Hot Electron Injection Efficiency in IGFET Structures

Abstract

A model is presented for predicting hot electron induced threshold shifts in N-channel Insulating Gate Field Effect Transistors (IGFET). Indirect measurements of electron injection currents are used to predict their dependence upon transistor parameters.These results are compared to long term stress tests on single transistors and large scale integrated cir- cuits. IntroductionThe present trend in N-channel Insulated Gate Field Effect Transistor (IGFET) technology is to increase performance by device scaling. This technique decreases all device dimensions and increases channel doping to increase speed and reduce power. If voltages are not scaled a potential reliability hazard can result from injection of hot electrons into the gate oxide.1 A fraction of the electrons injected into the gate oxide are trapped which cause a shift in device characteristics. The result is a long term wearout mechanism for devices with design rules that do not keep hot electron injection currents below a safe level.The reliability hazard incurred in a design can be determined by characterizing the parameters affecting hot electron trapping. The number of hot electrons injected into the gate oxide is dependent upon transistor geometry and bias conditions. Only a small fraction of the electrons injected into the gate oxide are trapped there to cause a shift in device I-V characteristics. A technique is presented to determine hot electron injection currents and the longterm effects on single transistors and LSI MOS circuits.In this paper, only the effect of bias conditions and channel length are considered. Other factors which affect hot electron trapping such as channel doping and gate dielectric composition are not evaluated. Highly doped channels concentrate the channel current close to the silicon-silicon dioxide interface, which increases the probability a sufficiently hot electron will be injected into the gate oxide.2 The composition of the gate dielectric has a large effect on trapping efficiency, with thermally grown silicon dioxide gate dielectrics having the lowest trapping efficiency.1 Hot Electron Injection ModelAn IGFET N-channel transistor in saturation has a large electric field in the drain depletion region. Electrons composing the channel current are accelerated by this field and gain kinetic energy. Energy is lost via impact ionization collisions and is manifested as substrate current.3 Impact ionization collisions and phonon scattering randomize the direction of the electrons' velocity, and direct some electrons to the Si-SiO2 interface. Electrons which have sufficient energy to surmount the silicon-silicon dioxide barrier may be injected into the gate oxide.4 The electron injection current Ie-is related to effective electron temperature (energy) kTe and the Si-SiO2 barrier potential EB = 3.1eV by equation (1).The actual electric fields in the channel are difficult to determine and are dependent upon transistor geometry, doping profiles, and transistor bias conditions. However as schematically shown in Figure 1, only ...