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TEMPERATURE DEPENDENT CHARACTERISTICS OF HYDROGENATED AMORPHOUS SILICON THIN FILM TRANSISTORS N. LUSTIG, J. KANICKI, R. WISNIEFF and J. GRIFFITH IBM T.J. Watson Research Center, P.O.Box 218, Yorktown Heights, NY 10598

ABSTRACT The characteristics of inverted staggered hydrogenated amorphous silicon/silicon nitride (a-Si:H/a-SiNx:H) thin film transistors (TFTs) are reported between 80 K and 420 K. The TFTs are found to have three distinct transport regimes. Between 80 K to approximately 260 K, the transport in the TFT channel is dominated by electrons hopping between localized gap states of a-Si:H and is analyzed using Mott's theory of variable- range hopping. As the temperature is increased above -260 K the current becomes thermally activated with an activation energy which depends on the gate voltage. The effective field effect mobility, as determined from the TFT characteristics in saturation, is activated in this regime, with an activation energy 0.10 to 0.15 eV. The various activation energies are found to be sensitive to annealing which can be explained by a reduction in deep and shallow states in the a-Si:H active layer. When operated above -360 K the TFTs become unstable due to rapid changes in threshold voltage under the applied gate field. The behavior of the threshold voltage is described over the entire temperature range and possible mechanisms are discussed.

INTRODUCTION The ability to deposit hydrogenated amorphous silicon over large areas at relatively low temperatures (250°C) makes it an important candidate for several microelectronic applications. Already, it has been widely adopted as a switching element in commercially available liquid crystal flat panel displays [1]. Other emerging applications of a-Si:H TFTs are in printing and document scanning [2] and in logic circuits [3]. In spite of this rapid technological progress there are still many fundamental issues which need to be investigated in order to improve TFT performance and reliability. Of major importance are device speed and stability. The first is limited by the inherently low electron mobility in a-Si:H. Since most of the current in TFTs is carried within 100 A of the interface between a-Si:H and the gate insulator, any improvements in mobility will require a better understanding and control of this region. The second major issue, that of TFT stability, has received a great deal of attention recently [4]. Understanding the different instability mechanisms is essential for predicting the long term performance of the TFTs. In the present work, the characteristics of inverted staggered TFTs are studied over a wide temperature range. Such studies provide us with a better understanding of the underlying transport mechanisms and are useful in material and device optimization. In addition, temperature dependent studies place important constraints on device modeling.

EXPERIMENTAL The TFTs used in this work have the inverted staggered configuration which is the most commonly used structure for flat panel display application. Three layers, a-S