Volt-Ampere Characteristics of Diode

       The diode current depends on the voltage applied to it. The relationship between applied voltage V and the diode current I is exponential and is mathematically given by the equation called diode current equation. It is expressed as,

       where     I₀  = Reverse saturation current in amperes

                     V = Applied voltage

                      η  = 1 for germanium diode

                          = 2 for silicon diode

                     V_T  = Voltage equivalent of temperature in volts

       The voltage equivalent of temperature indicates dependence of diode current on temperature. The voltage equivalent of temperature V_T for a given diode at temperature T is calculated as,

                              V_T  = KT volts

       where              K = Boltzmann's constant = 8.62 x 10^-5 eV/^oK

                              T = Temperature in ^oK

       Thus at room temperature of 27 ^oC i.e. T = 27 + 273 = 300 ^oK the value of V_T is,

                               V_T  = 8.62 x 10^-5 x 300 = 0.02586 V 26 mV

       The value of V_T also can be expressed as,

        Thus at room temperature of T = 300 ^oK, we get = 26 mV.

       The diode current equation is applicable for all the conditions of diode i.e. unbiased, forward biased and reverse biased.

       when unbiased,     V = 0 hence we get,

                                    I = I₀{e⁰ - 1} = 0 A

       Thus there is no current through diode when unbiased.

       For forward biased, V must be taken positive and we get current I positive which is forward current. For reverse biased, V must be taken negative and we get negative current I which indicates that it is reverse current. 

1.1 Nature of V-I Characteristics from Equation of Diode

        Consider a current equation of diode as,

       The above equation indicates that under reverse biased condition, the current is reverse saturation current which is negative indicating that it flows in opposite direction to that of forward current and almost constant. Such nature of diode characteristics is already been discussed and it is shown in Fig.1. The dashed portion represents breakdown region.

Fig. 1  V-I characteristics of p-n junction diode

Note : The entire V-I characteristics of a p-n junction diode depends on the temperature, as reverse current and V_T  are temperature dependent.
See solved examples 

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