# VI Characteristic of a Diode

As we know, forward bias produces current through a diode and reverse bias essentially prevents current, except for a negligible reverse current. Reverse bias preventsÂ current as long as the reverse-bias voltage does not equal or exceed the breakdown voltage of the junction. In this section, we will examine the relationship between the voltage and the current in the diode.

V-I Characteristic for Forward Bias

When a forward-bias voltage is applied across a diode, there is current. This current isÂ called the forward current and is designated IF. Figure illustrates what happens as theÂ forward-bias voltage is increased positively from 0 V. The resistor is used to limit the forward current to a value that will not overheat the diode and cause damage.Â With 0 V across the diode, there is no forward current. As you gradually increase theÂ forward-bias voltage, the forward current and the voltage across the diode gradually increase, as shown in Figure (a).

A portion of the forward-bias voltage is dropped acrossÂ the limiting resistor. When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward currentÂ begins to increase rapidly, as illustrated in Figure (b).Â As you continue to increase the forward-bias voltage, the current continues to increaseÂ very rapidly, but the voltage across the diode increases only gradually above 0.7 V. ThisÂ small increase in the diode voltage above the barrier potential is due to the voltage dropÂ across the internal dynamic resistance of the semiconductive material.

Graphing the V-I Curve If you plot the results of the type of measurements shown inÂ Figure on a graph, you get theÂ V-I characteristic curve for a forward-biased diode, asÂ shown in Figure (a). The diode forward voltage (VÂ horizontal axis, and the forward current (IFF) increases to the right along theÂ ) increases upward along the vertical axis.

Fig :Â Relationship of voltage and currentÂ in a forward-biased diode

As you can see in Figure (a), the forward current increases very little until the forward voltage across the pn junction reaches approximately 0.7 V at the knee of the curve.After this point, the forward voltage remains nearly constant at approximately 0.7 V, but IFÂ increases rapidly. As previously mentioned, there is a slight increase in VF above 0.7 V asÂ the current increases due mainly to the voltage drop across the dynamic resistance. The IFÂ scale is typically in mA, as indicated.

Three points A, B, and C are shown on the curve in Figure (a). Point A correspondsÂ to a zero-bias condition. Point B corresponds to Figure (a) where the forward voltageÂ is less than the barrier potential of 0.7 V. Point C corresponds to Figure (a) where theÂ forward voltage approximately equals the barrier potential. As the external bias voltageÂ and forward current continue to increase above the knee, the forward voltage will increaseÂ slightly above 0.7 V. In reality, the forward voltage can be as much as approximately 1 V,Â depending on the forward current.

V-I Characteristic for Reverse Bias

When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the pn junction. With 0 V across the diode, there is no reverseÂ current. As you gradually increase the reverse-bias voltage, there is a very small reverseÂ current and the voltage across the diode increases. When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdownÂ value (VBR), the reverse current begins to increase rapidly.

As you continue to increase the bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases very little above VBR. Breakdown, with exceptions, is not a normal mode of operation for most pn junction devices.

Graphing the V-I Curve

If you plot the results of reverse-bias measurements on a graph,Â you get the V-I characteristic curve for a reverse biased diode. A typical curve is shown in AboveÂ Figure. The diode reverse voltage (VR) increases to the left along the horizontal axis,Â and the reverse current (IR) increases downward along the vertical axis. There is very little reverse current (usually mA or nA) until the reverse voltage across theÂ diode reaches approximately the breakdown value (VBR) at the knee of the curve.

After thisÂ point, the reverse voltage remains at approximately VBR, but IR increases very rapidly, resulting in overheating and possible damage if current is not limited to a safe level. The breakdownÂ voltage for a diode depends on the doping level, which the manufacturer sets, depending onÂ the type of diode. A typical rectifier diode (the most widely used type) has a breakdown voltage of greater than 50 V. Some specialized diodes have a breakdown voltage that is only 5 V.

The Complete V-I Characteristic Curve

Combine the curves for both forward bias and reverse bias, and you have the complete V-IÂ characteristic curve for a diode, as shown in Below Figure.

Temperature Effects

For a forward-biased diode, as temperature is increased, the forward current increases for a given value of forward voltage. Also, for a given value of forward current, the forward voltage decreases. This is shown with the V-I characteristicÂ curves in Below Figure. The blue curve is at room temperature (25Â°C) and the red curve isÂ at an elevated temperature (25Â°C + Î”T). The barrier potential decreases by 2 mV for eachÂ degree increase in temperature.

Fig :Â Temperature effect on the diode V-IÂ characteristic. The 1 mA and 1 mAÂ marks on the vertical axis are givenÂ as a basis for a relative comparisonÂ of the current scales.

For a reverse-biased diode, as temperature is increased, the reverse current increases.Â The difference in the two curves is exaggerated on the graph illustration.Â Keep in mind that the reverse current below breakdown remains extremely small and canÂ usually be neglected.