Effects of Reverse Biased Floating Voltage at Source and Drain During High-Field Electron Injection on the Performance o
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high-field stress induced trapped charges in the gate oxide results in threshold voltage shift and even oxide breakdown. During high-field injection positive and negative trapped charges are generated [5]. In order to understand the degradation mechanism of the gate oxide both positive and negative trapped charge was monitored [6]. Although there are many reports on the highfield stress induced leakage current, threshold voltage shift, oxide trap charging and oxide catastrophic breakdown, the effects of reverse biased floating voltage at source and drain junctions during high-field injection on the performance of MOSFETs has not been discussed adequately in the literature. It is also possible that source and drain in a MOSFET can affect the oxide degradation during such stressing. Therefore, the purpose of present study is to see the effects of reverse biased floating voltage at source and drain junctions during high-field electron injection on NMOSFET characteristics. EXPERIMENT For the present study n-channel MOS transistors with gate length of 0.35l.Lm and oxide thickness of 60A were processed using a standard 0.25-ltrm CMOS technology. Wafers were annealed in forming gas ambient before any measurement was done. For gate injection and substrate injection transistors were stressed by constant current of-1OnA and lOnA for 3 seconds respectively. Fig. 1 shows the cross-sectional view of NMOSFET in stressing condition. In the 123
Mat. Res. Soc. Symp. Proc. Vol. 592 © 2000 Materials Research Society
present experiments source and drain terminals were biased at 0V, 2V and 4V during stress. Threshold voltage Vt and transconductance g,n, values measured before stress were quite uniform. Initial electron trapping slopes (IETS) were obtained from the gate voltage versus time curves [7]. Threshold voltage shift and transconductance shifts were obtained quickly through an automated measurement set up using HP4145 Semiconductor Parameter Analyzer to minimize the effects of detrapping.
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Fig. I The cross-sectional view of n-MOSFET under stress condition when floating voltages of V volts are applied to source and drain (for gate injection Ig=-lOnA and for substrate injection Vg=OnA). RESULTS The variation of AVt/Vt as a function of reverse biased floating voltage at source and drain junctions for both gate injection and substrate injection modes is shown in Fig. 2. It can be seen that threshold voltage shift for gate injection mode is almost independent of reverse biased voltage but for substrate injection mode increases with reverse biased voltage. Moreover, the threshold voltage degradation for substrate injection mode is higher than gate injection mode. It is well known that during high-field injection three mechanisms are responsible for the oxide charge build-up. These mechanisms are positive charge build-up due to hole trapping, negative charge build-up due to electron trapping and trap generation during stress [2]. For the gate injec
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