Shahrukh A. Khan

High-Frequency Half-Bit Shift Register With Amorphous-Oxide TFT

We have designed, fabricated, and successfully characterized a ten-stage half-bit shift register utilizing amorphous In-Ga-Zn-O (a-IGZO) thin-film-transistor (TFT) technology. High-performance bottom-gate staggered TFT structures with aluminum source, drain, and gate electrodes and RF-sputtered a-IGZO active material has been developed with a field-effect mobility of 16 cm2/V ·s. The half-bit shift register, which consists of only seven transistors per stage, has been successfully driven at a maximum clock frequency of 40 kHz (sufficient to drive a full VGA display at a frame rate of more than 80 kHz) and minimum rail-to-rail supply voltage of 20 V. This effort further demonstrates the potential of oxide-TFT technology to be employed for systems with higher integration level.

Origin of Low-Frequency Noise in the Low Drain Current Range of Bottom-Gate Amorphous IGZO Thin-Film Transistors

The low-frequency noise of bottom-gate amorphous IGZO thin-film transistors is investigated in the low drain current range. The noise spectra show generation-recombination (g-r) noise at drain currents Id <; 5 nA, attributed to bulk traps located in a thin layer of the IGZO close to the conducting channel. At higher drain currents, a pure 1/f noise is observed. It is shown that the carrier number fluctuations are responsible for the 1/f noise due to trapping/detrapping of carriers in slow oxide traps, located near the interface with uniform spatial distribution.

Effect of uniaxial tensile strain on electrical performance of amorphous IGZO TFTs and circuits on flexible Metal foils

Inexpensive and light-weight flexible displays and sensor electronics would be more rugged and portable than the more conventional rigid substrate-based electronics. Till date, large area flexible systems are enabled by a-Si:H or organics which suffer from low mobilities that limit their use in driver electronics that require higher current drive. This is where oxide-semiconductor (amorphous Indium Gallium Zinc Oxide, IGZO in particular) based thin-film transistors (TFTs) provide an attractive alternative to silicon-based TFTs. Therefore, one needs to study the interdependence of mechanical flexing and electrical performance of these devices as they find applications in flexible large area based electronics. This study systematically investigates the influence of tensile strain on IGZO TFTs and ring oscillators fabricated on flexible stainless steel substrates.

Comparative Study of Active-Over-Metal and Metal-Over-Active Amorphous IGZO Thin-Film Transistors With Low-Frequency Noise Measurements

Bottom-gate metal-over-active (MOA) and active-over-metal (AOM) amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs), with channel lengths varying from 1 to 16 μm, are investigated with low-frequency noise (LFN) measurements in the linear region of operation. In the low drain-current range, LFN originates from carrier number fluctuations, whereas in the high drain-current range, the noise is affected by the series resistance. The extracted gate-insulator trap densities near the interface lead to the conclusion that the AOM a-IGZO TFTs are superior compared to the MOA devices.

Electret field enhanced organic photovoltaic cells

Organic photovoltaic devices (OPVs) are realizing power conversion efficiencies that are of interest for commercial production. However, cell efficiencies are poor to modest when compared to their more traditional silicon and other inorganic based counterparts. Apart from material breakthroughs, there is immense need to therefore engineer novel approaches to improve OPV cell performance. In this work, we have demonstrated feasibility of “Electretic” organic solar cells with improved efficiency based on incorporating functional electret materials into optimized organic photovoltaic cells. Dielectric Materials with the capability of storing electrical excess charges and therefore generating an electric field to adjacent electrodes or semiconductors are known as electrets [1]. The most significant feature of electrets is that they can be charged, discharged and recharged in a very controlled manner after preparation of the whole device in which they are used. In this proof of concept work, we have demonstrated dramatic improvement achieved in the current-voltage characteristic of conventional, simple OPV cells by simulating the electric field effects of an embedded electret layer by external high voltage DC bias levels. The concept will be revolutionary in enabling organic solar cells to break through the efficiency/ cost barrier to become more broadly, more quickly adopted.

High performance IGZO TFTs on steel: Device stability and circuit integration

Oxide-semiconductor based thin-film transistors (TFTs) have advanced tremendously off-late and provides an attractive alternative to silicon-based TFTs. They are usually wide-gap materials, transparent in the visible spectrum and thus render possible ubiquitous transparent electronics. Furthermore, large carrier mobilities in the amorphous phase is achievable due to high degree of localization [1] and suggests that if the carrier concentration can be controlled, the properties of amorphous oxides are quite suitable for TFT applications. In this present study, we present room temperature fabrication and characterization of amorphous IGZO (Indium Gallium Zinc Oxide) based TFTs. Although numerous reports exist about IGZO TFTs, there is few that report high performance device characteristic without the need of post fabrication processes.