Chapter 6 Laser-induced Single Event Transients in BP MOSFETs
6.2 Experimental results and analysis
6.2.1 Signature of The Laser-induced SET Pulses in BP MOSFETs
Fig. 6.3 illustrates the recorded current transients at the source, drain and gate terminals when the laser (2.23 nJ) spot center is in the middle of the source and the drain electrodes. VG is biased at Vth= -0.18 V, the source is grounded, and the drain is biased at a DC voltage of -1 V.
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The measured source and drain transients have nearly the same magnitude but opposite polarity, which suggests that the transient current comes from the channel conduction. This is different from the traditional junction collection in Si devices [47]. The gate transients, if any, are indistinguishable from the background noise. The gate oxide effectively suppresses the gate transients because the barrier between semiconductor and HfO2 for both types of carriers in these devices is large enough. The peak of the SET is 80 µA with full width at half maximum (FWHM) around 100 ps. The collected charge is about 4 fC. The rise time of the measured pulse is about 35 ps and the fall time is about 75 ps. There is no obvious long term charge collection.
This suggests that diffusion-related charge collection is suppressed in these BP MOSFETs, which is likely due to the lack of a bulk region for carrier generation and collection in this device structure. In addition, the thickness of the channel BP materials is no more than 10 nm. Thus, collection volumes are much smaller than those of the planar bulk Si transistors. The small charge-collection region contributes to the relatively small magnitude of the SET peaks.
The laser-induced SETs of the BP MOSFETs are relatively fast transients with small peak current, and it is important to consider the limitations of the measurement setup discussed in section 6.1. First, the trigger level of the oscilloscope is preset to just above the background noise (1.75 mV in this case, which is equivalent to current magnitude of 35 µA) during the test for differentiation. Second, because all the terminals of the device are connected to analogous passive elements in the actual experiments, the capability to capture fast SETs is limited not only by the bandwidth of the oscilloscope, but also the external circuit and parasitic elements present
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including bond pads, bond wires, coaxial cables, bias tees, and oscilloscope input impedance [130]. It’s reported the narrow current spikes occurring within the first 20 ps of SETs are typically filtered out by parasitics [130], [131] in a similar measurement setup.
Fig. 6.3. Representative current transients recorded at the three terminals of the device when the laser spot center is at the center of the transistor. The laser wavelength is 1260 nm. The laser energy is 2.23 nJ.
6.2.2 Bias-dependence of The Laser-induced SET pulses
The bias dependence of the measured peak drain current was also investigated. First, VD
was fixed at 1 V, while VG was varied. The laser pulses are at the center of the device. The error bars in Fig. 6.4 reflect the standard deviations among the 50 transients recorded.
Fig. 6.4 (a) shows the measured peak source current versus gate bias. The SET magnitude shows negligible dependence on the overdrive voltage. Next, VG was fixed at 0.18 V and VD
was varied. The other conditions are the same as those in the gate bias dependence study. The data shown in Fig. 6.4 (b) imply that the SET magnitude increases slightly when VD changes
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from -0.6 V to -1 V. This is because the larger drain bias will induce a larger horizontal electric field. For the free carriers generated in the channel, larger horizontal electric field results in larger peak
current between the source and drain.
Fig. 6.4. Peak SET current magnitude at source for different (a) VG and (b) |VD|. The laser wavelength is 1260 nm and the center of the laser is at the center of the device. The laser energy is 2.23 nJ. The error bars are the standard deviations among the 50 transients recorded for each bias condition.
6.2.3 Position Dependence of The Laser-induced SET pulses
Line scans were performed to study the position dependence of the induced transients. A line scan of the laser spot from drain to source, parallel to the channel, was performed at the same bias conditions at VG = 0.18 V and VD = 1.0 V. The laser energy is 2.23 nJ. The center of the laser spot moves from x = 12 μm to x = 12 μm in the direction of drain to source. The center of the gate metal finger is set to x = 0. For each location at each bias condition, 50 SETs are captured and recorded to calculate the statistics of the transients. The measured SET peak current at the source is shown in Fig. 6.5 as a function of horizontal position. The maximum measured SET peak current and the longest SET current pulse are observed approximately at the
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center of the channel. The sensitive region is ~ 2 μm in size. The laser spot size is ~ 1 μm in diameter and the channel length is 0.5 μm, which suggests carriers are generated and observed as SETs only when the laser pulse is incident on the channel.
Fig. 6.5. Peak SET current at source along a line scan. The laser wavelength is 1260 nm. The shadow and error bars represent the standard deviation among the 50 transients recorded at each position. The bias conditions are VG =
0.18 V and VD = 1.0 V. The arrow indicates the movement direction of the laser spot during the line scan.
6.2.4 Laser Energy Dependence of measured SET pulses
Fig. 6.6 show the measured SET magnitude and FWHM dependence on the laser energy.
When the laser energy increases, more free carriers are generated at the same bias condition, which results in a larger SET current. The result also shows the slight increase of the laser energy induces an increase of the measured FWHMs.
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Fig. 6.6. Measured peak SET magnitude and FWHM as a function of laser energy. The laser wavelength is 1260 nm and the center of the laser is at the center of the device. The laser energy is 2.23 nJ. VG = -0.18 V, VD = -1 V and VS
=0 V. The error bar is the standard deviation among the 50 transients recorded at laser energy condition.