Fourth IEEE International Caracas Conference on Devices, Circuits and Systems, Aruba, April 17-19, 2002 0-7803-7381-2/02/$17.00 © 2002 IEEE D013 - 1 UNIFIED EXTRACTION METHOD FOR AMORPHOUS AND POLYCRYSTALLINE TFT ABOVE THRESHOLD MODEL PARAMETERS M. Estrada 1 , A. Cerdeira 1 , A. Ortiz-Conde 2 , F. J. García 2 , B. Iñiguez 3 1 Sección de Electrónica del Estado Sólido (SEES), Departamento de Ingeniería Eléctrica, CINVESTAV, Av. IPN No. 2508, Apto. Postal 14-740, 07300 DF, México. E-mail: mestrada@mail.cinvestav.mx 2 Laboratorio de Electrónica del Estado Sólido, Universidad Simón Bolívar, Apartado Postal 89000, Caracas 1080A, Venezuela. E-mail: ortizc@ieee.org 3 Depart. de Ing. Electrónica, Eléctrica y Automática, Universidat Rovira i Virgili, 43001 Tarragona, España. E-mail: binyigue@etse.urv.es Abstract - A unified procedure is presented to extract above-threshold model parameters in polysilicon and a- Si:H TFTs. It is based on the integration of the experimental data current, which has the advantage of reducing the effects of experimental noise. This method is applied to the linear and saturation regions for the above-threshold regime and allows the extraction of all the above-threshold parameters. We already presented a similar method applied to a-Si:H TFTs. In this work it is demonstrated that in the above-threshold regimen the mobility of polysilicon TFTs can be modeled also as a function of the gate voltage to the n power and a similar integration procedure can be used to extract the device modeling parameters. The unified extraction procedure provides in addition the possibility of monitoring the crystallization process of a-Si:H TFTs into polysilicon, which has become a widely used process of fabricating low temperature polysilicon TFTs. The process of polycristalization manifests itself by a variation and change in sign of one of the model parameters. Extracted parameters can be introduced in AIMSpice circuit simulator for device modeling. The accuracy of the simulated curves using the extracted parameters is verified with measured. I. INTRODUCTION The AIM-Spice levels 15 and 16 models for a-Si:H and polysilicon TFTs respectively [1] provides a well accepted way of simulating these devices. However, the number of parameters requested by this model is high. The parameters must be determined, but the way to do it is not straightforward. To extract these parameters, AIM-Spice provides a parameter extractor, which tries to optimize several of these parameters at the same time, in order to fit simulated curves to experiment. This is usually troublesome and a satisfactory fit of both simulated I DS -V GS and I DS -V DS curves to experimental curves using this extractor is not always possible to achieve for the same model parameters. In a previous paper [2] we presented a new simple and precise procedure to extract the above-threshold parameters for a-Si:H TFTs used in level 15 model AIM-Spice simulator. In this paper we demonstrate that the mobility of polysilicon TFTs can be modeled in the above threshold regime, using an expression to describe the decrease of the mobility with gate voltage as a negative power of the gate voltage. For this reason, an extraction procedure of the basic polysilicon TFTs model parameters, very similar to the one used for the a-Si:H in [2] can be implemented, using the same integral function H described in [2]. A unified extraction method (UEM) is achieved for both types of TFTs. This method has also the interesting feature that the transition from amorphous to polycrystalline TFTs can be observed by monitoring the value and sign of the parameter γ, which is the power to which the gate voltage is elevated in the mobility model. The extraction procedure is general and can be used to simulate the I-V characteristics under simplified charge control model [3], if the proposed mobility model for the polysilicon and a-Si:H TFT is introduced. This charge control model, with some modifications, is implemented in level 16 model of AIM-Spice. The parameters obtained by our UEM were correlated with input parameters of AIM-Spice level 15 and 16 models, in order to simulate the devices using this program. In this article we present a new unified method to extract all above-threshold parameters of a-Si:H and polysilicon TFTs required by the simplified charge control model and AIM-Spice level 15 and 16 models. They are determined from the transfer and output characteristics of the transistor without non-linear optimization or graphical data processing. Using the extracted parameters, I DS -V GS and I DS -V DS curves were calculated using analytical expressions and also simulated in AIM-Spice. Both results are compared with experiment.