Forward and Reverse Bias Characteristics of a p-n Diode

First identify diode terminals and battery polarity. Decide whether the junction is forward or reverse biased. Describe the effect on depletion width and barrier potential. Then connect this to the size and direction of current. For graph questions, read voltage on the x-axis and current on the y-axis before calculating ratios.

Show the circuit for forward and reverse bias and the I-V characteristic graph. Graph labels must include voltage V on the horizontal axis, current I on the vertical axis, forward region, reverse region, knee voltage, reverse saturation current, and breakdown voltage.

Using V/I from the origin as a constant diode resistance is usually wrong because a diode is non-ohmic. For a small part of the curve, use delta V divided by delta I.

The p-n diode characteristic is the graph of current through a diode against the voltage applied across it. In forward bias the diode offers low resistance after the knee voltage, while in reverse bias it offers high resistance with a small reverse saturation current until breakdown.

The p-n diode characteristic is the graph of current through a diode against the voltage applied across it. In forward bias the diode offers low resistance after the knee voltage, while in reverse bias it offers high resistance with a small reverse saturation current until breakdown. Circuit condition: forward bias has p connected to positive and n connected to negative; reverse bias has p connected to negative and n connected to positive. Graph relation: current is non-linear with voltage. The dynamic resistance in a small region is rd = delta V / delta I, where V is voltage in volt, I is current in ampere, and rd is in ohm. Use this only for a small portion of the I-V curve, not for the entire non-linear graph. For a silicon diode in forward bias, current remains small at very low voltage but increases sharply near about 0.7 V. In reverse bias, the current stays very small for a wide voltage range and rises suddenly only if breakdown voltage is reached.

A diode conducts mainly in one direction because external bias changes the depletion region and potential barrier. In forward bias, the p-side is connected to the positive terminal and the n-side to the negative terminal. The barrier reduces, the depletion region narrows, and majority carriers cross the junction more easily. Current rises sharply after the knee voltage. In reverse bias, the p-side is connected to the negative terminal and the n-side to the positive terminal. The barrier increases, the depletion region widens, and only a small reverse saturation current flows due to minority carriers until breakdown occurs at high reverse voltage. Circuit condition: forward bias has p connected to positive and n connected to negative; reverse bias has p connected to negative and n connected to positive. Graph relation: current is non-linear with voltage. The dynamic resistance in a small region is rd = delta V / delta I, where V is voltage in volt, I is current in ampere, and rd is in ohm. Use this only for a small portion of the I-V curve, not for the entire non-linear graph. A diode current changes from 8 mA to 18 mA when the forward voltage changes from 0.68 V to 0.72 V in a small operating region. Dynamic resistance rd = delta V / delta I = (0.72 - 0.68) V / (18 - 8) mA = 0.04 V / 0.010 A = 4 ohm. This means the diode offers small incremental resistance in that forward-biased region. Students may be asked to draw the I-V curve, compare forward and reverse bias, identify knee voltage or breakdown voltage, calculate dynamic resistance from graph data, or explain why a diode is unidirectional. Using V/I from the origin as a constant diode resistance is usually wrong because a diode is non-ohmic. For a small part of the curve, use delta V divided by delta I.