Examine the assertion — it is false. Rectification requires a device that conducts heavily in one direction and blocks in the other. A tunnel diode does the opposite of blocking in reverse: because both sides are degenerately doped, the depletion layer is only about 10 nm wide and electrons tunnel straight through it under reverse bias, so reverse current rises immediately and steeply. The device conducts well in both directions and therefore cannot rectify.

What it is actually used for. The forward characteristic rises to a peak current I_P at about 50 mV, then falls to a valley current I_V before rising again as normal injection takes over. Between peak and valley the slope is negative:

\(r_d=\dfrac{dV}{dI} \lt 0\)

That negative differential resistance can cancel the positive loss resistance of a tuned circuit, so the tunnel diode is used as a high-frequency oscillator, amplifier and fast switch — the tunnelling process is not limited by carrier transit time, so it works up to millimetre wavelengths.

Examine the reason — it is true. A tunnel diode is a p-n junction in which both regions are doped to about 10¹⁹–10²⁰ cm⁻³, so heavily that the Fermi level lies inside the conduction band on the n-side and inside the valence band on the p-side. Such material is called degenerate, and the resulting very narrow, very steep barrier is exactly what makes tunnelling possible.

Assemble. (A) is false and (R) is true, which is code 4.

The irony worth noting. The reason is not merely true independently — it is the very property that makes the assertion false. Degenerate doping produces the narrow junction, the narrow junction permits reverse tunnelling, and reverse tunnelling destroys any rectifying action.

Figure of merit. A tunnel diode is judged by its peak-to-valley current ratio I_P/I_V, typically about 10 in germanium and higher in gallium arsenide; a larger ratio means a stronger negative-resistance region.

Hence, (A) is false, but (R) is true.