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Modeling of Defect Related Reliability Phenomena
in SiC Power-MOSFETs

2.3 Measurement Setups

Extracting the channel conductance is most frequently performed by one of the two measurement principles shown in Figure 2.7. The constant gate voltage scheme uses a trans-impendance amplifier configuration to measure the source-drain current through the channel which is converted to a voltage proportional to the current by the amplifier circuit. The main advantage of this principle is its simplicity and high stability. However, the recorded change of the drain current has to be converted back to a threshold voltage shift by an initially recorded (math image)( (math image)) characteristics (see 2.1.1). Changes in the subthreshold-slope \( SS \) and trans-conductance \( g_\mathrm {m} \) of the transistor transfer characteristic during the experiment are thereby neglected and can lead to erroneous ∆Vth.

In the constant drain current method a feedback loop to the transistor gate from the amplifier output ensures constant conductance (at constant drain bias) in the channel, with the advantage that changes in the trans-conductance and shape of the (math image)( (math image)) do not lead to additional error due to post-processing utilizing an initially recorded characteristics. However, the circuit requires a setup designed in a way that stable operation due to the feedback loop is guaranteed at all measurement conditions, which requires additional passive elements [134].

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Figure 2.7: The basic measurement principles for the change of channel conduction are shown. In the constant gate voltage scheme (left) the operational amplifier is configured as a trans-impendance amplifier. At con- stant applied gate bias, the output voltage of the amplifier stage is proportional to the drain current. At the constant drain current configuration (right), a constant source current is forced through the channel ajdusted with the source resistor. The feedback loop ensures that the gate bias is adjusted in a way that the drain current is constant (reproduced from [134]).

Within this work, two custom built setups have been used. One is based on the constant voltage scheme, and termed Defect Probing Instrument (DPI) [163], and the other one on the constant current feedback loop method designed by Reisinger et. al [147]. Note that a comparison of the DPI with commercially available general purpose measurement instruments shows superior SNR at higher current resolution. The minimum measurement delay depends on the chosen gain region and is limited by the gain bandwidth product of the measurement region, with \( t_\mathrm {delay} \approx \SI {100}{\micro \second } \) at the highest resolution selected [163]. In the feedback loop setup, this delay is determined by the settling time of the output voltage of the OPAMP with a minimum of \( t_\mathrm {delay} \approx \SI {1}{\micro \second } \) [147].