Impact_of_MOSFETs_performance_on_its_thr - Milaim Zabeli Nebi Caka Myzafere Limani Qamil Kabashi WSEAS TRANSACTIONS on SYSTEMS and CONTROL Impact of

Impact_of_MOSFETs_performance_on_its_thr - Milaim...

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Impact of MOSFET’s performance on its threshold voltage and its influence on design of MOS inverters MILAIM ZABELI, NEBI CAKA, MYZAFERE LIMANI, QAMIL KABASHI Faculty of Electrical and Computer Engineering University of Prishtina 10110 Prishtina, Fakulteti Teknik, Kodra e Diellit, p.n. KOSOVA [email protected] , [email protected] , [email protected] , [email protected] Abstract: The aim of this paper is to research the impact of physical parameters which characterize the MOSFET transistors structure on the threshold voltage values and its influence on critical voltage values which characterize digital circuits that contain the MOSFET transistors. The results obtained emphasize the impact of each single physical parameter on the total value of the threshold voltage. By adjusting the values of MOSFET physical parameters the accepted threshold voltage can be achieved. Since the threshold voltage will have influence on critical voltage values which characterise MOS inverters and delay times during transfer logic states between stages, it must be taken into consideration during design phase of logic gates that contain the MOSFET transistors. Key words: Threshold Voltage, MOSFET Parameters, Enhancement-Type NMOS, Impurities, Doped Density, Short-Channel, Narrow-Channel, Voltage Level, Propagation Delay, Critical Values. 1 Introduction The important value which characterizes the MOSFET transistors is the value of threshold voltage, which can be positive and negative according to the MOSFET type. This value can be controlled during the fabrication process of MOSFET transistors. The physical structure of n-channel enhancement-type MOSFET (or NMOS) is represented in Fig.1. Because the enhancement-type NMOS have advantage over other type of MOSFET transistors, in the following we will focus on this analysis. Terminals of MOSFET transistors are indicated with S (source), D (drain), G (gate) and B (body). The value of the gate-to-source voltage V GS needed to create (induced) the conducting channel (to cause surface inversion) is called the threshold voltage ( V th or V t ). The value of the threshold voltage is dependent from some physical parameters which characterize the MOSFET structure such as: the gate material, the thickness of oxide layer t ox , substrate doping concentrations (density) N A , oxide–interface fixed charge concentrations (density) N ox , channel length L , channel width W and the bias voltage V SB [1, 2, 5]. Fig. 1 The physical structure of an n-channel enhancement-type MOSFET in perspective view. 2 The threshold voltage on MOSFET- transistors 2.1 The threshold voltage of NMOS- transistors with long-channel To calculate the threshold voltage we must consider physical parameters of MOSFET structure which have the impact in value of the threshold voltage by considering the various components of V t (when V SB =0V, the threshold voltage will indicate V t 0 ) [2]: WSEAS TRANSACTIONS on SYSTEMS and CONTROL Milaim Zabeli, Nebi Caka, Myzafere Limani, Qamil Kabashi ISSN: 1991-8763 259 Issue 4, Volume 3, April 2008
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