1/f Noise Behavior in Pentacene Organic Thin Film Transistors
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Rensselaer Polytechnic Institute, Dept. of Electrical, Computer and System Engineering, Troy, NY; 2 Pennsylvania State University, Dept. of Electrical Engineering, University Park, PA; 3 A.F. Ioffe Institute of Russian Academy of Sciences, Solid State Electronics Division, St. Petersburg, Russia ABSTRACT We studied the low frequency noise in top-contact pentacene Thin Film Transistors (TFTs). The relative spectral noise density of the drain current fluctuations SI /I2 had a form of 1/f noise in the measured frequency range 1Hz - 3.5kHz. Our studies of the noise dependencies on the gate-source VGS and drain-source VDS voltages showed that the dependencies differed from those observed for conducting polymers and resembled those reported for crystalline Si n-MOSFETs. To compare the device noise level with those of other devices and materials, we extracted the Hooge parameter α. In order to calculate the total number of carriers we used a model simulating the device DC characteristics, similar to that for amorphous Si TFTs. The extracted Hooge parameter was 0.04. For an organic material this is an extremely small value, which is three orders of magnitude smaller that the Hooge parameter values reported for conducting polymers and only several times higher than the values for amorphous Si TFTs. INTRODUCTION The demonstration of pentacene Thin Film Transistors (TFTs) with a field effect mobility as high as 2.1 cm2/V-s and the drain current on/off ratio as high as 108 [1] illustrates a high potential of this technology for active display applications. Pentacenebased five-stage ring oscillators with propagation delay 73 µsec [2] as well as complementary organic/inorganic circuits [3] have been demonstrated. In this paper we present the results of our experimental studies of 1/f noise in the pentacene TFT devices. 1/f noise in MOSFETs and TFTs is an important measure of the device quality that is linked to the carrier transport in the device channel [4]. EXPERIMENT In this work we measured 1/f noise in a top contact pentacene TFTs (Figure 1). The TFT had the gate length L and the gate width W of 30 µm and 220 µm, respectively. Gold source and drain contacts were deposited by vacuum evaporation through a shadow mask. A heavily doped Si substrate served as a common gate for all TFTs on the chip. The thickness of a thermally grown SiO2 gate dielectric layer was 250 nm. A pentacene
active layer of 50 nm thickness was deposited by evaporation in vacuum from prepurified commercially available pentacene powder. Details of the TFT fabrication
Source
Drain
Pentacene active layer SiO 2 Heavily doped Si Gate electrode
Figure 1. Schematic structure of the top-contact organic TFT with pentacene organic material as an active region.
IDS, µA
25
VGS = -60V
20
VGS = -50V 15
VGS = -40V
10
VGS = -30V 5
VGS = -20V 0 0
-10
-20
-30
-40
-50
-60
Drain voltage VDS, V
Figure 2. Measured output characteristic of the pentacene TFT. techniques can be found elsewhere [5]. We used an HP 4156B Precision Semiconductor Parameter Analy
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