11.7 : Modeling of Diode Reverse Characteristics

In electronic circuits, reverse-biased diode configurations are critical for regulating voltage levels. Zener diodes exploit the reverse breakdown phenomenon and exhibit a controlled breakdown at a specific Zener voltage (V_Z). They are designed to maintain a constant voltage across their terminals and are commonly used for voltage regulation in circuits.

When a reverse voltage applied to a Zener diode exceeds its breakdown voltage, the diode enters the breakdown region. At this point, the diode's current-voltage (I-V) characteristic curve is effectively vertical for currents above a certain threshold, known as the knee current (I_Z).

Kirchhoff's voltage law facilitates in providing a mathematical relationship between the source voltage, diode voltage drop, and diode current which is graphically represented by the load line and indicates all possible equilibrium points for the diode in the circuit. The intersection of the load line with the diode's characteristic curve denotes the steady-state operating point Q.

At currents above the knee, the Zener diode's voltage remains stable, changing little with further increases in current. This stability is a consequence of the diode's incremental resistance, defined as the reciprocal of the curve's slope at the operating point. Thus, the voltage variation across the diode is proportional to the incremental resistance and the current exceeding the knee current. The Zener diode's ability to maintain a constant voltage over a range of currents makes it invaluable for creating stable reference voltages and protecting circuits from overvoltage conditions.

Manufacturers provide the reverse breakdown voltage, the knee current, the incremental resistance, and the power rating for Zener diodes. The power rating allows one to estimate the maximum possible current that can flow through the Zener diode without damaging it.