Reza Shabanpour

A Compact a-IGZO TFT Model Based on MOSFET SPICE

This letter presents a compact model for flexible analog/RF circuits design with amorphous indium-gallium-zinc oxide thin-film transistors (TFTs). The model is based on the MOSFET LEVEL=3 SPICE model template, where parameters are fitted to measurements for both dc and ac characteristics. The proposed TFT compact model shows good scalability of the drain current for device channel lengths ranging from 50 to 3.6 μm. The compact model is validated by comparing measurements and simulations of various TFT amplifier circuits. These include a two-stage cascode amplifier showing 10 dB of voltage gain and 2.9 MHz of bandwidth.

{\rm Level}=3

Fully printed organic field effect transistors (OFETs) are fabricated on a flexible, 100-μm-thick, polyethylene terephthalate substrate using high-throughput printing techniques: 1) Cyflex; 2) gravure; 3) screen; and 4) flexographic printing without using a cleanroom, and below 130°C. The dependence of the transconductance g_m, transit-frequency f_T, and intrinsic-gain on the bias drain current I_D are measured. The OFETs show intrinsic gain for I_D >10 nA mm (per millimeter width), and reach f_T=64 kHz at I_D = 16 μA/mm, whereas the g_m loss with frequency is 10% up to f_T. Unlike silicon MOSFETs, the dependence of the OFET g_m on the f_T in the subthreshold region is found to be weaker than I_D^1.0. In addition, the overlap capacitance of the staggered-geometry OFET shows strong frequency dependence, and this is shown to be related to the overlap semiconductor. For the first time, it is found that the impact of process variations and bias stress on the OFET analog characteristics can be significantly attenuated by biasing the device at a fixed I_D. This approach is tested on an array of five amplifiers, reaching the gain-bandwidth product of 32 kHz, within ±3.7% variations.

Template for Analog/RF Circuit Designs

This paper presents a cascode amplifier for bendable analog and radio-frequency electronic systems in a flexible amorphous indium gallium zinc oxide (a-IGZO) TFT technology, featuring a minimum gate length of 5 μm. The design is optimized for large bandwidth. The circuit design was carried out with a MOSFET LEVEL=3 SPICE model template. The required model parameters were extracted from both DC and AC measured characteristics. Measurements results show 10.5 dB of voltage gain and a 3 dB bandwidth of 2.62 MHz; the small-signal performance was closely predicted by simulations. The presented circuit provides the highest frequency of operation reported for a single-stage cascode amplifier in a-IGZO TFT technology to date.

Analog Characteristics of Fully Printed Flexible Organic Transistors Fabricated With Low-Cost Mass-Printing Techniques

This paper presents two Cherry-Hooper amplifiers for bendable analog radio-frequency electronic systems fabricated in flexible self-aligned amorphous indium gallium zinc oxide (a-IGZO) thin-film-transistor (TFT) technology. The first circuit is a wideband single-stage Cherry-Hooper amplifier providing a voltage gain of 10.4 dB over a 3 dB bandwidth of 3.5 MHz and a gain-bandwidth product (GBP) of 11.6 MHz. For the second circuit, a two-stage Cherry-Hooper amplifier, a voltage gain of 33.3 dB with a 3 dB bandwidth of 400 kHz and a GBP of 18.5 MHz were measured. The simulations, which are based on a Rensselaer Polytechnic Institute-amorphous TFT (RPI-aTFT) model, are in good agreement with measurements.

A 2.62 MHz 762 µW cascode amplifier in flexible a-IGZO thin-film technology for textile and wearable-electronics applications

This paper presents an operational amplifier based on pseudo-CMOS blocks and integrated in a flexible a-IGZO TFT technology. The circuit consists of only nMOS transistors, and the pair of active loads is in a pseudo-CMOS configuration. These active loads allow various kinds of common mode feedback schemes or cross-coupled connection, typical for CMOS operational amplifiers. The proposed amplifier is fabricated on a flexible film, and characterized with 5 V supply voltage and an output load capacitance of 15 pF. The measured open-loop gain is 22.5 dB, which is the highest reported for operational amplifiers in metal-oxide TFT technology. The measured bandwidth and gain bandwidth products are 5.6 kHz, and 31 kHz, respectively with 160 μW power consumption, which is lowest among flexible operational amplifies.

