Q.Explain V-I Characteristics of a power diode.

The V-I characteristic of a power diode has a forward region that is linearised by the drift-region resistance and a reverse region of small leakage up to the breakdown voltage.

[!ANSWER]

The forward branch shows little current up to the knee $V_j$, then an almost straight rise of slope $1/R_{ON}$ ($V_{AK}=V_j+I_F R_{ON}$); the reverse branch shows a small leakage current up to $V_{BR}$, beyond which avalanche gives a steep reverse current.

[!TLDR]

Forward: small current up to $V_j$, then a linear rise set by $R_{ON}$; reverse: small leakage up to $V_{BR}$, then avalanche.

The static V-I characteristic of a power diode is drawn with forward current $I_F$ (and voltage) in the first quadrant and reverse current (and voltage) in the third quadrant.

Forward characteristic: For small forward voltages, below the knee (threshold) voltage $V_j$ (about 0.7 V for silicon), only a very small current flows. Once the junction is forward biased beyond the knee, the current increases, but because of the ohmic resistance $R_{ON}$ of the lightly doped drift region the increase is an almost straight line rather than the sharp exponential of a signal diode. The forward drop follows

so the forward branch is linearised with a slope determined by $1/R_{ON}$.

Reverse characteristic: When reverse biased, the diode carries only a small, nearly constant reverse leakage (saturation) current, since the depletion region blocks conduction. This continues up to the reverse breakdown voltage $V_{BR}$ (the peak reverse voltage rating). Beyond $V_{BR}$ avalanche multiplication sets in and the reverse current rises steeply, which can damage the device.

Key points marked on the curve are the forward knee $V_j$, the linear forward conduction region, the small reverse leakage, and the reverse breakdown $V_{BR}$.

[!ANSWER]

The power-diode V-I curve has a forward branch that stays low until $V_j$ and then rises almost linearly (slope set by $R_{ON}$, $V_{AK}=V_j+I_F R_{ON}$), and a reverse branch of small leakage up to $V_{BR}$, beyond which avalanche breakdown occurs.