Fig. 1. Schematic diagram of devices and their components. (a) Dev A is not covered by any passivation layer; (b) Dev B is only covered by Si₃N₄ thin film; (c) Dev C is covered with Si₃N₄ and silicone gel at the same time; (d) corresponding color of each component

Fig. 2. I-V curves under different annealing temperature. (a) Without RTA; (b) 300 ℃; (c) 350 ℃; (d) 400 ℃; (e) 450 ℃; (f) 500 ℃

Fig. 3. Laser reflectance of Si₃N₄ films with different thickness on top of GaAs substrate

Fig. 4. Impact of Si₃N₄ passivation layer and silicone gel encapsulation on device resistance. (a) Si₃N₄ passivation significantly increases device resistance; (b) silicone gel encapsulation slightly decreases device resistance

Fig. 5. Schematic of terahertz time-domain spectroscopy system for testing

Fig. 6. Time and frequency domain terahertz radiation spectrum. (a) Time domain terahertz waveforms of 3 devices with bias voltage of 65 V; (b) frequency domain spectrum of (a) after FFT; (c) time domain terahertz waveforms under extreme condition for Dev B and Dev C; (d) frequency domain spectrum of (c) after FFT

Fig. 7. Terahertz electric field intensity of Dev C varies with bias voltage and laser power. (a) Variation of peak electric field intensity with bias voltage at a fixed laser power of 40 mW; (b) variation of peak electric field intensity with laser power at a fixed bias voltage of 100 V