Question

Draw the I- V characteristic curve of p- n junction in forward and reverse bias

Answer 1

$\underline{\large{\textbf{Forward\: Characteristics\: of\: p-n\: junction\: diode}}}$

The circuit diagram to obtain the forward Characteristics of diode under forward bias is as shown in $\sf\textsf{Image1.}$

It consists of dc voltage source and a Resistor 'R' connected in series with the diode. This Resistor act as '$\textbf{current limiting Resistor}$' as it limits the forward current to a value that will not over heat the diode and cause damage.

By varying $V_{f}$, the readings of $V$ and $I$ are noted.

A graph is plotted by taking $V$ on X-axis and $I$ on Y-axis.

$\textit{Characteristics:}$

- Initially, current is small when the voltage across the diode is less than $V_{0}$(i.e., in region OA), because the potential barrier prevents the holes from p-region and electrons from n-region, to flow across the Diode resistance in the opposite direction

- For $V$ > $V_{0}$, the Diode resistance becomes thin and hence the holes cross the junction from p to n type and the electrons cross the junction in opposite direction, resulting in relatively large current flow in external circuit.

$\underline{\large{\textbf{Reverse\: Characteristics\: of\: p-n\: junction\: diode}}}$

The circuit diagram to obtain the forward Characteristics of diode under reverse bias is as shown in $\textsf{Image2.}$
(It is similar to earlier, except that polarities of battery $V_{f}$ are reversed, Let it be $V_{r}$ now)

By varying $V_{r}$, the readings of $V$ and $I$ are noted.

A graph is plotted by taking $V$ on X-axis and $I$ on Y-axis.

$\textit{Characteristics:}$

- As reverse voltage is increased, reverse current increases initially but after a certain voltage, the current remains constant and is equal to reverse saturation current ($I_{0}$), though reverse voltage ($V_{r}$) is increased.

- Therefore, $I_{0}$ is current due to minority charge carriers and depends on Temperature rather than reverse voltage current ($V_{r}$).

- But as the reverse voltage $V_{r}$ is further increased, diode breaks down which is shown by dotted line in the figure shown in attachment.

- Here, $V_{BD}$ is the $\textit{Breakdown voltage}$, which is the reverse voltage at which the junction breakdown occurs.