Caratterizzazione termica ed affidabilità di Power Amplifier in GaAs per applicazioni a radio-frequenza

In the thesis the thermal characterization of GaAs pHEMTS and GaAs power amplifiers for RF applications is analyzed. In the introduction the main characteristics of GaAs and an explanation of degradation mechanisms that affect p-HEMTs is reported. Moreover the process of manufacture of the devices under test is described. The first chapter analyses the pHEMT degradation due to thermal effects. Devices with several WG and gate pitch value (corresponding to structure EG7367W) have been stressed with several temperatures from 175°C to 350°C for several step stresses. The test lasted 2000 hours in low temperature conditions (from 175°C to 250°C). In high temperature conditions the stress has been continued until a current decrease of 15% was noticed. The pHEMT behavior due to thermal stress has been monitored after each step stress with DC and pulsed measurements. The degradation is studied through the evaluation of the following parameters of the transistor in DC conditions: (i) drain current evaluated in the saturation zone (i.e. VGS=0.4V and VDS=5V, VGS=0.4V and VDS=3V); (ii) maximum transconductance measured in linear zone (i.e. VDS=0.4V) and in saturation zone (i.e. VDS=5V), (iii) threshold voltage at VDS=5V and (iv) leakage current evaluated in OFF (VGS=-1.2V) and ON (VGS=0.4V) conditions. A MTTF value has been extrapolated from drain current decrease. Furthermore GS diode current has been measured in ON (VGS=0.4V) and OFF state (VGS=-1.2V). Finally current collapse considerations are evaluated through pulsed measurements. In the second chapter of the thesis thermal resistance is studied. A comparison between a DC and a pulsed technique is proposed. Firstly eight multifinger transistors (corresponding to structure EG7367W) characterized by several WG from 4x25μm to 12x80μm and different gate pitch have been analyzed. The study has then been extended to a four stage MMIC power amplifier on board (corresponding to structure EG7367K). Pulsed measurements have not been carried out on the power amplifier because of the presence of capacitors to avoid oscillations