Fabrication and Characterization of GaN Junctionfield Effect Transistors
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sec in N2 to activate the Mg dopant. Hall measurements showed a free carrier concentration of 1.3x1018, 2.4x1018, and 6.1x1014 cm-3 in the p-GaN, n-GaN, and SI-GaN, respectively. The electron mobility in the n-GaN active layer was 270 cm2/V-sec. The fabrication process began with a mesa isolation etch in an inductively coupled BCl3/Cl2/Ar plasma. Next, a gate metal of Ni/Au/Ni was e-beam evaporated on top of the mesa and used as the mask for the self-aligned source-drain etch in a BCl3/Cl2/Ar ICP discharge. The device was completed with Ti/Al source and drain ohmic contact metallization. Postmetallization annealing was not performed. A transmission line method (TLM) measurement showed an as-deposited source and drain ohmic contact resistance of 4.2 Ωmm, a specific contact resistance of 5x10-5 Ω-cm2, and a sheet resistance of 4700 Ω/square, respectively. These values were relatively large, possibly due to plasma induced damage and an overetched source and drain region. RESULTS AND DISCUSSION Table I summarizes the DC and microwave result of a 0.8 µm X 50 µm GaN JFET with a source-drain spacing of 3 µm. A maximum ID of 270 mA/mm and a maximum gm of 48 mS/mm were measured at VG=1 V. A RS of 8.5 Ω-mm and a RD of 13 Ω-mm were obtained using the end resistance measurement technique. From the above result, an intrinsic transconductance (gm0) of 81 mS/mm was calculated. The channel was completely pinched off at a threshold voltage of VG=-8 V, with an ID=210 µA/mm at VD=15 V. A gate-drain diode reverse breakdown voltage of 56 V was achieved, which corresponded to a breakdown field of 2.5x106 V/cm. The fact that the breakdown field approached the theoretical predicted breakdown field of GaN indicated that the relatively small breakdown voltage of the JFET as compared to the reported GaN MESFETs and MODFETs [4,12] was primarily due to the high doping concentration in the n-GaN. The gate leakage current of our JFETs was large as compared to GaN MODFETs [13,14], possibly due to the plasma induced damage to the junction [15, 16]. The forward turn-on voltage of the gate diode was ~ 1 V using a 1 mA/mm current criterion. This value is only 30% of the bandgap energy of GaN. Pernot, et. al. have also reported a low turn-on voltage of 1.2 V on GaN p-n diodes [17]. The cause of this low turn-on is not known at present but may result from defect levels in the GaN. maximum drain current (ID) threshold gate voltage (VT) knee voltage (Vknee) gate-drain diode breakdown voltage gate turn-on voltage extrinsic transconductance (gm) parasitic source resistance (RS) parasitic drain resistance (RD) cut-off frequency (fT) maximum frequency (fmax)
270 mA/mm -8 V 8V 56 V ~1V 48 ms/mm 8.5 Ω-mm 13 Ω-mm 6 GHz 12 GHz
Table I: DC and microwave characteristics of a 0.8 µm X 50 µm GaN JFET.
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Drain Current (mA)
VGS=1 to -7 V, -1 V steps
10
5
0 0
10
20
30
40
Drain Voltage (V) Figure 1: ID-VD characteristic showing the drain current collapse at VD< 25 V due to a partially depleted channel by electrons trapped in the SI-GaN. The
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