that organic TFT’s will fifind application, not only as pixel access elements in low-cost active matrix displays , but also to integrate logic circuitry and memory arrays into lowcost electronic products such as smart cards, smart price and inventory tags, and large-area sensor arrays.

Organic TFT’s based on vacuum-deposited fifilms of the fused-ring aromatic hydrocarbon pentacene have previously shown electrical characteristics comparable to those obtained with hydrogenated amorphous silicon devices, including fifield effect mobility as large as 1.5 cm /V-s and on/off current ratio larger than 10. For simplicity, these early devices were fabricated on single-crystal silicon wafers. To allow integrated TFT circuits on glass or plastic, we have developed a device process using selective metal gates and a low-temperature (80 C), ion-beam sputtered silicon dioxide gate dielectric. Using this process and a level-shifting circuit design, we have fabricated all-organic ring oscillators with minimum propagation delay below 75 sper stage. These are the fastest organic TFT circuits reported to date.

Although bulk pentacene is an excellent insulator with resistivity near 1014 Ω-cm, our pentacene TFT’s often have positive threshold voltage, indicating that a carrier channel is present in the pentacene, even with no gate fifield applied, and a positive gate voltage is necessary to deplete the channel. Wiring pentacene TFT’s in a circuit without introducing unacceptable current leakage therefore requires that the active TFT areas be isolated. Patterning of the active layer is problematic, however, since TFT characteristics tend to degrade signifificantly when the organic fifilm is exposed to solvents such as those commonly used in photolithographic processes.

Fig1

To adjust the logic levels so that they correspond properly to the on- and off-states of the drive TFT, we have integrated a level-shift stage with each logic gate. The level-shift stage requires two additional TFT’s, which act as saturated loads in a voltage divider confifiguration. An additional power supply (the level-shift bias) is used to adjust the output of the level-shift stage, thus allowing inverter operation over a wide range of threshold and supply voltages. The schematic diagram and the voltage transfer characteristics of a depletion-mode inverter circuit with integrated level-shift stage are shown in Fig. 2(b). Large voltage gain is obtained, and the input and output levels are compatible. The increased noise in the transfer characteristics is a result of the low current level selected for the level-shift stage design.

We have demonstrated integrated circuits based on pentacene organic TFT’s. Devices were fabricated on glass substrates using low-temperature ion-beam sputtered silicon dioxide as the gate dielectric. Using a level-shifting circuit design to allow circuit operation with depletion-mode TFT’s and applying a simple technique to pattern the organic active layer, we have demonstrated pentacene inverters with large voltage gain and ring oscillators with sub-75- μs propagation delay, limited by the level-shifting circuitry. When driven directly, pentacene inverters show sub-microsecond rise and fall time constants.