Inkjet-printed high performance oxide electronics and its application at sensor interfaces

Abstract

Over the last two decades, printing of oxide semiconductors has received special attention due to its advantages such as low processing temperature, even at room temperature; high electron mobility; good transparency due to the oxide semiconductor's wide bandgap; and excellent uniformity and surface flatness. However, due to the lack of a performance-matched high mobility p-type (hole conducting) oxide semiconductor, which is one of the primary hurdles for implementing all oxide CMOS. In contrast, a pseudo-CMOS inverter provides an alternate method for creating digital circuits. In this study, two alternative types of device designs are used to improve transistor characteristics, which play a crucial part in determining the performance of the circuits. Initially, the deep subthreshold regime of indium-gallium zinc oxide (a-IGZO) TFTs has been used to create a pseudo-CMOS inverter with a high voltage gain (η = 285) and a low power consumption (nW). Furthermore, electrolyte gated pseudo-CMOS inverters have been utilized to create fundamental circuits such as ring oscillators and SRAM that can function at frequencies in the kHz range. To enhance the very low input signal (from mV to V), this high gain inverter has also been employed as an amplifier and differential amplifier. Furthermore, an analog-to-digital converter (ADC) has been created using a proposed simple new circuit design, which is again coupled to a sensor (may be any sensor) for converting the analog input signal (from sensor) to digital output signal. The ADC circuit connecting with ARDUINO and ESP232 (wireless board) demonstrates hybrid electronics compatibility. Furthermore, by combining a current drive circuit with an ADC circuit, the entire circuit functionality becomes a current-based application in which a little change at the sensor side (input) results in a large current/power draw for the real-time application. These characteristic results give us a scope to move one step closer in developing full printed sensor includes biological sensor, pressure sensor, gas sensor, humidity sensor etc., patches with readout electronics on flexible substrates. Interestingly, the whole circuit operates at low supply voltage of 2 V. To improve further important key parameters of a TFT such as transconductance and subthreshold slope, a completely new device architecture i.e., nc-FET has been fabricated where gate insulator is being replaced by a stack of dielectric (Al2O3)/ ferroelectric insulator (PVDF-TrFE); it results into substantially reduction in subthreshold slope below Boltzmann’s limit which translates into a very huge voltage gain (η = 2691) as well as very low power consumption (nW) for depletion-load type inverter based on nc-FET. While the observed results are significantly superior when compared to the state of the art in printed electronics, it also paves way for many applications in the portable, wearable electronics domain, including interface circuits for sensors, displays etc.