(Invited) Ultra-Wide-Bandgap AlGaN Power Diodes and Transistors
Authors: Kaplar Robert J. , Allerman Andrew A. , Armstrong Andrew M. , Crawford Mary H. , Dickerson Jeramy R. , Fischer Arthur J. , King Michael P. , Baca Albert G. , Douglas Erica A. 
Published: February 2020

As the wide-bandgap materials SiC and GaN mature and become commercially available, “Ultra” Wide-Bandgap (UWBG) semiconductors with bandgaps exceeding 3.4 eV are gaining interest as the next-generation materials that will enable dramatic leaps in power electronics performance. This is because the Figure-of-Merit (FOM) comparing breakdown voltage (VB) and area-normalized “specific” on-resistance (Ron,sp), defined as VB
2/Ron,sp, scales as the cube of the critical electric field for vertical devices and the square of the critical electric field for lateral devices. The critical electric field for avalanche breakdown in turn scales approximately as the 5/2 power of the bandgap, implying a very strong dependence of FOM on bandgap. Representative UWBG materials include diamond, Ga2O3, and Al
x
Ga1-x
N. The AlGaN system is particularly attractive due to the availability of heterostructures as well as the ability to utilize polarization doping; both of these properties mitigate the issue of energetically deep impurity dopants and consequent incomplete ionization which universally affects UWBG semiconductors. Thus, our group’s focus has been on the development of AlGaN for the fabrication of next-generation power devices with both vertical and lateral architectures.

Quasi-vertical AlGaN PiN diodes were grown on thick (1.3 mm) sapphire substrates using Metal Organic Vapor Phase Epitaxy (MOVPE). The initial epitaxial layers consist of an AlN layer followed by an unintentionally-doped AlGaN layer to facilitate the transition from the substrate to the active device layers. Dislocation density is 1-2×109 cm-2; additional reduction in dislocation density is possible using patterned AlGaN and overgrowth techniques. The transition layer is followed by a heavily-doped n-type AlGaN:Si contact layer and the thick lightly-doped n-type AlGaN:Si drift layer. The drift layer is critical since it supports the high voltage in the reverse-bias blocking mode and is also responsible for the on-state resistance in the forward-bias conducting mode. This is followed by a p-type AlGaN:Mg anode layer and finally graded to a very thin GaN:Mg p+ contact layer. Ion implantation into the p-layers is utilized to form a Junction Termination Extension (JTE) to prevent premature breakdown. A mesa etch is performed down to the n-contact layer, and Ohmic contacts (TiAlMoAu) are deposited; this is necessary since the substrate is insulating and, due to the lateral current transport in this layer, the device configuration is termed “quasi-vertical” (the conduction through the pn junction and drift layer is in the vertical direction). The top Ohmic contact is PdAu, the surface is passivated with silicon nitride, and a shallow trench isolates the JTE from the mesa sidewalls. For a device utilizing Al0.3Ga0.7N for the active layers, and with a 4.3 um thick drift region doped in the mid-1016 cm-3 range, VB exceeded 1.5 kV (Figure 1a); combined with the measured Ron,sp of 16 mOhm·cm2, a FOM of approximately 150 MW/cm2 is achieved. To our knowledge this is the first time a vertical kV-class AlGaN PiN diode has been demonstrated, and the FOM is the highest reported for an AlGaN diode. Further, the current density under forward bias exceeds 3.5 kA/cm2, and the critical electric field for the Al0.3Ga0.7N is estimated to be 6.0 MV/cm.

Lateral High Electron Mobility Transistors (HEMTs) were also grown and fabricated in Al-rich AlGaN alloys. The epitaxial layers are likewise grown on sapphire substrates by MOVPE. Following the growth of an AlN nucleation layer, a 400 nm thick Al0.85Ga0.15N buffer/channel layer is grown, which is capped by a 48 nm thick AlN barrier layer. All layers are unintentionally doped. A combination of mercury-probe capacitance-voltage and contactless resistance measurements indicate a sheet resistance of approximately 4200 Ohm/square and a channel mobility of 250 cm2/V·s, and the pinch-off voltage is approximately -4.0 V. Circular HEMTs were fabricated using a six-mask photolithographic process. The key step in the process is the fabrication of the source and drain Ohmic contacts, for which n+ GaN:Si is re-grown in trenches etched through the AlN barrier, and a TiAlNiAu metal stack is afterwards deposited. The Schottky gate is NiAu and the surface is passivated with silicon nitride; no field plates are utilized. Drain current vs. drain voltage curves are shown in Figure 1b and clear gate control is evident. However, the current density is much lower than expected based on the measured sheet resistance and known device geometry, indicating that further optimization of the Ohmic contacts is necessary (the contact resistivity is estimated to be 0.1 Ohm·cm2). Despite this, the devices show excellent performance with Ion/Ioff > 107 and VB > 800 V, and to our knowledge are the highest-bandgap transistors ever reported.

Figure 1