Observation of ground loop signals in GaN monolithically integrated devices

The observation of ground loop signals in nonelectrically isolated GaN monolithic systems has prompted an investigation on its origins. The study is carried out with devices comprising monolithic light-emitting diodes (LED) and photodetectors (PD) that are either electrically isolated by completely etching through the GaN epitaxial layers, or nonelectrically isolated devices where the etch terminates at the n-GaN layer, through TCAD simulations and experiments. While the devices behave identically to DC input signals, a signal can be observed across the PD of the nonelectrically isolated devices when an AC signal is fed to the LED, even at voltages below the LED’s turn-on voltage. The ∼180° phase difference of the output PD potential with respect to the input LED potential indicates that the signal, regarded as a ground loop signal, couples through the junction capacitance of the LED and PD. The ground loop signal increases with increasing frequency due to the frequency-dependence of the junction impedance. The insertion of a grounded metal line between the LED and PD reduces the ground loop signal, but not to a sufficient extent not to affect the photovoltage. The findings illustrate the necessity of electrical isolation among devices for GaN monolithic systems, especially those operating at higher frequencies, such as photonic integrated systems.