Deep Trap Levels Responsible for Current Collapse in AlGaN/GaN MISHFET
Authors: Seol Jeong-hoon , Kang Hee-Sung , Lee Gil-Ho , Lee Jung-Hee , Hahm Sung-Ho 
Published: February 2020

GaN-based heterostructure field-effect transistors (HFETs) have been attracted considerably as candidates for next-generation devices for both high-power and high frequency applications, because of its excellent material properties such as high breakdown field, high polarization-induced two dimensional electron gas (2DEG) and high saturated electron velocity [1-3]. Despite the advantages, current collapse has been concerned for reliability of AlGaN/GaN HFETs. In this work, we investigated origins of the current collapse in AlGaN/GaN MISHFETs after negative gate bias by observing the spectral photo-responsive drain current and gate-drain spacing dependent effect.
Schematic structure of the fabricated MISHFETs is shown in Fig. 1(a), in which the epitaxial layers were grown on the sapphire (0001) substrate by MOCVD. After mesa isolation by TCP-RIE, a 5 nm-thick Al2O3 layer was deposited using PAALD. Si/Ti/Al/Ni/Au (1/25/10/40/100 nm) were deposited for ohmic metal stacks by e-beam evaporator and rapid thermal annealing (RTA) was performed at 850 ˚C for 30 s in N2flow. Subsequently, Ni/Au (40/50 nm) was deposited as the schottky gate contacts. The defined gate length and width of the fabricated devices were 3 μm and 50 μm, respectively. Fig 1(b) shows typical output characteristics of the fabricated MISHFET.
Fig. 2 shows the drain currents measured before and after the bias stress when the monochrome lights of different energy were illuminated. Under the illumination condition, it showed only slight collapses of drain current for 500 ~ 600 nm light after the negative bias stress, while showed serious collapse for the other wavelengths. It means that the photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep level which was attributable to the current collapse due to the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer as widely known as a culprit in previous researches in AlGaN/GaN HFET [4]. Fig. 3 shows a schematic band diagram for the current collapse caused by a negative bias stress. When a high negative bias stress is applied to the gate, the energy band near AlGaN/GaN junction will be bended and the electrons be released from crystal interface which have been trapped in acceptor like states such as Ga vacancy levels as shown in Fig. 3 (a).
Fig. 4 shows the drain current suppression at a bias condition of Vgs=1 V and Vds=7V after negative bias stress of -20 V, in which the current decreases as the gate-drain spacing increases. For wide gate-drain spacing, even the electric field between gate and drain decreases, more states seemed to be exposed and contributed to the current collapse at the AlGaN or GaN layer.
In conclusion, the current collapse in AlGaN/GaN MISHFET was significant for the wide gate-drain spacing. The photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep levels which were attributed to the current collapse induced by the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer.
Reference
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Figure 1