GaN/InGaN Heterojunction Bipolar Transistors with Collector Current Density > 20 kA/cm2
We report new development of GaN/In0.03Ga0.97N npn double-heterojunction bipolar transistors (DHBTs) grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) with state-of-the-art high collector current density (JC) and low knee voltage (Vknee). The common-emitter I-V characteristics show a high JC of 19.8 kA/cm2 and JC = 28.6 kA/cm2 from the Gummel plot with a offset voltage (Voffset) of < 0.22V and a Vknee < 2.1 V. The measured BVCEO is 110 V. These values are among the best values reported to date for III-nitride (III-N) HBTs, suggesting that these DHBTs would be viable for high-power radio-frequency (RF) applications.
Compared to their III-V counterparts, III-N npn HBTs are suitable for harsh environment operation because of the wider bandgap properties. Recent developments in MOCVD epitaxial material growth and fabrication technologies have enabled a number of GaN/InGaN npn HBT demonstrations that used a single-pass epitaxial growth scheme [1-7]. These results showed that high-performance III-N HBTs can be achieved without the need for complex re-growth schemes as reported earlier (e.g. reference [8 9]). To further develop III-N HBTs for RF power amplifications, high JC, low Voffset, and low Vknee are required. Previously, the reported highest JC for III-N HBTs was limited at 10 kA/cm2 , and Vknee were typically in the range of 5~15 V due to certain fabrication processing issues. In this study, the GaN/InGaN HBT fabrication process is further optimized through the low-resistive metallization and low-damage etching processing. As a result, dramatically enhanced JC and lower Voffset and Vknee were achieved.
The GaN/InGaN npn DHBT structure is grown on a c-plane sapphire substrate in a Thomas-Swan MOCVD system. The material growth optimization and epitaxial structure were reported earlier . The fabrication processing involves three-step mesa etching and emitter, base and collector metallization. Benzocyclobutene (BCB) passivation was used as device passivation and metal 1 is used to provide the interconnect of via holes. The fabrication processing is compatible with the conventional GaAs-based HBT fabrication procedure without the use of extensive base regrowth or emitter regrowth schemes.
Fabricated GaN/InGaN npn DHBTs are characterized using a Keithley SCS-4200 system at room temperature. Fig. 1 shows a set of common-emitter I-V family curves of a DHBT with an AE = 3×3 Mm2. The base current (IB)increases from 5 μA to 70 μA with an increment of 5 μA, and the collector voltage (VCE) is swept from 0 to 5 V. Voffset is 0.11 V for IB = 5 μA and which value slightly rises to 0.22 V for IB = 70 μA. At IB = 70 μA, Vknee is as low as 2.1 V, and the JC reaches > 19.8 kA/cm2 (IC = 1.78 mA) for VCE > 2.1 V and the d.c. current gain (β= IC/IB) is 25. The inset of Fig. 1 shows a microscope picture of a fabricated GaN/InGaN HBT. Fig. 2 shows that BVCEO is 110 V and the off-state leakage JC is 80 mA/cm2 (or IC = 7 nA) near the device breakdown. Fig. 3 is the Gummel plot of the same device under test at VCB = 0 V. The maximal differential current gain (hfe=OIC/OIB) is 38 at VBE = 9.5 V and IC = 1.75 mA. At VBE = 10 V, hfe is 30, and the JC as high as 28.6 kA/cm2 (IC = 2.58 mA) is observed. In terms of d.c. power handling capability, a record power density of 300 kW/cm2 was also achieved at VCE = 20 V and JC = 15 kA/cm2. Preliminary RF measurement on HBTs also shows that the cut-off frequency (fT) is greater than 2.5 GHz (plots not shown here).
In summary, we report high-performance GaN/In0.03Ga0.97N npn DHBT grown on sapphire substrates. An AE=3×3 Mm2 device shows a high JC of 19.8 kA/cm2 with a low Voffset of 0.22V and a low Vknee of 2.1 V. These values are among the best values reported to date for III-N HBTs. To the best of our knowledge, it is also the first report of III-N HBT with fT > 2.5 GHz. More details on the device fabrication, d.c. characteristics, and RF performance will be presented in the conference.