D1 Current-Voltage Curves (I-V)

Topic

The measured current-voltage curve of this diode—representing the basic experimental method underlying this model—is fitted on a semi-logarithmic scale to extract the Shockley model parameters (which are described in greater detail elsewhere in this set of models). On this semi-logarithmic scale, the segment of the measured curve where the diode still behaves ideally appears as a straight line; the slope and y-intercept of this line yield the Shockley model's ideality factor and saturation current, respectively. Any deviation of the measured curve from this ideal straight line on the semi-logarithmic scale means that the corresponding segment falls outside the range where the semi-logarithmic fit can reliably extract the Shockley model parameters.

The Cheung method exploits this very deviation—observed at high forward currents—to extract the diode's series resistance. At high forward currents, an increasing fraction of the total applied voltage drops across the series resistance rather than across the junction itself (which the Shockley model describes); this additional voltage drop across the series resistance causes the measured curve to deviate from the ideal straight line on the semi-logarithmic scale. Consequently, a higher series resistance causes the measured curve to deviate from the ideal straight line at a lower forward current. For this reason, the model—utilizing the diode's current-voltage curve and the series resistance identified earlier—applies this same electrical characterization method to a different MOSFET. The goal is to extract that MOSFET's transconductance from its current-voltage curve; this transconductance—representing the slope of the MOSFET's current relative to its controlling gate voltage—is a parameter that, like the diode's ideality factor, saturation current, and series resistance, can only be directly extracted through this specific current-voltage measurement technique. The fact that this current-voltage curve—whether measured on the diode or the MOSFET—serves as the fundamental experimental technique for extracting all these distinct parameters makes this model the common experimental starting point for the other two models in the set.

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