High-power and high-temperature FET technology

The performance of heterojunction metal-semiconductor-field-effect-transistors (MESFETs) and junction-field-effect-transistors (JFETs) fabricated with different buffers is presented. For the JFET, carbon was chosen as the p-type dopant because of its relative low diffusivity compared to other doping elements. The viability of heterojunction MESFET and JFET devices operating at 400 degreesC have been demonstrated. Two key factors contributing to the reduction of drain leakage currents were the use of a high resistivity, undoped AlAs buffer layers and the gate contacting layers: n-type AlGaAs for the MESFET and p-type AlGaAs for the JFET. A two LT- Al0.3Ga0.7As layer scheme were used for the first time specifically for use in high temperature applications. Even at 400 degreesC, the gate leakage current density for a gate length of 2 mum was 9x10(-7) A/mum at Vds = 3V and Vgs = -7V. The high resistance of LT-AlGaAs materials after annealing was responsible for such low gate leakage currents. The p-HEMTs became leaky at high temperature because of the parallel conduction and buffer design. The gate diode performed better when contacted to the undoped AlGaAs layer. DC and high-temperature performance of GaN-based MESFETs and MODFETs were compared. Al0.3Ga0.7N/GaN MODFETs with a gate-length of 2 mum exhibited high transconductance, 47 mS/mm, high de power, 2.9 W/mm, and high current on/off similar to 100 ratio at 400 degreesC. The peak transconductance was 47mS/mm, and dropped by 12% of its initial value to 41.4 mS/mm at 350 degreesC. The decrease in transconductance with temperature can be explained by the temperature dependence of the electron mobility. The large conduction band discontinuity in this material system may play an important role in terms of better electron confinement thus resulting in less degradation in transconductance.