Pulsed-Bias Pulsed-RF Harmonic Load Pull for GaN and WBG Devices

Introduction

Wide Band-Gap devices, such as GaN offer several advantages over GaAs, including higher operating voltage (over 100V breakdown), higher operating temperature (over 150°C channel temperature), and higher power density (5-30W/mm). Despite these obvious advantages, the large output power capability presents a great deal of heat dissipation. SiC has an impressive thermal conductivity, but for large periphery GaN devices, it is not sufficient for eliminating thermal effects. GaN HEMTs can also suffer from the effects of trapping in the surface passivation along the gate width.

Pulsed Measurements

Because GaN devices tend to self-heat and are susceptible to trapping effects, it is important to pulse voltages between a quiescent and hot value and define appropriate pulse-widths. Pulsing the voltage will result in a lower average power being delivered to the device and reduced self-heating. Such a measurement allows for near-isothermal performance.

Pulsed Load Pull

Load pull consists of varying or "pulling" the load impedance seen by a device-under-test (DUT) while measuring its performance under actual operating conditions. This method is important for large signal, nonlinear devices where the operating parameters may change with power level or impedance. Because the device will operate differently under DC and pulsed- bias conditions, a difference in load pull contours is expected. Since commercial and military pulsed applications are being considered, the load pull setup and results should perfectly describe the application.

Many higher-power GaN devices have source impedances around or below 1-3Ω because of their large peripheries. Achieving these impedances at the DUT reference plane (taking into account the losses of a fixture, adapters, cabling, probes...) requires a tuner, or a combination of tuners, capable of presenting a VSWR in the range of 100:1 - 200:1. Load matching requirements are not as high due to the output geometry of the device designed specifically for higher current operation, and a 15:1 - 20:1 VSWR is often enough to meet matching requirements.