Apply Proper Bias Sequencing With GaN HEMTs

SPECIAL ATTENTIOIN IS NEEDED for the biasing of high-power RF devicesespecially if they are based on gallium nitride (GaN). Specifically, instabilities and oscillations can be minimized in GaN devices by enabling large current drains with small voltage drops. GaN decoupling circuits also must be properly biased so that interference with RF matching circuitry is reduced while limiting the influence of linearity on the device. The issues associated with biasing, bias sequencing, and temperature compensation of a GaN highelectron mobility transistor (HEMT) are detailed in a nine-page application note, "AN-009: Bias Sequencing and Temperature Compensation for GaN HEMTs."

The biasing sequence is especially critical for GaN HEMT devices. Areas sensitive to a device's potential instability (for example, where VDS is low and IDS is high) must be avoided. The note offers a recommended bias and turn-off sequence. It then tackles how to handle a positive gate current, which will arise when the device is driven into saturation. If a resistor is connected across the power-supply terminals, for example, that resistor can enable the power supply to always provide a negative current while allowing the device to source or sink current. For temperature compensation, consistent performance can be maintained if a bias circuit is designed around the device in a way that maintains a constant IDSQ.

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