Q.Explain briefly with the help of a circuit diagram, charges and depletion layer, how V-I characteristics of a p-n junction diode are obtained in reverse bias. Draw the shape of the curves obtained. Why is the current under reverse bias almost independent of the applied potential up to a critical voltage? OR Why should a photodiode be operated at a reverse bias? Draw the biasing circuit of an illuminated photodiode and characteristic curves.

To trace the forward V-I characteristic, the diode is forward-biased through a variable-voltage source and a current-limiting resistor, with a voltmeter across the diode and a milliammeter in series to record corresponding V and I readings.

A labelled circuit description (since this is a diagram question, the arrangement is described precisely below):

• A p-n junction diode is connected in forward bias: its p-side (anode) connected through a rheostat (variable resistor, to limit/control current) to the positive terminal of a DC battery, and its n-side (cathode) connected to the battery's negative terminal, completing the loop.
• A milliammeter (mA) is connected in series in this loop, to measure the forward current $I_f$ through the diode.
• A voltmeter (V) is connected in parallel directly across the diode terminals, to measure the forward voltage $V_f$ across it.
• A key/switch is included in series to open/close the circuit. …

A hole carries a positive charge $+e=+1.6\times10^{-19}$ C; the diode symbol is an arrowhead (p, anode) pointing to a bar (n, cathode).

Charge on a hole. A hole is the vacancy left when a covalent bond loses an electron. It behaves exactly like a free particle carrying a positive charge equal in magnitude to the electron's, i.e.

$q_{hole}=+e=+1.6\times10^{-19}\ \text{C}.$

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The diode's V–I curve is not a straight line through the origin, so it does not obey Ohm's law.

Ohm's law, $V = IR$ with constant $R$, holds only for conductors whose V–I graph is a straight line through the origin (ohmic devices, e.g. a metal wire at constant temperature). A PN junction diode's V–I characteristic is strongly non-linear: negligible current flows until the forward voltage crosses the barrier potential (~0.6–0.7 V for silicon), after which current rises st …

The arrow-into-bar symbol is the circuit symbol of a p-n junction diode.

In the standard circuit symbol of a semiconductor diode, the triangle (arrow) represents the p-side (anode) and the bar represents the n-side (cathode). The arrow points in the direction …

A diode's V-I characteristic shows how strongly it conducts as a function of the voltage across it — in forward bias, it stays almost off until the barrier potential is overcome, then conducts very freely.

Definition

The $V$-$I$ (voltage-current) characteristic of a diode is a plot of the current $I$ flowing through it against the voltage $V$ applied across it, showing how the diode's conduction depends on bias.

Forward-bias characteristic (diagram description)

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Reverse bias pulls majority carriers further away from the junction, widening the depletion region and drastically reducing current.

In reverse bias, the applied voltage attracts the majority carriers (electrons in the n-side, holes in the p-side) away from the junction, widening the depletion region and increasing the potential barrier. This makes it harder for majority carriers to cross, so majority-carrier current becomes negligibl …