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Forward and Reverse Bias of P N Junction
Forward Biased P-N JunctionWhen we connect p-type region of a junction with the positive terminal of a voltage source and n-type region with the negative terminal of the voltage source, then the junction is said to be forward biased. At this condition, due to the attraction of positive terminal of source, electrons which participated in covalent bond creations in p-type material, will be attracted towards the terminal. As result numbers of covalent bonds are broken and, electrons are shifted towards the positive terminal. As a result, the concentration of electrons in the crystal nearer to the terminal increases and these electrons recombine with holes here. In this way, the number of holes increases in the portion of the p-type region away from the junction, and it is reduced in the portion of p-type region nearer to the terminal. As such holes are shifted from terminal to junction.
Due to the higher concentration of holes adjacent to negative impurity ions layer the electrons of negative ions come out and recombine with those holes and create new holes in the layer. Consequently, the width of this negative ions layer is reduced and finally this layer vanishes. Similarly due to the negative terminal of source the free electrons in the n-type region will repeal towards junction where they will find the layer of positive impurity ions and start recombine with these ions and generate free electrons inside the layer. Consequently, the width of positive impurity ions is reduced, and finally, it vanishes. In these ways, both layers of ions disappear, and there will be no more depletion layer. After the depletion layer disappeared, free electrons from the n-type region can easily drift to p-type region and holes from p-type region to n-type region in the crystal. Hence, ideally there will be no obstruction of flowing current and the pn junction behaves as the short circuit.
Reverse Biased PN JunctionWhen positive terminal of a voltage source is connected to the n-type region and the negative terminal of the source is connected to the p-type region then the pn junction is said to be in reverse biased condition. When there is no voltage applied across the p n junction, the potential developed across the junction is 0.3 volts at 25oC for germanium pn junction and 0.7 volts at 25oC for silicon p n junction. The polarity of this potential barrier is same as the polarity of voltage source applied during reverse biased condition. Now if reverse biased voltage across the pn junction is increased the barrier potential developed across the pn junction is also increased. Hence, the pn junction is widened. When positive terminal of the source is connected to the n-type region, the free electrons of that region are attracted towards positive terminal of the source because of that more positive impurity ions are created in the depletion layer which makes the layer of positive impurity ions thicker. At the same time since negative terminal of the source is connected to the p-type region of the junction, electrons are injected in this region. Due to the positive potential of the n-type region the electrons are drifted towards the junction and combine with holes adjacent to the layer of positive impurity ions and create more positive impurity ions in the layer. Hence, the thickness of the layer increases. In this way over all width of the depletion layer increases along with its barrier potential. This increment of the width of depletion layer will continue till the barrier potential reaches to applied reverse biased voltage. Although this increment of barrier potential will continue up to applied reverse biased voltage but if the applied reverse biased voltage is sufficiently high then the depletion layer will disappear due zener breakdown and avalanche breakdowns.
It is also to be noted that after completion of reverse biased depletion layer there is no more drift of charge carriers (electrons and holes) through the junction as the potential barrier opposes the applied voltage which has the same value as the potential barrier. Although tiny current flow from n-type region to p-type region due to minority carriers that is thermally generated electrons in p-type semiconductor and holes in n-type semiconductor.
Forward Current in PN JunctionWhen battery voltage is applied across the junction in the forward bias, a current will flow continuously through this junction. IS is Saturation Current (10-9 to 10-18 A) VT is Volt-equivalent temperature (= 26 mV at room temperature) n is Emission coefficient (1 ≤ n ≤ 2 for Si ICs) Actually this expression is approximated.
Reverse Current in PN JunctionWhen a p-n junction is connected across a battery in such a manner that its n-type region is connected to the positive potency of the battery and the p-type region is connected to the negative potency of the battery the p n junction is said to be in reverse biased condition and ideally there is no current flowing through the junction. But practically there will be a tiny reverse bias current iD which is expressed as. iD drops to zero value or very small value. iD can be written as i0. IS is Saturation Current (10-9 to 10-18 A) VT is Volt-equivalent temperature (= 26 mV at room temperature) n is Emission coefficient (1 ≤ n ≤ 2 for Si ICs) Actually this expression is approximated.
General Specification of PN JunctionA p-n junction is specified in four manners.
- Forward Voltage Drop (VF): Is the forward biasing junction level voltage (0.3V for Germanium and 0.7V for Silicon Diode )
- Average Forward Current (IF): It is the forward biased current due to the drift electron flow or the majority carriers. If the average forward current exceeds its value the diode gets over heated and may be damaged.
- Peak Reverse Voltage (VR): It is the maximum reverse voltage across the diode at it reverse biased condition. Over this reverse voltage diode will go for breakdown due to its minority carriers.
- Maximum Power Dissipation (P): It is the product of the forward current and the forward voltage.
V-I Characteristics of A PN JunctionIn the forward bias, the operational region is in the first quadrant. The threshold voltage for Germanium is 0.3 V and for Silicon is 0.7 V. Beyond this threshold voltage the graph goes upward in a non linear manner. This graph is for the dynamic Resistance of the junction in the forward bias.
In the reverse bias the voltage increases in the reverse direction across the p-n junction, but no current due to the majority carriers, only a very small leakage current flows. But at a certain reverse voltage p-n junction breaks in conduction. It is only due to the minority carriers. This amount of voltage is sufficient for these minority carriers to break the depletion region. At this situation sharp current will flow through this junction. This breakdown of voltage is of two types.
- Avalanche Breakdown: it is not properly sharp, rather inclined linear graph i.e. after break down small increase in reverse voltage causes more sharp current gradually.
- Zener Breakdown: This breakdown is sharp and no need to increase reverse bias voltage to get more current, because current flows sharply.
Resistances of p-n Junction
Dynamic Resistance of p-n JunctionFrom V-I characteristics of a p-n junction, it is clear that graph is not linear. The forward biased p-n junction resistance is rd ohm it is called AC resistance or dynamic resistance. It is equivalent to slope of voltage-current of the PN junction.
Average AC Resistance of p-n JunctionAverage AC resistance is determined by the straight line that is drawn linking the intersection of the minimum and maximum values of external input voltage. Some important terms related to p-n Junction
Transition Capacitance of p-n JunctionWhen depletion region exist in the common junction around, the diode acts as a capacitor. Here the depletion region is the dielectric and two regions (p-type and n-type) at both ends act as the charged plates of a capacitor. As the depletion layer decreases the capacitance value goes down.
Diffusion Capacitance of p-n JunctionIt the capacitance of the diode in forward biased condition and it is defined as the ratio of transiting charge created to the differential change in voltage. When the current through the junction increases the diffusion capacitance also increases. Along with this increase in current, the forward biased resistance also decreases. This diffusion capacitance is somewhat greater than the Transition capacitance.
Storage Time of PN JunctionIt is the time taken by the electrons to move from n-type region to p-type region and p-type region to n-type region by applying simultaneous forward and reverse bias voltage during switching.
Transition Time of p-n JunctionIt is the time taken by the current to decrease to reverse leakage current. This transition time can be determined by geometry of P-N junction and concentration of the doping level.
Reverse Recovery Time of p-n JunctionIt is sum of the storage time and transition time. It is the time for diode to raise applied current to get 10% of the constant state value from the reverse leakage current.
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