Forward Resistance of a diode:
The resistance offered by p-n junction diode in forward biased condition is called forward resistance.

Static forward resistance : this is forward biased resistance of p-n junction diode when p-n junction is used in d.c. circuit and the applied forward voltage is d.c. this resistance is denoted as Rf and is calculated at a particular point on a the forward characteristic.
Dynamic  forward resistance : the resistance offered by the p-n junction under a.c. condition is called dynamic resistance denoted as rf
the dynamic resistance is reciprocal of the slop of the forward biased characteristics.

Reverse resistance of a diode :
P-N junction offer large resistance in the reverse biased condition called reverse resistance

Reverse static resistance:  this is reverse resistance under d.c. condition. it is the ratio of applied reverse voltage to the reverse saturation current Io.
Reverse dynamic resistance: this is the reverse resistance under a.c. condition

it is the ratio of incremental in reverse voltage applied to the corresponding change in the reverse current
Diode current Equation and Nature of characteristics:
The relationship between applied voltage V and the diode current I is exponential and is mathematically given by the equation called diode current equation. 
the voltage equivalent of temperature indicates dependence of diode current temperature .

the voltage equivalent of temperature for given diode at temperature is calculated 
where k = boltzmann's constant 
          T = Temperature in K

The diode current equation is applicable for all condition of diode unbiased , forward biased and reverse biased.

Nature of V-I Characteristics from equation of diode:
The current equation of diode is
For forward biased condition, the bias voltage V consider positive and hence exponential index has positive sign.
this indicates that once bias voltage exceeds cut in voltage , the forward current increase exponentially.

In reverse biased condition, the bias voltage is treated negative and and due to this exponential index has negative sign.
the above equation indicates that under reverse biased condition , the current is reverse saturation current which is negative indicating that it flow in opposite direction to that of forward current and almost constant.


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