Field effect transistors (FETs) are active devices that use electric fields to control the conductivity of solid materials. Because of its small size, light weight, low power consumption, good thermal stability, no secondary breakdown phenomenon and wide safe working area, it has become one of the important components in the microelectronics industry.Field effect transistors (FETs) are active devices that use electric fields to control the conductivity of solid materials. Because of its small size, light weight, low power consumption, good thermal stability, no secondary breakdown phenomenon and wide safe working area, it has become one of the important components in the microelectronics industry. At present, inorganic field effect transistors are close to the natural limit of miniaturization, and the price is high. There are still many problems in the preparation of large surface area devices. Therefore, people naturally think of using organic materials as active materials of FET. Since 1986, when the first organic field effect transistor (OFET) was reported, the research of OFET has developed rapidly and made significant breakthroughs.Field effect transistors (FETs) are active devices that use electric fields to control the conductivity of solid materials. Because of its small size, light weight, low power consumption, good thermal stability, no secondary breakdown phenomenon and wide safe working area, it has become one of the important components in the microelectronics industry.
What is an Organic Field Effect Transistor–Basic Structure of an Organic Field Effect Transistor
Traditional organic field effect transistors mainly consist of bottom gate and top gate structures. The bottom gate and top gate structures include top contact and bottom contact, respectively, as shown in Figure 1.
OFET generally adopts bottom gate structure with bottom grid, i.e. the two structures shown in Fig. 1 (a) and (b), which are bottom gate-top contact structure and bottom grid-bottom contact structure respectively. The biggest difference between the two is whether the organic layer is in front of the plating electrode (top contact) or after the plating electrode (bottom contact). The source and drain electrodes of the top contact structure are far away from the substrate, and the organic semiconductor layer is directly connected with the insulating layer. In the process of fabrication, modification of the insulating layer can be adopted to change the film-forming structure and morphology of the semiconductor, so as to improve the carrier mobility of the device. At the same time, the area of the semiconductor layer affected by the grid electric field is larger than that of the device structure with the source and drain electrodes at the bottom, so it has higher carrier mobility. The main characteristic of bottom contact OFET is that the organic semiconductor layer is evaporated on the source and drain electrodes, and the device structure of source and drain electrodes at the bottom can be fabricated by lithography at one time, which can simplify the fabrication process. Moreover, for organic sensors, semiconductor layers need to be exposed to the test environment uncovered, so the use of the bottom structure has a greater advantage. However, due to the large contact resistance between the semiconductor layer and the metal electrode, the carrier injection efficiency is reduced and the performance is affected. At present, there are some improvements in this aspect, such as the use of gold electrodes coated with polyethylenedioxythiophene and polystyrene sulfonate (PEDOT: PSS) materials can reduce the contact resistance between organic semiconductor and pentacene materials. The resistance of carrier injection between them decreases directly from 0.85 eV to 0.14 eV, resulting in an increase in field mobility from 0.031 cm2/(V.s) to 0.218 cm2/(V.s).
Fig. 1 (c), (d) is a top gate structure. First, organic semiconductor layer is fabricated on the substrate, then source and drain electrodes are fabricated, then insulating layer is fabricated, and finally gate is fabricated on the insulating layer. There are not many reports about the top gate structures of these two kinds of gates.
Fig. 2 is a vertical channel OFET structure. It is a new type of field effect transistor designed to shorten the channel length. It takes semiconductor layer as channel length, evaporates leak-source-Shan electrodes in turn, and controls the current change of source and leak electrodes by changing gate voltage.
The main characteristics of this structure are that the channel length is reduced from micron to nanometer, which greatly improves the working current of the device and reduces the open voltage of the device. The disadvantage of this kind of transistor is that the leakage-source-gate are in the same vertical plane, and the parasitic capacitance between them makes the zero current drift, which can be avoided by discharge treatment.
Application of Organic Field Effect Transistors
As early as 1954, people began to study organic crystal materials, but the crystal quality is very low. Nowadays, high quality organic crystal materials have been prepared, which create favorable conditions for the research of organic crystal semiconductor devices. J. H. Schon, Bell Laboratory, USA, etc. made organic crystal field effect transistors (FETs) from crystal materials of pentacene and Tetracene. In the MISFET structure shown below, the crystal semiconductor material is pentacene, and the insulating layer is Al2O3 film with a thickness of 250 nm. The source and drain electrodes are gold with a thickness of 100 nm.
This organic crystal material has high resistivity. Under the action of gate voltage, a high concentration of holes or electrons is induced on the surface of the organic crystal film to form a high conductivity layer, thus forming ohmic contact with the gold electrode. The characteristic curve of the field effect at room temperature is shown below. The field effect mobility of holes and electrons at room temperature is 2.7 and 1.7 cm2/V.s. With the decrease of temperature, the field effect mobility increases exponentially. When the temperature is below 10K, the field effect mobility of holes and electrons increases to 1200 and 320 cm2/V.s, respectively.