A THEORETICAL STUDY OF A TERNARY Ill-V COMPOUND MIS-FIELD EFFECT DEVICE

Abstract

The growing need for high-speed digital circuits and high-frequency applications are generally met by devices made on high carrier mobility III - V semiconductors such as GaAs, InP, InGaAsP and InGaAs. Considerable attention needs to be focused on the development of III - V compound MIS-technology. In view of the reports that the fabrication of n-channel inversion-mode MISFETs in InP, InGaAsP and InGaAs substrates is a possibility. Experimental investigations show that of these, the InGaAs-based MISFETs are of highest inversion layer mobility and best insulator-semiconductor interface properties. As InGaAs has turned out to be the most useful semiconductor for photodetector applications, InGaAs MISFET is also a competent device for applications such as integrated optics. An analysis of the properties of inversion layer formed on p-type InGaAs shows that in an inversion-mode InGaAs MIS device an excellent control of surface potential can be achieved by the application of a gate voltage. Onset of strong inversion in InGaAs MIS device requires much lower effective gate voltage than other known MISFET materials. Normal effective field in an inverted InGaAs substrate is relatively higher than in an inverted silicon surface. The effective carrier mobility in an n-type inversion layer in InGaAs is extremely high because of its lower electron effective mass and thicker inversion layer. Furthermore, the lower electron effective mobility in n-inversion layer on InGaAs degrades severely with increase in gate voltage. The analysis shows that this is caused by a high rate of decrease of channel thickness with increase in gate voltage. InGaAs is the ternary limit of the quaternary alloy system of Ini_xGaxAsyPl_y. Considering the effective mobility in InGaAs MISFET is compared with that in other members of the quaternary family. In the past, the effective mobility of InGaAsP MISFETs have been reported to be alloy composition dependent. A degradation of effective mobility with increase in gate voltage was also reported. In order to explain these observations, a Matthiessen's rule approach is taken in the present thesis. In the process two factors, namely, the scattering and screening factors have been identified. These factors are dependent on the gate as well as on alloy composition. In agreement with the experimental observations available in literature, the analysis shows that the effective mobility degradation with increase in gate voltage becomes more and more pronounced if the y-composition parameter is increased. The study of the drain characteristics (drain current-drain voltage characteristics) of the n-channel inversion-mode long-channel InGaAs MISFETs is carried out through the development of an appropriate MISFET-model. The model uses an effective mobility model that has been widely used in silicon MISFET analysis. This mobility expression involves a number of mobility parameters which are generally determined by elaborate laboratory experiments. It has been shown that using the basic MISFET-model developed, the mobility parameters can be approximated with nominal experimental data. The model is then used to study the drain characteristics of InGaAs MISFET. The validity of the model is first checked by comparing the calculated drain characteristics with experimental data available in literature. The MISFET-model is then used to study the effects of substrate doping level on the drain characteristics of an InGaAs MISFET. It is found that for higher doping levels, the saturation drain current and the drain voltage for pinch-off are both lower. For the sake of comparison, the model is applied to InP and silicon MISFETs. It is observed that the level of operating drain current in InGaAs MISFET is comparatively higher than in InP and silicon MISFETs. A high drain conductance in InGaAs MISFET is also indicated by the study. The MISFET-model is self-sufficient to give a picture of potential and electric field variation along the channel. It is also capable of determining the distribution of gate induced and drain induced carriers along the channel. It has been shown that these quantities vary in a similar manner as in silicon MISFETs. However, the magnitudes of each of these quantities are higher in InGaAs MISFET. The analysis also considers the variation of effective mobility along the channel. It is shown that effective mobility decreases towards the drain end. This mobility degradation is enhanced by lowering of gate voltage. In a MISFET the channel carrier concentration is enhanced by impact ionization at high drain voltages and also there is possibility of avalanche breakdown in the p-n junction formed by the drain and substrate. In order to account for the influence of current multiplication by the above two processes, it is necessary to deal with drain breakdown mechanism and its effect on the characteristics of an InGaAs MISFET. The analysis of the drain breakdown mechanism has been carried out in the light of the well established impact ionization theories. To this effect the breakdown parameters for InGaAs and related semiconductors are first analysed. It is shown that for Ini_xGaxAsyPi_y, the breakdown voltage and breakdown fields are both dependent on the alloy composition parameters. InGaAs, which is the ternary limit (iv) of the quaternary alloy system has the lowest breakdown voltage and breakdown field. The breakdown field in InGaAs is a slowly varying function of the doping level. A most important breakdown parameter is multiplication factor. Two distinct multiplication factors are defined for a MISFET, viz., the channel multiplication factor and drain-diode multiplication factor or substrate multiplication factor. The channel multiplication factor is related to impact ionization in the channel of the MISFET, while the other multiplication factor arises out of avalanche breakdown in the drain-diode at high drain voltage. It has been shown analytically that the channel multiplication factor is controlled by'the gate voltage. It decreases with increase in gate voltage. The drain-diode multiplication factor on the other hand is independent of gate voltage. For the analysis of the influence of impact ionization on the drain characteristics of the InGaAs MISFET, the drain current is assumed to be composed of two components. One of the component is the multiplied channel current and the other is the drain-diode current. The multiplied channel current is obtained by the product of channel multiplication factor and the drain current obtained from the MISFET-model developed. The analysis shows that influence of impact ionization in channel and drain-diode depletion layer on the drain characteristics of an InGaAs MISFET is similar to what has been observed in silicon MISFETs. The analysis also shows that the drain breakdown voltage at which avalanche breakdown occurs either in the channel or drain-diode, increases with increase in gate voltage. Furthermore, the drain breakdown voltage in a silicon MISFET is comparatively lower than that in a comparable InGaAs MISFET.