Cherry-Hooper amplifiers with 33 dB gain at 400 kHz BW and 10 dB gain at 3.5 MHz BW in flexible self-aligned a-IGZO TFT technology

In this paper, a printed audio amplifier, which is a new application for organic electronics, is suggested. The amplifier consists of several fully-printed bendable components including: a loudspeaker, organic field effect transistors (OFETs), capacitors, and resistors. All components are fabricated on polyethylene terephthalate (PET) substrate by means of high-throughput printing techniques. A complete self-biased circuit is reported consisting of large multi-finger OFETs with channel length of 20 μm and total width of 0.475 meter. The amplifier provides a peak voltage gain of 18 dB at 400 Hz, can reproduce sound pressure level of 36-60 dBA over 700 Hz to 12.5 kHz at one meter distance, and has a unity-gain-bandwidth of 17.7 kHz/5.2 kHz when driving 0 nF/~39 nF load at VDD = 80 V, respectively. The impact of bias-stress effects on the amplifier performance is measured to be ~3 dBA sound loss after 5 hours of continuous operation. The whole circuit is packaged and laminated on a separate PET sheet. In addition, the intrinsic electrical impedance of the printed PVDF-TrFE piezoelectric polymer used in the loudspeaker is characterized, and is modeled by a complex dielectric constant.

22.5 dB open-loop gain, 31 kHz GBW pseudo-CMOS based operational amplifier with a-IGZO TFTs on a flexible film

This paper presents an AM receiver implemented in a flexible a-IGZO TFT technology. The circuit consists of a four-stage cascode amplifier at the RF input, a detector based on a source follower, and a common source circuit for the baseband amplification. The measured conversion gain is very flat and exceeds 15 dB from 2 to 20 MHz carrier frequency range, which covers a relevant portion of the shortwave radio band. The 3 dB-bandwidth of the audio signal is 400 Hz to 10 kHz, which is comparable to the so-called voice band, and it is also suitable to low-rate data communication. In addition, an integrated demonstration of the AM receiver and textile antennas is carried out. The flexible a-IGZO receiver successfully detected the baseband signal through the textile antennas, demonstrating for the first time wireless transmission for this class of technologies.

A Fully-Printed Self-Biased Polymeric Audio Amplifier for Driving Fully-Printed Piezoelectric Loudspeakers

To our knowledge, this paper presents the first high-gain amplifiers fabricated in flexible self-aligned amorphous indium gallium zinc oxide (a-IGZO) thin-film-transistor (TFT) technology. For the common source amplifier applying positive feedback a voltage gain of 17 dB, a bandwidth of 79 kHz and a DC power of only 0.76 mW were measured. For the cascode amplifier a voltage gain of 25 dB voltage gain, a bandwidth of 220 kHz and a DC power of 2.32 mW were measured. The simulations based on a RPI-aTFT model are compared with measurements. The chip areas are 8 and 10 mm2, respectively.

15 dB conversion gain, 20 MHz carrier frequency AM receiver in flexible a-IGZO TFT technology with textile antennas

The quest for high-performance flexible circuits call for scaling of the minimum feature size in thin-film transistors (TFTs). Although reduced channel lengths can guarantee an improvement in the electrical properties of the devices, proper design rules also play a crucial role to minimize parasitics when designing fast circuits. In this letter, systematic computer-aided design simulations have guided the fabrication of high-performance flexible operational amplifiers (opamps) and logic circuits based on indium-gallium-zinc-oxide TFTs. In particular, the performance improvements due to the use of an additional third metal layer for the interconnections have been estimated for the first time. Encouraged by the simulated enhancements resulting by the decreased parasitic resistances and capacitances, both TFTs and circuits have been realized on a free-standing 50-μm-thick polymide foil using three metal layers. Despite the thicker layer stack, the TFTs have shown mechanical stability down to 5-mm bending radii. Moreover, the opamps and the logic circuits have yielded improved electrical performance with respect to the architecture with two metal layers: gain-bandwidth-product increased by 16.9%, for the first one, and propagation delay (tpd) decreased by 43%, for the latter one.

High gain amplifiers in flexible self-aligned a-IGZO thin-film-transistor technology

In this paper we present the design, measurements, simulation, and modelling of a flexible textile loop antenna. Using embroidering techniques, a stranded copper wire is integrated onto a textile substrate, which in turn can be part of a garment or wearable accessory. The targeted application is a textile antenna integrated into the sides of a messenger bag next to fully flexible a-IGZO TFT receiver circuitry. The carrier frequency is dictated by the relatively slow TFTs, which limit the operation frequency of the receiver. As a result, a compact antenna for frequencies in the range of 100 kHz to 10 MHz is required. For this purpose loop antennas are suited. They usually provide only a limited gain, yet they allow for a low profile. Since the performance of thin and organic large area electronics (TOLAE) technologies improves steadily, we also investigate their applicability for higher carrier frequencies, which will enable smaller antenna dimensions. Finally we present a fully flexible TFT circuit to tune the antenna characteristics.