Silicon Field Emitter Arrays Integrated with MOSFET Devices
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Silicon Field Emitter Arrays Integrated with MOSFET Devices Ching-yin Hong and Akintunde I. Akinwande Massachusetts Institute of Technology, Microsystems Technology Laboratories, Cambridge, MA 02142, U.S.A. ABSTRACT We report a metal oxide semiconductor field effect transistor (MOSFET) controlled field emission array (FEA) device. The device uses a lateral double diffused MOSFET (LD-MOSFET) to control electron supply to the surface. The FEAs were fabricated using isotropic etch of silicon, oxidation sharpening and chemical mechanical polishing. The LD-MOSFETs have a threshold voltage of 0.48V while the FEAs have a turn-on voltage of 28V. Analysis using the FN formulation indicates that a silicon tip radius of 11 nm would fit the FEA IV data. The tip radius from the electrical data is very close to the average tip radius of 10 nm obtained from transmission electron microscope (TEM) analysis. The IV characterization of the MOSFET/FEA demonstrated control of electron emission by the MOSFET gate voltage. INTRODUCTION Silicon-based field emitter arrays (FEAs) are being studied as electron emitters in field emission displays (FEDs) due to their compatibility with silicon IC technology. [15]. Silicon FEA technology, while adequate for a number of applications, has two major shortcomings: high addressing voltages and non-uniform, unstable emission current. Most FEAs use a feedback “ballast” resistor to achieve spatial uniformity and stabilize the current by creating a load line. The ballast resistor provides a high dynamic resistance that keeps the operating current constant. However, a more efficient approach for achieving the high dynamic resistance required is to use a voltage controlled current source such as a MOSFET, which could in turn be used to modulate the emission current [6,7]. A typical field emitter structure consists of a conducting conical tip located within a conducting annular gate. When a voltage is applied to the gate, a high surface electric field makes the surface barrier width narrow enough for electrons to tunnel out [8]. In a silicon field emitter, electron emission could be viewed as consisting of two sequential processes: (i) the flux of electrons to the inversion layer or accumulation layer at the surface followed by (ii) the transmission of the electrons through the energy barrier. Either of these two processes could control the emission current. When the electron supply to the surface is large, emission current is controlled by electron transmission through the barrier. When the electron supply is low, the emission current is determined by the flux of electrons to the tip surface. Thus it is possible to vary the FEA gate voltage or the MOSFET gate voltage to change the electron emission current. When the FEA gate voltage is varied, the electron transmission probability is modulated. When the MOSFET gate voltage is varied the electron flux (supply) to the surface is modulated. Noise in field emission devices is caused by barrier height fluctuations due to adsorption and desorption of gasses. A d
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