Limitations of Ohm's Law:
1. Non-linear devices: For diodes, transistors — $V$-$I$ relationship is not linear. The $I$-$V$ graph is a curve, not a straight line.
2. Non-unique relationship: In devices like thyristors, for a given $V$, there can be more than one value of $I$.

I-V Characteristics:
- Ohmic conductor: straight line, $I \propto V$
- Non-ohmic device (diode): exponential curve $I = I_0(e^{V/V_T} - 1)$
- Thyristor type: $I$-$V$ curve folds back (negative resistance region)