Vertical Buffer Leakage and Temperature Effects on the Breakdown Performance of GaN/AlGaN HEMTs on Si Substrate

GaN-based lateral high electron mobility transistors (HEMTs) are highly attractive for high-power-switching applications, because they are foreseen to contribute significantly towards improving the efficiency and miniaturizing the power supplies, due to the low on-state resistance (R_ON) for a given blocking breakdown voltage (V_br) and the low gate charge required for fast switching.

Transistors with high lateral V_br and low R_ON have been demonstrated by several groups. Most of the excellent results achieved for the lateral breakdown voltage (three terminal off-state breakdown) are reported for the substrate being in floating mode. As soon as the lateral V_br measurement is performed within a substrate in grounded mode, the HEMT’s lateral V_br degrades and affects the device performance. It is due to the vertical isolation of the buffer (two terminal off-state breakdown) through the leakage current which affects and dominates the lateral V_br. The HEMTs breakdown voltages are limited by GaN-to-Si vertical leakage. Moreover, in practice the transistor back side needs to be grounded.

Alike, little works are reported concerning the temperature effect on the HEMT’s lateral V_br and the vertical buffer isolation, which is a reliability requirement for components operating at high temperature, as in automotive applications.

To improve the lateral V_br, thicker epitaxial buffer layers are used, but it causes a severe strain, increasing the wafer’s bow and leads to cracks formation. An alternative would be to optimize the carbon doping in the buffer layers, in combination with an appropriate thickness of the buffer’s layers.

In this talk, we will present our recent results reporting to the electrical characterization of the 600 V designed GaN/AlGaN HEMTs, grown on 4-inch silicon substrate, including the breakdown voltage measurements of the lateral buffer isolation (substrate grounded) and the vertical buffer isolation. We compare the HEMTs off-state V_br for substrate grounded or in floating mode. The HEMT breakdown voltage does not degrade up to 600 V bias (grounding substrate). We have also studied the effect of the substrate’s heating (up to 150°C) on the vertical buffer isolation characteristics, and on the V_br performance of the HEMTs.

The AlGaN/GaN HEMT heterostructure is grown on (111)-oriented p-type Si substrates by metal–organic chemical vapor deposition, with a total thickness of 5.6 µm. The epitaxial structure consists of a thin AlN nucleation layer, a thin AlGaN layer, followed by a carbon doped AlN/GaN superlattice and a carbon doped GaN buffer. Afterwards, we grow a 400 nm thick undoped GaN channel layer and a 20 nm unintentionally doped Al_0.18Ga_0.82N barrier layer. The normally-on HEMTs were fabricated using our standard technology, implantation for the isolation, annealed Ti/Al/Ni/Au stack as ohmic contacts, Ni/Au as Schottky gate metallization, and SiN_x as a passivation layer. HEMTs with a gate length of 2 μm, gate–drain spacing of 15 μm, and gate width of 10 mm; 100 × 100 μm² isolated contacts to characterize the vertical top-to-substrate leakage and two metallic lines with 15 µm distance for the lateral buffer isolation were fabricated.

Figure 1 exhibits the mapping measurement comparison, across the 4-inch wafer, between the different blocking voltages of the buffer. As shown, the lateral buffer isolation (substrate floating) reaches 1700 V (blue curves), at an absolute current of 0.1 µA. The lateral blocking voltage decreases to 830 V (substrate grounded), due to the contribution of the vertical leakage current (red curves). The vertical two-terminal isolation blocking voltage (green curves) is also 830 V (substrate grounded), at a corresponding absolute leakage current of 0.1  µA or a current density of 10 µA/mm². It confirms the domination of the vertical leakage current over the lateral leakage current, and the necessity to grow a buffer with a high vertical isolation to avoid the degradation of the HEMT’s breakdown voltage (grounded mode).

The 10 mm gate periphery HEMTs achieve a (V_br)  of  600 V, at gate-source voltage of – 3 V, with corresponding I_DS currents between 0.3 and 0.55 µA/mm (floating substrate), as depicted in the mapping measurements of Figure 2 (blue curves). When the substrate is grounded, no current degradation is observed and the HEMTs achieve again a V_br of 600 V, with corresponding I_DS currents between 0.2 and 0.4 µA/mm (red curves). The extracted vertical bulk current (green curves) is too low at 600 V bias, and does not deteriorate the lateral leakage current (grounded mode). This demonstrates the quality of our epitaxial structure.

The influence of the substrate temperature, up to 150°C, on the buffer’s isolation voltage/current and HEMT’s V_br will be discussed.