Subthreshold Characteristics and Interface State Density of a-Si:H TFT
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SUBTHRESHOLD CHARACTERISTICS AND INTERFACE STATE DENSITY OF a-Si:H TFT
KEIJI MAEDA, HIROKI KOYANAGI AND TOSHIHIDE JINNAI Science University of Tokyo, Noda, Chiba 278, Japan
ABSTRACT Inverted-staggered a-Si:H TUT was prepared by successive PECVD of a- SiN 1 .7 :H and aSi:H layers. Drain current ID vs gate voltage VG1 characteristics of the TFT were investigated. The gatc-voltage swing defined by S=dVG/d(log ID) in the subthreshold region was 1.4 V at room temperature. If the observed S value is attributed to the bulk gap state density, the space- charge layer width is estimated to be about 450 A. This value is too small compared with the a-Si:H layer width of about 3000 A in the TFT, which exhibits good performance. On the other hand, if the S value is attributed to the interface states, a state density of 1.5x10 12 (cm 2 eV)-1 is necessary. Nearly the same density, (1-2)xlO 12 (cm 2 eV)-1, nearly independent of the energy level, was obtained in our a-SiNt. 7 :H/c-Si interface by capacitance measurements. Therefore, it is concluded that the interface states are the main origin of the subthreshold characteristics in our a-Si:H TFT.
INTRODUCTION Hydrogenated amorphous silicon thin film transistors (a-Si:H TPT) are now being extensively developed as an active matrix addressing for liquid-crystal display. A steep increase in drain current ID with gate voltage VG is necessary for the application. The field effect was the first method [11 which demonstrated the dramatic decrease in the gap state density in a-Si:H achieved by the glow discharge deposition technique. It yielded the energy distribution of the rather high density of gap states [2]. It was assumed that there are no surface states in analyzing the field effect data. The density of gap states was found to be very small in undoped a-Si:H by deep-level transient spectroscopy (DLTS) 131. This result is consistent with a small concentration of the spin density in undoped a-Si:H detected by electron spin resonance (ESR) 141. Actually an undoped a-Si:H layer is used at the insulator/semiconductor interface of TFT. In crystalline (c-)Si field-effect transistors (FET), the interface state density is the dominant factor determining the subthreshold characteristics [5]. It is of fundamental importance to understand whether the dominant factor is the bulk gap state density or the interface state density in a-Si:H TFT. Hitherto, experimental verification has been difficult since the complicated analytical process leading to the gap state density hampered insight and the resistivity of undoped a-Si:H is too high to investigate the interface properties [6]. In this paper, TFT samples were prepared and the subthreshold characteristics were analyzed by a simple analytical process. Relevant necessary conditions to be able to explain the characteristics by either mechanism were investigated. Finally, the interface state density was concluded as the principal factor for the subthreshold characteristics Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
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