Bias Conditions of a Diode
Zero Biased Junction Diode
No external potential energy is applied to the PN junction during connecting a diode in a Zero Bias condition. Therefore, if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with adequate energy to overcome the potential barrier will travel across the junction against this barrier potential. This is called the “Forward Current” and is referenced as IF.
Similarly, holes generated in the N-type material (minority carriers), find this situation favorable and travel across the junction in the reverse direction. This is called the “Reverse Current” and is referenced as IR. This relocation of electrons and holes back and forth across the PN junction is called diffusion, as shown below.
Existing potential barrier prevents the flow of additional majority carriers across the junction. Nevertheless, the potential barrier assists minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction.
Then an “Equilibrium” or balance will be formed when the majority carriers are equal and both moving in reverse directions, so that the net result is zero current flowing in the circuit. When this happens the junction is called in a state of “Dynamic Equilibrium".
The minority carriers are frequently generated due to thermal energy so this state of equilibrium can be shattered by increasing the temperature of the PN junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow since no circuit has been connected to the PN junction.
Reverse Biased PN Junction Diode
A positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material during connecting a diode in a Reverse Bias condition.
The positive voltage applied to the N-type material attracts electrons towards the positive electrode outside the junction, while the holes in the P-type end are also attracted outside the junction towards the negative electrode.
The net outcome is that the depletion layer develops broader due to a lack of electrons and holes and presents a high impedance path, almost an insulator and a high potential barrier is formed across the junction thus averting current from flowing through the semiconductor material.
Increase in the Depletion Layer due to Reverse Bias
This circumstance denotes a high resistance value to the PN junction and basically zero current flows through the junction diode with an upsurge in bias voltage. However, a slight reverse leakage current travel through the junction which is measured in micro-amperes, (μA).
If the reverse bias voltage Vr applied to the diode is very high, it will cause the diode’s PN junction of the diode to overheat and fail due to the avalanche effect around the junction. This effect makes the diode shorted and causes the maximum circuit current movement, and this shown as a step descending incline in the reverse static characteristics curve below.
Reverse Characteristics Curve for a Junction Diode
Occasionally this avalanche effect has concrete applications in voltage stabilizing circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a predetermined maximum value thereby creating a fixed voltage output across the diode. These types of diodes are called Zener Diodes.
Forward Biased PN Junction Diode
A negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material during connecting a Forward Bias. When this voltage is more than the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overwhelmed and current will begin to flow.
As the negative voltage pushes electrons to the junction giving them the energy to cross over and associate with the holes being pushed in the reverse direction to the junction by the positive voltage. This consequence in a characteristics curve of zero current flowing up to this voltage point, known as “knee” on the static curves and then a high current flow through the diode with slight rise in the outside voltage as shown below.
Forward Characteristics Curve for a Junction Diode
A forward biasing voltage’s application on the junction diode causes the depletion layer becoming very tinny and thin which signifies a low impedance path through the junction thereby permitting high currents to move. The point at which this sudden upsurge in current takes place is denoted on the static I-V characteristics curve above as the “knee” point.
Reduction in the Depletion Layer due to Forward Bias
This circumstance signifies the low resistance path through the PN junction permitting huge currents to drift through the diode with only a little upsurge in bias voltage. The actual potential difference across the junction or diode is retained constant by the depletion layer’s action at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.
As the diode is able to conduct “infinite” current above this knee point, it causes a short circuit. Hence, resistors are used in series with the diode to limit the flow of current. Higher than its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for causing rapid failure of the device.