Bulk Breakdown in AlGaN/GaN HFETs
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t. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
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in most cases, such an electrical/light emission response is associated with a destructive damage of the tested device. The present breakdown definition is independent of the breakdown mechanisms leading to the device failure. 0rtýe3A)~In most cases, with no premature on-surface breakdown, our test method allows us to observe bulk impact ionization processes up to large VDs (90 to 160 V), very close (< 5 %) to the VDwsdefined here.
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signal. This isa definition of the "destructive" or total breakdown, because,
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RESULTS
5 4
Source RelatedBreakdown
The results below correspond to a typical high operational voltage HFET (device #H9) having gate length La = 2.5 pmn, gate width w = 50 pm, and gate-to-drain gap LM = 1.5 pm. Three-terminal pulsed measurements were performed for Vos = -6 to -2 V with 0.5 or I V steps up to VDs - 150 V. The tested device had VWq = -3.5 V. Examples of time-resolved characteristics in the subthreshold regime are presented in Figs. 2a and 2b. For VGS = -6 V (Fig. 2a) it is observed that Is = 0 up to Vns = 85 V. At that voltage a small source current (-1 pA) appeared which increased with VDs with an increasing rate. In this example VDs was limited to 120 V for which the peak currents Isp= 65 pA and 'Op = 20 pA, while the intensity of the visible light emission signal was very low (
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Flg.2. Tilme-resolved electrdcal and visible enision characteristics for a AlGaN/GaN with L=..5 pn, w = 60 po, and LID = 1.5 pmn(a) For VYs = -6 V and Va=120 V: Isp = 65 pA; IG = 20 pA; Lv, < 10 mV;(b) For: VYs = -5 V and Vosp = 132 V: Isp= 0.96 pA; IG = 38 p.; Lv, = 110 mV.
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Fig. 3. Current and light emission signal variations with drain voltage for gate biases: different (a) Is vs. VDSfor Vcs = -6 to -2 V ; (b)IG vs. Vns (3-T, solid lines) and I. yS. VD (2-T, dashed lines; (c)Lvs vs. V0 (3-T, solid lines) and Lvjs vs. VD (2-T, dashed lines) characteristics.
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Fig. 4. Time-resolved characteristics for a device with LG = 2 pmi, w = 100 pm, and LCD =1.4 pm showing a
bulk channel breakdown in the subthreshold reglme(VYs=-6V): (a) For Vmp = 96 V: s6pf2.1 mA; IGp= 147 pA; Lvys = 156 mV; (b) For Vosp = 97 V; Isp = 5 mA; IGp = 264 pA; Lvys = 336 mV; (c) Total breakdown response at VYsp = 97 V (following applied pulse after the above in (b)). The Is = f(Vos) characteristics for VG = -6 to -2
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