Graduate Thesis Or Dissertation
 

Capacitance-voltage analysis, SPICE modeling, and aging studies of AC thin-film electroluminescent devices

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  • Alternating-current thin-film electroluminescent (ACTFEL) devices are metal-insulator-semiconductorinsulator- metal (MISIM) structures which emit light under high field, pulsed excitation. One aspect of ACTFEL operation that is not well understood is the aging of such devices with operating time. One of the primary goals of this thesis is to characterize the kinetics of ACTFEL aging and to determine the associated activation energy which is then used as an aid in identifying the physical mechanism responsible for aging. Toward this end a new method, the capacitance-voltage (C-V) technique, for electrical characterization of ACTFEL devices is developed and refined. The threshold voltage ,Vth, total capacitance Ctot, and the insulator capacitance, Cthg, as well as some information about the relative interface state density are available from C-V analysis. SPICE simulation and a discrete ACTFEL model are used to verify and refine the C-V technique. The heart of the SPICE model is the standard ACTFEL circuit model which consists of three capacitors representing the phosphor and two insulator layers and a pair of back-to-back Zener diodes which account for conduction in the phosphor above threshold. Model enhancements are the inclusion of five resistances which account for 1) the resistance of the transparent conducting layer, Rito, 2)and 3) the bulk insulator resistances, R1 and R2, 4) the phosphor layer bulk resistance, R, and 5) a diode resistance, Rd, which is in series with the back-to-back Zener diodes and is termed a hot electron resistor since it is associated with the emission of electrons from interface trap states. The refined SPICE equivalent circuit is found to give good agreement with experimental C-V curves and with C-V curves generated using a discrete ACTFEL model which is built using discrete capacitors and Zener diodes. Aging experiments are conducted using the C-V technique to monitor the threshold voltage as a function of aging time over a temperature range of -50 °C to 80 °C. An incubation period, in which the threshold voltage is constant, occurs for temperatures below 20 °C but no incubation period is observed for temperatures above 20 °C. After the incubation period, if any exists, the threshold voltage increases logarithmically with time to a saturated value which is temperature-dependent; logarithmic and saturated aging are collectively referred to as constituting short-term aging. Short-term aging is characterized by Ctot, C8, and phosphor threshold voltage which are independent of aging time, rigid shifts in the C-V transition to higher threshold voltages with aging time, and decreases in the conduction and polarization charges with aging time. A kinetic analysis of the variable-temperature ACTFEL aging characteristics results in an activation energy of 0.2 eV. Such experimental observations lead to a model for ACTFEL aging in which conduction electrons are trapped in deep level, fixed charge states which arise from atomic rearrangement at the interface. This trapped charge reduces the polarization charge with a corresponding increase in the threshold voltage.
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