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Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used not only to optimize these devices but also to enable new design principles as well. Here, a novel type of OFET is reported—the vertical organic tunnel field-effect transistor. Based on heterogeneously doped drain and source contacts, charge carriers are injected from an n-doped source electrode into the channel by Zener tunneling and are transported toward a p-doped drain electrode. The working mechanism of these transistors is discussed with the help of a tunnel model that takes energetic broadening of transport states in organic semiconductors and roughness of organic layers into account. The proposed device principle opens new ways to optimize OFETs. It is shown that the Zener junction included between the source and drain of the vertical organic tunnel field-effect transistors suppresses short channel effects and improves the saturation of vertical OFETs. 

Organic electrochemical transistors (OECTs) are becoming a key device in the field of organic bioelectronics. For many applications of OECTs, in particular for enzymatic sensing, a complex mixture of room temperature ionic liquids (RTILs) combined with other electrolytes is used as a gate electrolyte, making the interpretation of experimental trends challenging. Here, the switching mechanism of OECTs using such RTILs is studied. It shows that ions smaller in size than the ions contained in the RTIL (e.g., Na+) have to be added to the ionic liquid to ensure switching of the OECTs. Furthermore, it is shown that OECTs based on RTILs exhibit noticeable gate‐bias stress effects and a hysteresis in the electrical transfer characteristics. A model based on incomplete charging/discharging of the effective gate capacitance during operation of the OECT and a dispersion in the ion mobilities is proposed to explain these instabilities, and thus it shows that the hysteresis can be minimized by optimizing the geometry of the device. Overall, a better understanding of the underlying mechanisms of switching and stability of OECTs based on RTILs is the first step toward various applications such as lactate acid sensors and neurotransmitter recording. 

Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C₆₀metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C₆₀OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 10⁷are realized. The devices have low subthreshold swing in the range of 80 mV dec⁻¹and a large transconductance of up to 8 mS mm⁻¹.

 

Research in Organic Permeable Base Transistors (OPBTs) has led to a significant increase in their performance. However, despite this progress, understanding of the working mechanism of OPBTs is still limited. Although first numerical models of OPBTs are able to describe the switching mechanism of OPBTs correctly, they neglect currents injected at the base electrode, which leads to unrealistically low off-currents and high ON/OFF ratios. Here, a tunneling model is developed that is capable of describing injection of charges through a thin oxide layer formed around the base electrode of OPBTs. With the help of this injection model, the performance of the base-collector diode of OPBTs is discussed. In particular, the model is used to explain the reduction in backward currents due to an exposure to ambient air by an increase in the thickness of the oxide layer. Furthermore, the tunnel model is used to show that the reduction in backward currents of the base-collector diode leads to a decrease in off-currents of complete OPBTs, which in turn leads to an increase in their ON/OFF ratio. 

Organic permeable‐base transistors (OPBTs) show potential for high‐speed, flexible electronics. Scaling laws of OPBTs are discussed and it is shown that OPBT performance can be increased by reducing their effective device area. Comparing the performance of optimized OPBTs with state‐of‐the‐art organic field‐effect transistors (OFETs), it is shown that OPBTs have a higher potential for an increased transit frequency. Not only do OPBTs reach higher transconductance values without the need for sophisticated structuring techniques, but they are also less sensitive to parasitic contact resistances. With the help of a 2D numerical model, the reduced contact resistances of OPBTs are explained by a homogeneous injection of current across the entire emitter electrode, compared to injection in a small area along the edge of the source of OFETs.