Low-voltage organic transistors with high transconductance

This thesis presents the development of low-voltage organic thin-film transistors with high transconductance. This was achieved by employing ultra-thin bi-layer gate dielectric consisting of aluminium oxide (AlOx) and a self-assembled monolayer of octadecyl phosphonic acid (C18PA) and by increasing the channel width of the transistors through the implementation of the multi-finger source/drain contacts. The transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) exhibited low turn-on voltage and a.c. transconductance around 30 to 60 µS. Transistor amplifiers based on such transistors exhibited voltage gain approaching 10 V/V and a gain of about 2 V/V when the supply voltage was limited to 5 V. Next, a series of [n]phenacenes ([n] = 5, 6, or 7) was used for the first time in combination with the thin AlOx/C18PA dielectric bi-layer. Regardless of the substrate and the source-drain contact geometry, the field-effect mobility of such transistors was found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene with the largest average field-effect mobility of 0.27 cm2/V⋅s for transistors on glass and 0.092 cm2/V⋅s for transistors on flexible PEN. The highest transconductance of 12.2 µS was achieved for [7]phenacene transistors on glass, which was lower than that achieved for DNTT transistors. In addition, nearly hysteresis-free behaviour, improved charge carrier injection/extraction properties, and reduced threshold voltage were achieved. Finally, a semi-empirical transistor model was developed in Matlab. The model was validated using d.c. and a.c. measurements obtained on DNTT transistors with high transconductance. Four fitting parameters were extracted by optimising a fitting function using genetic algorithm. The model reproduces the d.c. transistor measurements with high accuracy. The error between the measured and simulated peak-to-peak a.c. transconductance values ranged from 1.7% to 11.6%.This thesis presents the development of low-voltage organic thin-film transistors with high transconductance. This was achieved by employing ultra-thin bi-layer gate dielectric consisting of aluminium oxide (AlOx) and a self-assembled monolayer of octadecyl phosphonic acid (C18PA) and by increasing the channel width of the transistors through the implementation of the multi-finger source/drain contacts. The transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) exhibited low turn-on voltage and a.c. transconductance around 30 to 60 µS. Transistor amplifiers based on such transistors exhibited voltage gain approaching 10 V/V and a gain of about 2 V/V when the supply voltage was limited to 5 V. Next, a series of [n]phenacenes ([n] = 5, 6, or 7) was used for the first time in combination with the thin AlOx/C18PA dielectric bi-layer. Regardless of the substrate and the source-drain contact geometry, the field-effect mobility of such transistors was found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene with the largest average field-effect mobility of 0.27 cm2/V⋅s for transistors on glass and 0.092 cm2/V⋅s for transistors on flexible PEN. The highest transconductance of 12.2 µS was achieved for [7]phenacene transistors on glass, which was lower than that achieved for DNTT transistors. In addition, nearly hysteresis-free behaviour, improved charge carrier injection/extraction properties, and reduced threshold voltage were achieved. Finally, a semi-empirical transistor model was developed in Matlab. The model was validated using d.c. and a.c. measurements obtained on DNTT transistors with high transconductance. Four fitting parameters were extracted by optimising a fitting function using genetic algorithm. The model reproduces the d.c. transistor measurements with high accuracy. The error between the measured and simulated peak-to-peak a.c. transconductance values ranged from 1.7% to 11.6%

Organic thin film transistors with multi-finger contacts as voltage amplifiers

Low-voltage p-type organic transistors with two types of multi-finger source/drain contacts were fabricated on glass and polyethylene naphthalate (PEN). They exhibited threshold voltage between −0.3 and −0.5 V and field-effect mobility between 0.2 and 0.4 cm2/Vs. Their transconductance in saturation operation varied from 25 to 60 uS and scaled with the gate dielectric capacitance and transistor dimensions. All transistors operated beyond 1 kHz, while the transistors with the shortest channel length (L = 20 um, W = 4.03 mm) exhibited a cut-off frequency of 13.4 kHz. The transistors were used to build simple voltage amplifiers by adding a resistor Rd on the drain side of the transistor. Higher Rd required higher supply voltage Vdd but resulted in increased voltage gain. A voltage gain in excess of 8 V/V was obtained for Vdd of −12 V and Rd of 220 kohm when transistor with medium value of transconductance of 37 uS was used. Consequently, the voltage gain of 10 V/V is achievable, making such transistor structures viable for sensor development

Low-voltage high-transconductance dinaphtho-[2,3-b:2',3'-f]thieno [3,2-b]thiophene (DNTT) transistors on polyethylene naphthalate (PEN) foils

