GaN metal–semiconductor–metal photodetectors (MSM PDs) on silicon substrates and sapphire substrates were fabricated and characterized. We found that the current–voltage (I–V) characteristics of MSM PDs on the silicon substrate could be approximated by the Poole–Frenkel conduction behavior. This phenomenon was attributed to the presence of micro-grain structure in the silicon-substrate epitaxy layer. The voltage-dependent responsivity of GaN MSM PDs on the silicon substrate was also evidence of micro-grains inside the epitaxy layer. At a low frequency, the 1/f-form noise was a main contribution to both PDs. Moreover, the extremely low β (~0.7) extracted from GaN MSM PDs on the silicon substrate was first reported. Based on the current–voltage behavior, the extremely low β was believed to originate from the silicon-substrate-induced micro-grain.
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