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    Device Physics and Compact Modeling

 

Almost-Off Transistor Electronics: 

 

The quest for low power becomes highly compelling in newly emerging application areas related to wearable devices in the Internet of Things. Here, we report on a Schottky-barrier indium-gallium-zinc-oxide thin-film transistor operating in the deep subthreshold regime (i.e., near the OFF state) at low supply voltages (<1 volt) and ultralow power (<1 nanowatt). By using a Schottky-barrier at the source and drain contacts, the current-voltage characteristics of the transistor were virtually channel-length independent with an infinite output resistance. It exhibited high intrinsic gain (>400) that was both bias and geometry independent. The transistor reported here is useful for sensor interface circuits in wearable devices where high current sensitivity and ultralow power are vital for battery-less operation (See also our Science paper).

 

 

Carrier Transport Mechanisms: 

Amorphous oxide semiconductor (AOS) exhibits high electron mobility even when fabricated at room temperature, making them the leading candidate for the next generation thin film transistor (TFT) technology. One of key requirements for design of flat panel systems are physically-based TFT models. This in turn requires good knowledge of the underlying carrier transport mechanisms in the TFT, and in particular, the DOS and field effect mobility. The AOS material has conduction band fluctuations in the amorphous phase, this leads to localized states in the sub-gap and potential barriers above the conduction band minima (CBM). Thus when carriers have sufficient thermal energy, they are released into the conduction band, and transport is governed by multiple trapping and release events, i.e. by trap-limited conduction (TLC).


Density of States Analysis: 

   

We extract density of localized tail states from measurements of low temperature conduction in the amorphous oxide semiconductor thin film transistors (TFTs). At low temperatures, trap-limited conduction prevails, allowing extraction of the trapped carrier distribution with energy. Using a test device with a-InGaZnO channel layer, the extracted tail state energy and density at the conduction band minima are 20 meV and about 1019 cm-3eV-1, respectively, which are consistent with the values reported in the literature. Also, the field-effect mobility as a function of temperature from 77K to 300K is retrieved for different gate voltages, yielding the percolation threshold.

 

Oxygen Defects and Visible Light Detection:

       

We investigate optical characteristics of the AOS TFTs, in terms of photoconductivity, and present analytical and theoretical way to explain it. Since the AOS has oxygen vacancies due to disorder and imperfection of the atomic structure, the AOS can exhibit a optical absorption even at visible light illumination. However, this elude to a persistent photoconductivity associated with the ionized oxygen vacancies, leading to a light-induced threshold voltage shift. This can be turned around and used as the basis of a high gain photo image sensor, with higher sensitivity that the amorphous silicon equivalent. Here, we analyze the basis of photoconductivity in AOSs and use this to maximize the performance of AOS image sensors.

 

Physically-based TFT Modeling:

  

A physically-based transistor model for oxide TFTs is derived. Since oxide TFTs have shallow tail states, the relevant mobility model is for the case of kTt<kT. Based on this, current-voltage relations, which are to describe terminal characteristics of the transistor, are derived to illustrate the above-threshold regimes, showing a good agreement with the experimental results. For the operation when the gate voltage is smaller than the threshold voltage, sub-threshold characteristics are investigated considering diffusion and drift current components. Additionally, an unified model is discussed to cover both sub- and above-threshold regimes at the same time.