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Soon after its anode current falls to zero, the device is not is a state to block the forward voltage due to the presence of carriers in its four layers its still in conduction mode. Thus at first it takes some time to remove excess charge from the four layers.

Thus just after its anode currents decays to zero it is unable to block the forward voltage.

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'why can't we determine barrier potential by using a voltmeter connected across the ends of a diode'?

The barrier potential is not a voltage created inside the diode. instead, it refers to the depleted zone around the juncture. Since this region is deplete from carriers (electrons or holes), it became a virtual isolator. In order to make the depleted zone conductive, you need to apply an external voltage to the diode terminals. If the voltage is in forward bias (+ to the anode and - to the cathode), you will need 0.2/0.3 V for germanium diodes and 0.6/07 V for silicon diodes. You need an external diode to keep the forward current with safe limits. If the voltage is in reverse mode (- to the anode and + to the cathode), you will need to apply much more voltage to achieve conduction, although this could permanently damage the diode. Zenner diodes, for instance, always work in reverse bias to create a stable voltage, which is used for regulation purposes.


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A variety of techniques can be used. Node-Voltage and Mesh-current (or Loop-current) methods, for example. See related link for examples. If there is a single voltage source in the model, then find the current supplied, and Resistance = Voltage/Current. {R = V/I} You could simulate/ model the circuit on a computer then apply the power profile and a current value will be calculated. Or if you have the circuit working, Place an amp meter into the circuit and measure the current. V/i = resistance.

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