SOME MODELS FOR THRESHOLD VOLTAGE STUDIES OF ION-IMPLANTED GaAsMESFET

Abstract

GaAs-MESFETs are devices of proven abilities for applications in microwave and millimeter wave frequencies, high-speed digital circuits, GaAs-integrated circuits and high temperature electronics. A key parameter for the design of a MESFET is its threshold voltage. In the case of GaAs-MESFET, the properties of the semi-insulating substrate in which the device is built, greatly influence the threshold voltage. This thesis deals with models for the calculation of ion-implanted GaAs-MESFET threshold voltage. There are ample evidences which show that a depletion region formed at the channel-to-substrate interface of a MESFET influences the threshold voltage of the device. The theoretical models which are available in literature, treats this depletion region as equivalent to that of a step p-n junction . Since, impurity concentration varies across the channel-to-substrate interface of an ion-implanted MESFET, this approximation deviates the calculated threshold voltage from its actual value. In view of this, the present work is first devoted to the development of an improved threshold voltage model which accounts for the gradient of impurity concentration across the channel-to-substrate interface of an ion-implanted GaAs-MESFET. The superiority of this improved model over the existing ones is shown by comparison with threshold voltage experimental data available in literature. The dependence of threshold voltage on operating temperature, substrate bias and substrate doping are studied with the help of the improved model. The model so developed is also used to study the Zero Temperature Coefficient (ZTC) operating point and temperature dependence of small-signal transconductance at ZTC -operating point. The improved model, however, does not have any provision to account for the effect of any deep-level trap which might be present in the substrate. Thus, this model is applicable to those GaAs-MESFETs which use a buffer-layer between the channel and semi-insulating substrate to isolate the channel from the substrate. Further more, for the sake of simplicity some of the temperature dependent parameters like band gap, carrier effective mass, Schottky barrier height and ionized impurity concentration have been assumed to be temperature independent. The strong temperature dependence of threshold voltage as seen experimentally by previous workers, suggest that the threshold voltage model developed by considering impurity distribution gradient across the channel-tosubstrate interface can be further improved. To achieve this the improved model is further modified into a more comprehensive model. The comprehensive model, besides accounting for temperature dependence of band gap, carrier effective mass, carrier thermal velocity, Schottky barrier height, ionized impurity concentration and intrinic carrier concentration, also includes the effects of deep-level traps, such as EL2 and chromium. The influence of temperature on the ionization of these traps is also taken into account. By the use of this model, the contributions made by temperature dependence of Schottky-gate built-in potential, channel-to-substrate junction built-in-potential and Schottky barrier height at the gate metal-to-GaAs junction, towards the temperature dependence of threshold voltage have been estimated. Comparison of threshold voltage calculated by using the comprehensive model, previous improved model and experimental data available in literature, showed that the comprehensive model yields the best results.

The threshold voltage of an ion-implanted GaAs-MESFET depends on the dopant profile that results from the ion-implantation process. The profile is characterized by the projected range and the range straggle. Thus, these two parameters are important in the determination of the threshold voltage. The projected range and range straggle are in turn determined by implantation process parameters of ion dose, ion energy and cap-thickness ( in case of through-cap implantation). When through-cap implantation is carried out the stopping power of the cap material also matters. The simple threshold voltage model which is developed first in the present work is used to study the effects of the ion-implantation parameters on threshold voltage of GaAs-MESFET. Ion-implantation produces lattice damage. For the removal of the damage, the sample is annealed at an elevated temperature. For GaAs samples, this temperature lies in the range of 800-900°C. At such high temperatures, the implanted dopants diffuse, thereby changing the projected range and range straggle of the dopant profile. Thus, post-annealing threshold voltage is different from the pre-annealing threshold voltage. First, the simple model which was initially developed is used to analyze the effect of annealing time on the threshold voltage of GaAs-MESFET. These effects for different ion-energy and cap-thickness are also studied. Since the simple model does not take into account, the reflection of out-diffusing dopant atoms from the cap-to-substrate interface, a third model for threshold voltage calculation is developed. This model considers the reflection of out-diffusing dopants at the cap-to-substrate interface. It is found that these reflections have profound influence on threshold voltage.