The three currents that make up a tunnel diode characteristic. Walking along the forward I–V curve from zero bias, each region is dominated by a different mechanism:

1. Tunnelling (band-to-band) current. Because both sides are degenerately doped, the junction is only about 10 nm wide and filled states on the n-side face empty states at the same energy on the p-side. Electrons tunnel straight through the barrier, giving a current that rises to the peak I_P and then falls as the bands slide out of alignment — the negative-resistance region.

2. Excess current. At the valley the tunnelling current should fall to zero, but a measurable current remains. It flows by tunnelling via defect and impurity states inside the forbidden gap, and it is what sets the valley current I_V and hence the peak-to-valley ratio.

3. Thermal (normal diode injection) current. Beyond the valley the bands no longer overlap, ordinary minority-carrier injection takes over, and the curve climbs like a conventional diode following \(I=I_S\left(e^{qV/\eta kT}-1\right)\).

So the complete static characteristic contains all three, which is statement (d). Statement (a) is incomplete (it omits the excess current) and (c) is plainly wrong, so the option that says "(d) is correct but (a) is wrong" is the answer.

Reading the option list. Note the unusual phrasing: three of the four options are claims about the statements rather than lists of them. Option 4 ("(b) and (d) are wrong") fails because (d) is right; option 2 fails because (c) is wrong and (d) is right — the exact reverse of what it asserts.

Why the shape matters. The negative-resistance stretch between peak and valley lets the tunnel diode act as an amplifier or oscillator at microwave frequencies, and since tunnelling is not transit-time limited it works into the gigahertz range. The figure of merit is the peak-to-valley current ratio, which is precisely why the excess current — the smallest of the three components — is worth naming separately.

Hence, the correct choice is (d) is correct but (a) is wrong.