A STUDY OF THE INFLUENCE OF SCHOTTKY BARRIER ENHANCEMENT ON GaAs FIELD EFFECT TRANSISTORS

Abstract

The GaAs MESFETs have proven abilities over a wide range of applications. The basic operation of these devices consists of the control of channel conductivity by a depletion layer resulting from a Schottky contact, known as the gate. The drain and source are simple ohmic contacts. GaAs is preferred over Si, as the former has higher electron mobility, direct and wider band gap. The advantages offered by these properties are higher switching speed, higher operating frequency and temperature, and, more efficient response to optical radiations. As electron mobility is much higher than hole mobility, the n-channel GaAs-MESFETs generally find most applications. However the advances in GaAs digital ICs have given rise to interest in complementary GaAs-MESFET • (CMESFET) logics. As such circuits comprise both n- and p-MESFETs, there exists interest in p-GaAs MESFETs also. A major problem with p-GaAs MESFET is that, it is difficult to realize a Schottky barrier of sufficient height. As the gate of a MESFET is a Schottky contact of reasonably high barrier, which does not allow any significant gate current, p-GaAs MESFETs need special technological considerations. Schottky barrier enhancement by the use of ion-implantation was first demonstrated by Shannon [154]. Applying this technique, useful p-GaAs MESFETs have been developed ever since. Shannon-implantation is now Schottky barrier height enhancement depends on the undesirable carrier accumulation under the gate that can be tolerated. It is found that if a lower concentration of accumulated carriers is tolerable, then the maximum dose of the Shannon-implant is also lower. Both n- and p-channel devices have been considered. As stated, the main purpose of this study is to analyze the effects of Shannon-implant on the various parameters of the gate-barrier enhanced GaAs MESFETs. Considering the requirements for barrier enhancement, it is concluded that Shannon dose is the only Shannon-implantation parameter, which may be used to control the electrical properties of the various implant regions. After analysis of Shannon dose limit, the effects of Shannon-implant on device parameters such as pinch-off voltage, threshold voltage, channel current, substrate current and small-signal transconductance are analyzed. It is found that increase in Shannon dose decreases pinch-off voltage in magnitude, increases threshold voltage for n-channel devices, and decreases threshold voltage of p-channel devices. Increase in Shannon dose decreases channel current, substrate current and degrades small-signal transconductance. The analysis of the device current-voltage relationships has been further extended to study the effects of Shannon-implant on the terminal current-voltage relationships. Such current-voltage relationships of greatest importance are the output characteristics or the drain characteristics of the device. To analyze effects of Shannon-implant on this characteristics, existing model of Chen and Shur [31] is modified to account for iii