Low-power,High-gain Two-dimensional Organic Transistors And Circuits

Organic thin film transistors(OTFTs)have been extensively pursued for printable and wearable electronic applications due to their intrinsic flexibility and low-cost processing.A variety of applications,including wearable electronics and Internet of Things(IoT),require electronic devices to provide high enough current to drive the circuits or to have high gain to amplify the signal at a lower operating voltage.However,there are still huge challenges to make OTFTs more competitive with carbon nanotube films,two-dimensional materials and oxides.First,the mobility of organic semiconductors is generally lower than that of inorganic semiconductors.This leads to low transconductance(gm)and intrinsic gain(Ai=gm·r0,where r0 is output resistance).Second,the contact resistance,which partly arises the vertical access resistance introduced by the finite film thickness and disorders brought by conventional fabrication processes(such as lithography and metal deposition),has become the major limiting factor for its high-frequency operation.Third,the switching of OTFTs is usually not ideal,which leads to a large operating voltage(Vdd).Despite the tremendous efforts,it is still difficult to keep subthreshold swing(SS)close to the Boltzmann thermionic limit(ln(10)(kBT)/q?60 mV/dec,where q is elementary charge,kB is Boltzmann's constant,and T is temperature)over extended range and Vdd low enough to realize battery/wireless operation.Recently,negative capacitance(NC)effect from ferroelectric hafnium oxides provides a promising solution for low-power and complementary metal-oxide-semiconductor compatible electronics.Experimentally,NC transistors have been implemented on silicon,germanium and two-dimensional materials,showing evidence of sub-60 mV/dec switching and enhanced gm.In addition,both theories and experiments have reported the transistion from negative to positive drain induced barrier lowing(DIBL)in NC transistors.This would result in infinite r0 and intrinsic gain near the transition region,which is conductve to the realization of high-gain analogue amplifiers.However,so far,this unique advantage of NC transistors has not been fully exploited.In this thesis,we combine solution-processed two-dimensional organic crystal with ferroelectric HfZrOx(HZO)gating to demonstrate flexible sub-thermionic organic transistors as well as ultra-high-gain amplifier circuits.We further demonstrate battery-powered,integrated wearable electrocardiogram,electromyography and pulse sensors that can amplify human physiological signal with high fidelity.The main contents are as follows:1)By solution shearing method,wafer-scale uniform monolayer 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene(C10-NDTT)films were grown on various substrates,including silicon,quartz and polymide.The single crystal size of the film can reach millimeter level,and the orientation of each crystal domain is close.Meanwhile,the technology of transferring large area monolayer organic semiconductor crystal(OSC)had been developed,which can transfer ultra-thin OSC from original substrate to target substrate without any damage.2)Based on the monolayer C10-NDTT film,ferroelectric HZO gating and the lossless van der Waals(vdW)metal-transfer technique,the sub-thermionic organic thin film transistor(ST-OTFT)had been fabricated.Our technology has several unique features.First,the monolayer organic crystal film ensures excellent gate control while maintaining high channel mobility up to 10.4 cm2V-1s-1.Second,the double-well energy landscape of ferroelectric gating and associated NC effect break the thermionic limit in SS(60 mV/dec)and transconductance efficiency(38.7 S/A)and simultaneously enhances on-current and gm by saving overdrive voltage loss.Third,the uniform film morphology and vdW integration of metal electrodes ensures direct and damage-free contact with the channel,giving contact resistance below 60 ?·cm.3)It is proved that the monolayer ST-OTFT has high stability at air environment with room temperature,good bias stress and stable switching characteristics.In addition,the monolayer ST-OTFT has an ultra-high intrinsic gain of 4.7 × 104,which is more than one order of magnitude higher than ultrathin indium tin oxide and Schottky barrier FETs,and more than two orders of magnitude higher than two-dimensional materials and metal oxide FETs.Other than gm,an important reason for the high Ai is large r0?1010 ? due to near-zero DIBL in the subthreshold region.This can be understood as a consequence of the transition from negative to positive DIBL,leading to infinite r0 at the transition.To gain further insight of the device physics,we performed quantitative modeling using an equivalent circuit model.4)By designing proper electrode patterns,we could build functional circuits such as inverter and logic gates using local backgate devices.The inverter exhibited full swing output near zero input voltage with peak power of?50 nW.Remarkably,we obtained giant voltage gain(Av)of 4.1 × 103(1.1×104)under Vdd=-1 V(-3 V).The gain of our devices again outperforms those similar amplifier structures based on organics,metal oxides,two-dimensional materials and carbon nanotubes by orders of magnitude while maintaining low Vdd.Further,we demonstrated the ST-OTFTs and circuits on flexible substrates.Importantly,the steep slope and high gain were preserved on polyimide substrate,which hold promise for wearable electronics and IoT applications.5)We further demonstrate battery-powered,integrated wearable electrocardiogram,electromyography and pulse sensors that can amplify human physiological signal by 900 times with high fidelity.The sensors are capable of detecting extremely weak electrocardiogram waves(undetectable even by clinical equipment)and diagnosing arrhythmia and atrial fibrillation.Our sub-thermionic OTFT is promising for battery/wireless powered yet performance demanding applications such as electronic skins,radio-frequency identification tags,among many others.