In practice, no diode is an ideal diode, this mean neither it acts as a perfect conductor when forward biased nor it acts as an insulator when it is reverse biased. In other word an actual diode offers a very small resistance( not zero) when forward biased and is called a forward resistance. Whereas, it offers a very high resistance ( not infinite) when reverse biased and is called as a reverse resistance.
There are various type of diode resistance :
Forward Resistance : In forward biasing , the diode will not conduct until it reaches a minimum threshold voltage level. After the applied voltage exceeds this thresholds level, the diode start to conduct. In this case the resistance produced by the diode is called forward resistance of the diode. The forward resistance is classified into two types are under below :
- Static or DC forward resistance.
- Dynamic or AC forward resistance.
Static or DC Resistance: Static resistance of P-N junction diode when P-N junction is used in D.C circuit and applied forward voltage is D.C. this resistance is denoted by Rf and is calculated at a particular point on forward characteristic.
Curve that does not alter with time. The static resistance at the knee of the curve and below of it much greater than the resistance value of the vertical rise section of characteristic curve. Minimum is the current passing through a diode maximum is the level of DC resistance.
AC or dynamic Resistance : The dynamic resistance measured when input source is A.C signal varies continuously the resistance offered also change continuously, the ration of change in voltage to change in current gives dynamic resistance. The dynamic resistance is defined the diode resistive nature when AC source is which is depends on the DC polarization of the P-N junction diode is connected to it. In short , it is equivalent to slope of voltage- current of the PN diode.
Average AC Resistance: If the input signal is sufficient enough to produce a large swing, then the resistance related to the diode for this is called AC average resistance. It is determines by the straight line that is drawn linking the intersection of the minimum and maximum values of external applied input voltage.
Average AC Resistance:
Under the Reverse biasing condition, the opposition offered by the diode to the reverse current is known as Reverse Resistance. Ideally, the reverse resistance of a diode is considered to be infinite. However, in actual practice the reverse resistance is not infinite because diode conducts a small leakage current (due to minority carriers) when reverse biased. The value of reverse resistance is very large as compared to forward resistance. The ratio of reverse to forward resistance is 1 00 000 : 1 for silicon diodes, whereas it is 40 000 : 1 for germanium diode
Reverse static Resistance :The reverse static resistance under D.C condition, it is the ratio of applied reverse voltage to the reverse saturation current I0.
Reverse dynamic Resistance : This is the reverse under A.C condition, it is the ration of incremental in reverse applied to the corresponding change in the reverse current.