Low threshold voltage, high transconductance DNTT transistors (OTFTs) with interdigitated source/drain contacts can provide low-voltage transistor amplifiers with a.c. cut-off frequency in excess of 10 kHz [1], making them suitable for wearable sensors. This paper presents an in-depth study of the geometry of such transistors fabricated on PEN. Changes in channel width-to-length ratio W/L were achieved by varying the W from ~12 to ~18 mm and L from 20 to 50 μm, leading to W/L of ~300 to ~900. The OTFTs exhibit threshold voltage from −0.33 to −0.74 V, field-effect mobility from 0.17 to 0.42 cm2/V·s, on-current from 28 to 67 μA (at VGS = VDS = −2 V), off-current from 6×10-12 to 7×10-8 A, and subthreshold slope from 65 to 266 mV/decade. While the OTFTs exhibit large on-state drain current and a.c. transconductance, smaller L leads to a slightly reduced mobility. In addition, the OTFTs with the largest W of 18.23 mm possess the lowest off-state drain current and subthreshold slope

Compact modeling of organic transistors with multi-finger contacts

Organic thin-film transistors (OTFTs) with multi-finger contacts based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thio-phene (DNTT) exhibit near-zero turn-on voltage, hysteresis-free behavior, and high transconductance of 30-80 μA at VDS = VGS = −2 V. [1] In addition, common-source amplifiers based on such transistors deliver voltage gain even when the supply voltage is limited to 5 V, making them attractive for flexible/wearable analog sensors. This paper presents the results of compact modeling, implemented in Matlab Simulink, applied to such transistors. The measured transistor transfer characteristics are used to extract the parameters for the semi-empirical model. The model was validated in 3 ways on 8 OTFTs with varied geometries and substrates (glass or PEN). The validation included calculations of (a) transistor output characteristics, (b) a.c. drain currents for 1 Hz sinusoidal gate voltages, and (c) output voltages of the common-source amplifier, and their comparison to the measured data

Low-voltage organic thin-film transistors based on [n]phenacenes

Low-voltage p-channel organic thin-film transistors based on [n]phenacene (n = 5, 6 or 7) were fabricated on glass and on flexible poly(ethylene 2,6-naphthalate) (PEN) substrates. For the first time, these phenacenes were combined with two ultrathin gate dielectrics based on aluminium oxide and a monolayer of octadecyl-phosphonic acid in three different transistor structures. Regardless of the substrate and the transistor structure, the field-effect mobility is found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene. The largest average field-effect mobility we have obtained is 0.27 cm2/V·s for transistors on glass and 0.092 cm2/V·s for transistors on flexible PEN

Interplay between Vacuum-Grown Monolayers of Alkylphosphonic Acids and the Performance of Organic Transistors Based on Dinaphtho[2,3-b:2?,3?-f]thieno[3,2-b]thiophene

Monolayers of six alkylphosphonic acids ranging from C8 to C18 were prepared by vacuum evaporation and incorporated into low-voltage organic field-effect transistors based on dinaphtho[2,3-b:2?,3?-f ]thieno[3,2-b]thiophene (DNTT). Similar to solution-assembled monolayers, the molecular order for vacuum-deposited monolayers improved with increasing length of the aliphatic tail. At the same time, Fourier transform infrared (FTIR) measurements suggested lower molecular coverage for longer phosphonic acids. The comparison of FTIR and vibration frequencies calculated by density functional theory indicated that monodentate bonding does not occur for any phosphonic acid. All monolayers exhibited low surface energy of ?17.5 mJ/m2 with a dominating Lifshitz?van der Waals component. Their surface roughness was comparable, while the nanomechanical properties were varied but not correlated to the length of the molecule. However, large improvement in transistor performance was observed with increasing length of the aliphatic tail. Upon going from C8 to C18, the mean threshold voltage decreased from ?1.37 to ?1.24 V, the field-effect mobility increased from 0.03 to 0.33 cm2/(V·s), the off-current decreased from ?8 × 10?13 to ?3 × 10?13 A, and for transistors with L = 30 ?m the on-current increased from ?3 × 10?8 to ?2 × 10?6 A, and the on/off-current ratio increased from ?3 × 104 to ?4 × 106. Similarly, transistors with longer phosphonic acids exhibited much better air and bias-stress stability. The achieved transistor performance opens up a completely “dry” fabrication route for ultrathin dielectrics and low-voltage organic transistors