Experimental and theoretical analysis of 1/f noise in polysilicon thin film transistors

Abstract

ABSTRACT A study of low frequency noise is made in Solid Phase Crystallised (SPC) polysilicon Thin Film Transistors (TFTs) issued from low temperature process ( 600 °C). The study is performed in both below and above threshold regions. At first, a static electrical caracterisation of the transistors is carried out. Analysis of the low frequency noise in the TFTs shows that it can be related both to the Meyer- Neldel MN effect, and to the flatband voltage fluctuations due to the trapping/ detrapping processes of carriers at the S i O 2 /Poly-S i interface. Furthermore, a new method of the channel carrier number calculation is proposed. Then, the apparent noise parameter app , based on the Hooge formula, is deduced. At low gate voltages app increases and reaches a maximum value close to the threshold voltage. This app singular behavior is then discussed. Keywords: TFTs, 1/f noise, Meyer- Neldel effect, interface defect density, apparent noise parameter. 1. INTRODUCTION Thin film transistors are useful for addressing pixels and driver integrated circuits in active matrix liquid crystal displays (AMLCDs) and in active matrix organic light emitting displays (AMOLEDs). These transistors require high electrical performances such as a high mobility carriers and particularly a low noise level. Carrier transport through the channel is strongly affected by the trapping effect of carriers at the grain boundaries, causing electrical discontinuities in the channel. In this case, the electrical properties in po lysilicon TFTs and those in MOSFETs are different in an important way. For this reason, the conduction and fluctuation existing models which treat a polysilicon channel like homogeneous material, are incompetent to give actual conduction and fluctuation gate bias dependence, in particular from the subthreshold to the on state regions. This limitation led us to study low frequency noise in polysilicon TFTs related to static electrical conduction in the two (operating) regions. Furthermore, in this context we propose in this paper a new method to extend the determination of the apparent noise parameter