Low-contact-resistance (Rc) non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contacts were realized on an undoped AlGaN/GaN high-electron-mobility transistor (HEMT) grown on a silicon substrate. Optimization of the rapid thermal process reveals that Rc decreases drastically from the annealing temperature of 700 to 850 °C and slightly increases from 875 to 900 °C. The sample annealed at 850 °C exhibited the lowest Rc of 0.22±0.03 Ω·mm [specific contact resistivity, ρc=(0.78±0.22)×10-6 Ω·cm2] with a smooth surface morphology (RMS roughness ∼5.5 nm). The low Rc is due to the formation of TixSiy and the intermixing of TixSiy with the bottom Ta layer at the metal/semiconductor interface.
The distribution of trap energy (DTE) levels was observed in the energy band gap of buffer GaN by temperature-dependent current transient measurements on AlGaN/GaN HEMTs under fully ON drain-stress (VD[ON]_Stress) conditions. The activation energies (Ea’s) obtained from current transients increase with increasing VD[ON]_Stress. Using a multitrap energy (MTE) model, the applied-VD[ON]_Stress-dependent Ea is attributed to DTE levels in the GaN energy band gap, rather than to discrete single trap energy levels. An effective activation energy (Ea_eff) corresponding to trap energy levels activated by the applied VD[ON]_Stress is thus obtained. This observation is validated with two-dimensional numerical simulations. This study will help device designers develop a “DTE-dependent” emission time constant model that is readily applicable for the reliability modelling of future GaN-based circuits.
The influence of electric field (EF) on the dynamic ON-resistance (dyn-RDS[ON]) and threshold-voltage shift (ΔVth) of AlGaN/GaN high electron mobility transistors on Si has been investigated using pulsed current-voltage (IDS-VDS) and drain current (ID) transients. Different EF was realized with devices of different gate-drain spacing (Lgd) under the same OFF-state stress. Under high-EF (Lgd = 2 μm), the devices exhibited higher dyn-RDS[ON] degradation but a small ΔVth (∼120 mV). However, at low-EF (Lgd = 5 μm), smaller dyn-RDS[ON] degradation but a larger ΔVth (∼380 mV) was observed. Our analysis shows that under OFF-state stress, the gate electrons are injected and trapped in the AlGaN barrier by tunnelling-assisted Poole-Frenkel conduction mechanism. Under high-EF, trapping spreads towards the gate-drain access region of the AlGaN barrier causing dyn-RDS[ON] degradation, whereas under low-EF, trapping is mostly confined under the gate causing ΔVth. A trap with activation energy 0.33 eV was identified in the AlGaN barrier by ID-transient measurements. The influence of EF on trapping was also verified by Silvaco TCAD simulations.
This work investigates the conduction mechanism of non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contact in un-doped AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on Si. The temperature dependent I-V measurement reveals that the conduction mechanism is primarily via Thermionic Emission(TE). The extracted mean barrier height(Φ B ) values are 0.113 and 0.121 eV and the mean contact resistance(R c ) values are 0.24 and 0.28 Ω•mm respectively for annealing temperature at 850 ºC and 900 ºC. The low R c is attributed to the formation of low work function Ti x Si y at the metal-semiconductor interface. The HR-TEM and EDX analysis also provide structural evidence to support the TE mechanism.
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