A STUDY OF CHARGE INJECTION AND REMOVAL IN A FLOATING GATE IONISATION-INJECTION MOS(FIMOS) CELL

Abstract

A study is made of the charge injection and removal in c floating gate ionisation-injection MOS (FIMOS) device based on Texas Instruments' TMS 2532 EPROM Memory cell. This thesis starts with a literature survey which reviews the work done in the past on floating gate devices starting with the devices first discussed by Kahng and Sze up to the present FIMOS devices, also known as stacked gate memory cell. A model of the hot electron injection mechanism is introduced, based on approximate computations of FIMOS channel current, lateral electric field in the channel, electric field across the floating gate oxide and effective injection area of channel, as functions of top gate and source/drain voltages. By using the dimensions and parameters of the TMS 2532 memory cell, theoretical values of the floating gate injection current and all the other aforementioned parameters are generated. Experimental measurements of the floating gate injection current of single TMS 2532 memory cells on test vehicles are carried out. The gate injection current is determined by measuring the shift in the threshold voltage of the FIMOS device, when very short voltage pulses of known pulse widths are applied to the top gate with all the other terminals held at constant voltages. A study is made of the effects of top gate and drain voltages, continuous voltage application, and temperature variation on the gate injection current. In the study of charge removal from the floating gate, models for charge removal by exposure to ultraviolet light and elevated temperatures are presented. The effects of oxide traps on charge removal are discussed. Experimental measurements are made on the charge removal rate when the device is separately exposed to ultraviolet light and elevated temperatures. Since little work has been reported on the high temperature charge remova1 rate of a single FIMOS device, focus of the work in this thesis is on high temperature. During high temperature charge removal experiments. anomalies are observed in one set of results which are attributed to mobile ionic contaminants.