Electric Breakdown in Nitride PN Junctions
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Mat. Res. Soc. Symp. Proc. Vol. 395 ©1996 Materials Research Society
determined by C-V measurements ranged from 2.2 to 2.6 V. The impurity gradient in the pn junction was calculated to be from 2x1022 to 2x10 23 cm-4 for different samples. The diodes had good rectifying I-V characteristics (Fig. 1). The reverse I-V characteristics showed an abrupt electric breakdown having a microplasmic nature. The breakdown voltage, Vb, ranged from 40 to 150 V. At reverse voltages less than breakdown voltage (V < 0.9xVb), the leakage current density at room temperature was less than 10-2 A/cm 2 . Taking into account that the pn junctions were linearly graded, the strength of electric breakdown field was estimated using the well-known equation [6]: Vbi,
Vb = 2WE= 4E1.5 (2Es)o.5(a).0.5
3
3
(1)
q
where E is the strength of the electric breakdown field, W is the width of the space charge region at the breakdown voltage, es is the dielectric constant of the semiconductor, and a is the impurity gradient. The values for W and a were extracted from C-V characteristics. Using equation (1), it was found that the strength of the electric breakdown field for the GaN pn junctions ranged from 1.5 to 3 MV/cm (Fig. 2). This value is approximately 4 times higher than the maximum breakdown field for a linearly graded GaAs pn junction at the same impurity gradient (see ref. [6]). The maximum breakdown field of 3 MV/cm was measured for an impurity gradient of 2x10 23 cm- 4 . It was found that Vb increases with temperature. The temperature coefficient of Vb ranged from 1x10- 2 to 2x10- 2 V/K (Fig. 3).
1.0 0.5 son
0.0
-----
--------.-.-.------.-----
i..li........ -.-.-.--..
emi
-0.5 -1.01 -100
-50
0 Voltage (V)
5
10
Figure 1. Current-voltage characteristics of a GaN pn diode (300 K). 910
10
11 10&2
i
I
I
I
I
I
I
I i
I
.
.
.
.
.
.
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102 4
1P2 C(cm4 )
Figure 2. Breakdown field as a function of impurity gradient in a GaN pn junction.
1101
100
90
80
I
300
I
I
400
I
......
5W
600
Temperature(K Figure 3. Temperature dependence of the breakdown voltage for two GaN pn structures.
911
AlGaN pn junctions. The structures investigated were as follows: n-GaN / n-AlGaN / p-AlGaN / p+GaN. The structure was grown in a single epitaxial run. First, the n-GaN layer was grown and then the AlGaN pn junction was formed. The AIN concentration in the alloy was from 2 to 8 mol.%. Thickness of the AlGaN layers ranged from 0.1 to 0.5 jtm. A p+GaN cap layer about 0.5 11m thick was grown on the top of the AlGaN pn junction. A detailed description of the structures was given in [2]. C-V characteristics indicated that these junctions were also linearly graded. Reverse I-V characteristics showed an abrupt breakdown at 40 - 100 V. The breakdown field was estimated to be not less than 2 MV/cm. CONCLUSIONS The electrical characteristics of GaN and A1GaN pn junctions were investigated. The CV characteristics showed that the junctions were linearly graded having an impurity gradient ranging from 2x10 22 to 2x10 23 cm-4 . For GaN pn
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