V - I Characteristics of Diodes

Dr. Hari V S
Department of Electronics and Communication
College of Engineering Karunagappally

What You will Learn

• You will plot the V-I characteristics of Si and Ge diodes and understand the difference in cut in voltage and slope of the characteristics.

Theory

Semiconductor diode is a $p-n$ junction that conducts well on forward bias and conducts very little on reverse bias. The relation between diode current and voltage is \begin{equation}\label{diodeeqcurrent} I=I_{o}[e^{\frac{V_{D}}{\eta V_{T}}}-1] \end{equation} where $I_{o}$ is the reverse saturation current, $V_{D}$ is the voltage across the diode and $V_{T}=\frac{kT}{q}$. $k$ is the Boltzmann constant and $T$ is the absolute temperature. The ideality factor $\eta$ varies from $1$ to $2$.

Experiment - Forward Characteristics

Qucs is launched and the components in the schematic, shown below, are entered into the editor.

A Si diode ($1N4001$ here) is selected from $\fbox{Tools}$ $\longrightarrow$ $\fbox{Components Library}$.

The bias voltage $V_d$ is selected from $\fbox{Sources}$ and a current probe $I_{d}$ is selected from $\fbox{Probes}$ from the $\fbox{Components}$ tab in the left pane.

$V-I$ characteristics are drawn by varying the bias voltage and measuring currents in discrete steps. This accomplished by dragging and dropping the $\fbox{Parameter Sweep}$ from the $\fbox{Simulations}$. Right click on the $\fbox{Parameter Sweep}$ and set variation of voltage and the number of iteration points. In the schematic shown, the voltage source is set to to vary linearly from $1\mu$V to $10$V in $1000$ steps. A $\fbox{DC simulation}$ is also dragged and dropped in the schematic.

The components are connected together by pressing $\fbox{Ctrl E}$ and mouse clicks. Any procedure can be stopped by hitting $\fbox{Esc}$.

Save the schematic as name.sch in the current working directory.

Once the connections are made, go to $\fbox{Simulation}$ $\longrightarrow$ $\fbox{Simulate}$ or as short cut, press $\fbox{F2}$, to start the simulation.

When the simulation is over, it launches an output window titled name.dpl with the left pane pointed at $\fbox{diagrams}$. There is a rich variety of outputs here such as Cartesian, polar, Smith chart, truth table etc.

Drag and drop Cartesian diagram to the dpl window and a dialog box with the data sets for the name.sch schematic appears. Select $Id.1$ vs $v_{d}$ and click $\fbox{OK}$ and the $V-I$ characteristics are displayed on the dpl window. The plot can be saved or the data can be exported. A typical plot in format is shown below.

Observe the linear and exponential regions in the characteristics as modeled by the current equation. Observe the cut in voltage of the diode.

Experiment - Reverse Characteristics

The Qucs schematic for plotting reverse characteristics of Si diode is shown below. In the $\fbox{Parameter Sweep}$, set the diode reverse voltage between $0$ V and $10$ V to study the saturation of reverse current. Run the simulation and observe the dpl file as shown below.

The transient simulation with parameter sweep results in the reverse characteristic prior to break down as shown below. If the diode voltage is allowed to sweep to $100\,V$, then the break down happens at around $50\,V$, as shown below. Repeat the experiment for a Ge diode as well.

Observations

• Forward resistance of Si diode =$\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\, $ $\Omega$
• Forward resistance of Ge diode =$\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\,$ $\Omega$
• Reverse resistance of Si diode =$\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\,$ $k\,\Omega$
• Reverse resistance of Ge diode =$\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\,$ $k\,\Omega$

What You Learned

• You plotted the V-I characteristics of diodes under forward and reverse condition on the simulator
• You measured the cut in voltage, forward and reverse resistances of Si and Ge diodes