Question

Find static and dynamic resistances of a PN junction germanium diode for an applied forward bias of $0.2$ volts, if the temperature is ${300}^o$K and reverse saturation current of $1A$.

• A. $2.85\Omega ,22.5\Omega$
• B. $22.85\Omega ,2.5\Omega$
• C. $5.85\Omega ,26.5\Omega$
• D. $5.63\Omega ,55\Omega$

Answer 1

The correct option is A $2.85\Omega ,22.5\Omega$

Forward current through diode is
$I=I_s\left[exp\right(\frac{eV}{kT})-1]$
$I_s$ saturation current at room temperature for Germanium diode ${10}^{-6}A$
T=300K V=0.2 volts , k=$1.38\times {10}^{-23}$
$I={10}^{-6}\times \left[exp\right(\frac{1.6\times {10}^{-19}\times 0.2}{1.38\times {10}^{-23}\times 300})-1]$=2.27mA
Static resistance $r_s=\frac{v}{I}=88\Omega$
Dynamic resistance= $r_d=\frac{vt}{I}=\frac{kT}{eI}=\frac{26\times {10}^{-3}}{2.27\times {10}^{-3}}$=11.4 $\Omega$