As we know, forward bias produces current through a diode and reverse bias essentially prevents current, except for a negligible reverse current. Reverse bias preventsĀ 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 isĀ called the forward current and is designated IF. Figure illustrates what happens as theĀ 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.Ā With 0 V across the diode, there is no forward current. As you gradually increase theĀ 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 acrossĀ 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 currentĀ begins to increase rapidly, as illustrated in Figure (b).Ā As you continue to increase the forward-bias voltage, the current continues to increaseĀ very rapidly, but the voltage across the diode increases only gradually above 0.7 V. ThisĀ small increase in the diode voltage above the barrier potential is due to the voltage dropĀ 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 inĀ Figure on a graph, you get theĀ V-I characteristic curve for a forward-biased diode, asĀ shown in Figure (a). The diode forward voltage (VĀ horizontal axis, and the forward current (IFF) increases to the right along theĀ ) increases upward along the vertical axis.
Fig :Ā Relationship of voltage and currentĀ 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 IFĀ increases rapidly. As previously mentioned, there is a slight increase in VF above 0.7 V asĀ the current increases due mainly to the voltage drop across the dynamic resistance. The IFĀ scale is typically in mA, as indicated.
Three points A, B, and C are shown on the curve in Figure (a). Point A correspondsĀ to a zero-bias condition. Point B corresponds to Figure (a) where the forward voltageĀ is less than the barrier potential of 0.7 V. Point C corresponds to Figure (a) where theĀ forward voltage approximately equals the barrier potential. As the external bias voltageĀ and forward current continue to increase above the knee, the forward voltage will increaseĀ slightly above 0.7 V. In reality, the forward voltage can be as much as approximately 1 V,Ā 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 reverseĀ current. As you gradually increase the reverse-bias voltage, there is a very small reverseĀ 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 breakdownĀ 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,Ā you get the V-I characteristic curve for a reverse biased diode. A typical curve is shown in AboveĀ Figure. The diode reverse voltage (VR) increases to the left along the horizontal axis,Ā 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 theĀ diode reaches approximately the breakdown value (VBR) at the knee of the curve.
After thisĀ 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 breakdownĀ voltage for a diode depends on the doping level, which the manufacturer sets, depending onĀ 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-IĀ 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 characteristicĀ curves in Below Figure. The blue curve is at room temperature (25Ā°C) and the red curve isĀ at an elevated temperature (25Ā°C + ĪT). The barrier potential decreases by 2 mV for eachĀ degree increase in temperature.
Fig :Ā Temperature effect on the diode V-IĀ characteristic. The 1 mA and 1 mAĀ marks on the vertical axis are givenĀ as a basis for a relative comparisonĀ of the current scales.
For a reverse-biased diode, as temperature is increased, the reverse current increases.Ā The difference in the two curves is exaggerated on the graph illustration.Ā Keep in mind that the reverse current below breakdown remains extremely small and canĀ usually be neglected.
