Sunday, May 20, 2012

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     I= Reverse saturation current in amperes
                     V = Applied voltage
                      η  = 1 for germanium diode
                          = 2 for silicon diode
                     V= Voltage equivalent of temperature in volts
       The voltage equivalent of temperature indicates dependence of diode current on temperature. The voltage equivalent of temperature VT for a given diode at temperature T is calculated as,
                              V= 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 VT is,
                               V= 8.62 x 10-5 x 300 = 0.02586 V 26 mV
       The value of VT 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 Vare temperature dependent.
See solved examples 

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