Physics · Ch 14 — Semiconductors
Static and dynamic resistance of a diode
Static and dynamic resistance of a diode
One of the most practically important properties of a diode is its resistance -- and this differs sharply between forward bias and reverse bias. An IDEAL diode is defined as offering exactly ZERO resistance when forward biased and exactly INFINITE resistance when reverse biased (Fig. 14.27); a REAL diode's forward I-V characteristic (as in Fig. 14.24) is instead used to define two distinct, genuinely useful resistances at any chosen operating point.
The STATIC (or DC) RESISTANCE, , of a diode at a particular point on its forward characteristic is defined as the ratio of the DC voltage across the diode to the DC current flowing through it at that point: . …
What this figure shows. A current-versus-voltage graph for an IDEAL diode, drawn as two straight line segments meeting exactly at the origin, forming a right-angle ('L'-shaped) characteristic. Along the entire reverse-voltage (negative horizontal) side, the curve is a perfectly flat horizontal line sitting exactly on the voltage axis at zero current, representing INFINITE resistance (no current flows for any reverse voltage). Along the forward-voltage (positive) side, the curve is a perfectly vertical line rising straight up the current axis at zero voltage, representing ZERO resistance (current can rise to any value with no voltage drop). This idealised right-angle shape is explicitly contrasted …
What this figure shows. The real (non-ideal, smoothly curving) forward I-V characteristic of a diode, as in Fig. 14.24, with a single operating point P marked somewhere on the curve at a chosen forward voltage and current. Two distinct lines are drawn to define the two resistances at P: a straight CHORD line drawn from the origin (0,0) up to point P, whose slope gives the static/DC resistance at that point; and a short straight TANGENT line drawn touching the curve only at point P, whose slope (found from small changes and around P) gives the dynamic/AC resistance at that same point. The two lines are drawn with visibly different slopes, illustrating that the DC and AC resistances of a dio …
Worked out. A resistor network contains an ideal diode together with two 30-ohm resistors arranged such that one path (through the diode) and one plain resistive path both connect points A and B. (1) When the diode is FORWARD biased, an ideal diode behaves as a perfect conductor (zero resistance), so the circuit reduces to two 30-ohm resistances in parallel across A and B: . (2) When the diode is REVERSE biased, an ideal diode behaves as a perfect open switch (infinite resistance), so the path through the diode carries no current at all, and the only resistance left in the circuit between A and B is the other 30-ohm resistor on its own path: $ …