Current controllability and stability of multi-mesa-channel AlGaN/GaN HEMTsT. Hashizume and K. Ohi
Research Center for Integrated Quantum Electronics (RCIQE), Hokkaido Universitye-mail: hashi@rciqe.hokudai.ac.jp
We have recently developed a multi-mesa-channel (MMC) AlGaN/GaN HEMT, as shown in Fig.
1 [1, 2]. By forming a periodic trench, the MMC HEMT has parallel mesa-shaped channels with twodimensional electron gas (2DEG) surrounded by the gate electrode. In the present study, we report thebasic DC characteristics, including threshold voltage control, subthreshold behavior, transfercharacteristics and off-state breakdown properties, of such mesa-channel HEMTs.
Typical drain current-voltage (IDS-VDS) characteristics of conventional and MMC HEMTs
measured at RT are shown in Fig. 2. The MMC HEMT has a mesa-top width, Wtop, of 80 nm and amesa/trench period of 400 within a gate-electrode width, W, of 100 µm. Thus, the effective gate width,i.e., the sum of Wtop values, was 32 µm for the MMC structure. The MMC HEMT demonstrated goodI-V behavior with complete pinch-off behavior. In particular, the MMC HEMT showed a shallower
threshold voltage and a lower knee voltage than the conventional planar HEMT. The shallower VTH
dominantly arose from the lateral field effect through the mesa gate. In addition, the high impedanceof each mesa channel in the MMC HEMT appears to weaken the effect of the access resistance
between the drain and gate electrodes on knee voltage.
The transfer characteristics of both devices in the linear region (VDS = 0.5 V) are compared in Fig.
3. As the mesa width decreased, a systematic shift of the threshold voltage was observed in the MMC
HEMTs, probably owing to the surrounding field effect. In spite of the fact that the effective gate
width of the MMC device is only 10.5 µm (about 20 % of that of the planar device), in addition, theMMC HEMT exhibited a current and a transconductance (gm) almost equal to those of the planardevice.
To investigate the gate controllability of the MMC device, we measured the subthresholdcharacteristics of the device. Figure 4 shows a comparison of the drain currents for the planar andMMC HEMTs. The gate leakage currents are also plotted in Fig. 4 by broken lines. For both devices,
drain current is dominated by reverse-biased gate current in the pinch-off region, as typically reported
for Schottky-gate AlGaN/GaN HEMTs. Reflecting a lower gate leakage current, the drain-current ON/
OFF ratio is about 109 for the MMC device. In addition, a small subthreshold slope of 76 mV/dec wasobserved in the MMC HEMT. In MMS structure, the potential modulation through the mesa edge andthe undoped GaN layer becomes marked near pinch off. Thus, a combination of normal and lateralfields results in a field surrounding 2DEG, which can effectively control the depletion of 2DEG.
The MMC device also showed excellent current stability in the saturation region. Figure 5 shows
the drain I-V curves of the planar and MMC HEMTs with LGD = 10 µm. The inset in Fig. 5 shows
enlargement of drain current in the saturation region. For the planar device, the current reached itsmaximum and thereafter gradually decreased with increasing drain voltage. This result can beexplained by the thermal effect in the channel. On the other hand, as shown in the inset of Fig. 8, theMMC device exhibits excellent current stability.
References[1] T. Tamura, J. Kotani, S. Kasai, and T. Hashizume: Appl. Phys. Express 1 (2008) 023001.
[2] K. Ohi and T. Hashizume, Jpn. J. Appl. Phys. 48, 081002 (2009).

3source

AlGaN
GaN

drain
60 ~ 400 nm

AlGaN gate
GaN

trenchDrain current (mA)trenchNi/Au gate

planar2MMC
70 nm

1050 nmtrench2DEG

VDS= 0.5V-6-4MMC50 nm-202Gate voltage (V)Fig. 3 Transfer characteristics inthe linear region.

Fig. 1 Schematic illustration ofMMC AlGaN/GaN HEMT.

10-3planar

ID and IG (A)10-510-776 mV/dec

10-9IG

10-11

VDS=0.2V
300 K

10-13-6MMC
50 nm-5-4-3-2-10Gate voltage (V)Fig. 4 Drain I-V characteristics inthe subthreshold region.

MMC (Wtop= 80nm)

Drain current m(A)25

Weff= 32µm

VG= 0 V

2015-1 V

1050-2 V-3 V02468Drain voltage (V)Fig. 2 Comparison of drain I-V
characteristics.

10Fig. 5 Drain I-V curves of planar and
MMC HEMTs. The inset indicates anenlargement of drain current in thesaturation region.