InterviewSolution
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InterviewSolution
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In a p-n junction diode, the current I can be expressed as I=I_(0)"exp"((eV)/(2k_(B)T)-1) where I_(0) is called the reverse saturation current, V is the voltage across the diode and is positive for forward bias and negative for reverse bias, and I is the current through the diode, k_(B) is the Boltzmann constant (8.6xx10^(-5)eV//K) and T is the absolute temperature. If for a given diode I_(0)=5xx10^(-12)A and T = 300 K, then (a) What will be the forward current at a forward voltage of 0.6 V ? (b) What will be the increase in the current if the voltage across the diode is increased to 0.7 V ? (c ) What is the dynamic resistance? (d) What will be the current if reverse bias voltage changes from 1 V to 2 V ? |
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Answer» Solution :Current passing through junction diode, `I=I_(0)["exp"((EV)/(k_(B)T))-1]` where `I_(0)=` Reverse saturated current `=5xx10^(-12)A` `T=300K` `k_(B)=8.6xx10^(-5)eVK^(-1)` `=8.6xx10^(-5)xx1.6xx10^(-19)JK^(-1)` `=13.76xx10^(-24)JK^(-1), 1 eV=1.6xx10^(-19)JK^(-1)` (a) When V = 0.6 V then `I=I_(0)["exp"((eV)/(k_(B)T))-1]""...(1)` but `(eV)/(k_(B)T)=(1.6xx10^(-19)xx0.6)/(13.76xx10^(-24)xx300)=0.23255xx10^(2)` `~~23.26` `therefore` From equation (1), `I_(1)=I_(0)["exp"(23.26)-1]A` `=5xx10^(-12)["exp"(23.26)-1]A` `=5xx10^(-12)[e^(23.26)-1]A` `=5xx10^(-12)[log(0.4343xx23.26)-1]A` `=5xx10^(-12)[1.2586xx10^(10)-1]A` Neglecting 1 compare to `1.2586xx10^(10)` `=5xx10^(-12)xx1.2586xx10^(10)A` `therefore I_(1)=0.06293 A` (b) When V = 0.7V then From equation (1), Taking `(eV)/(k_(B)T)=(1.6xx10^(-19)xx0.7)/(13.76xx10^(-24)xx300)=27.13` `I_(2)=I_(0)["exp"((eV)/(k_(B)T))-1]A` `=5xx10^(-12)[27.13log(0.4343)-1]A` `=5xx10^(-12)[6.07xx10^(11)-1]A` Neglecting 1 compare to `6.07xx10^(11)` `=5xx10^(-12)xx6.07xx10^(11)A` `therefore I_(2)=3.035A` `therefore ` Increase in current `DeltaI=I_(2)-I_(1)` `=3.0350-0.06293` `=2.97207A` `~~2.972`A (c ) `DeltaV=0.7-0.6=0.1V` and `DeltaI=2.972A` `therefore` Dynamic variable resistance, `r_(d)=(DeltaV)/(DeltaI)=(0.1)/(2.972)=0.03364Omega` `therefore r_(d)~~0.0336Omega` (d) When voltage varies from 1 V to 2V then current `I_(0)=5xx10^(-12)`A remains constant it SHOWS that in reverse bias the dynamic resistance is infinite. |
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