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
VT = 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,
VT = 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,
VT = 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 VT are temperature dependent.
