Tunnel Diode

The tunnel diode is a heavily doped p-n junction device that exploits quantum mechanical tunneling to exhibit a negative resistance region in its I-V characteristic. This property makes it useful in oscillators, amplifiers, and switching circuits operating at microwave frequencies.

Core Concept Explanation

In a conventional p-n junction diode, both the p-side and n-side are lightly doped, so the depletion region is wide and carriers must gain enough energy to cross the potential barrier. In a tunnel diode, both sides are doped extremely heavily, typically above 10^19 cm^-3. This extreme doping narrows the depletion region to a few nanometers, thin enough for conduction band electrons on the n-side to tunnel quantum mechanically into empty valence band states on the p-side without requiring any thermal energy.

At zero bias, some tunneling occurs in both directions and the net current is zero. As a small forward bias is applied, the energy band alignment becomes favorable and tunneling current rises rapidly to a peak current Ip at peak voltage Vp. Beyond Vp, the band overlap decreases, so tunneling current falls even as voltage increases. This fall of current with increasing voltage defines the negative resistance region. At the valley point (Iv, Vv) tunneling becomes negligible and normal diffusion current begins to dominate, causing current to rise again like an ordinary diode.

The negative resistance behavior is not due to any destructive mechanism. It is a direct quantum mechanical consequence of band structure alignment changing with bias. This is why tunnel diodes can respond at extremely high frequencies, limited only by quantum transition times rather than carrier transit times through a drift region.

Mathematical Expression

The negative resistance of a tunnel diode is defined as the slope of the I-V curve in the negative resistance region. Mathematically it is written as:

rn = dV/dI, which yields a negative value between Vp and Vv. The magnitude |rn| typically ranges from 30 ohms to 100 ohms for germanium tunnel diodes. The peak-to-valley ratio (Ip/Iv) is a key figure of merit. A higher ratio means a more pronounced negative resistance region, which is desirable for oscillator applications. Germanium tunnel diodes achieve peak-to-valley ratios around 8:1, while gallium arsenide types can exceed 15:1.

Practical Understanding

Because tunneling is a quantum process with no time delay comparable to minority carrier storage, tunnel diodes switch in picoseconds and operate well into the microwave range. They were historically used in early microwave oscillators and low-noise amplifiers. Their main limitation is low output power since the voltage swing is confined to the small range between Vp and Vv, typically a few hundred millivolts.

In GATE context, the key examination points are the shape of the I-V curve, the location of the negative resistance region, and understanding that the negative resistance arises from quantum tunneling and not from heating or avalanche. Questions often test whether students can identify the peak and valley points and state the doping requirement.

Given:
Ge tunnel diode: Ip = 10 mA at Vp = 0.065 V, Iv = 1 mA at Vv = 0.35 V

Why this formula applies:
Negative resistance is slope of V vs I in the negative resistance region

Formula:
rn = (Vv - Vp) / (Iv - Ip)

Substitution:
rn = (0.35 - 0.065) / (1 - 10) mA
rn = 0.285 V / (-9 mA)

Calculation:
rn = 0.285 / (-0.009)

Final Answer: rn = -31.7 ohms (negative resistance magnitude approximately 31.7 ohms)

Exam Tip: Tunnel diode requires degenerate doping (both p and n sides above ~10^19 cm^-3). If a question states lightly doped junction, tunneling cannot occur. Also, negative resistance means dI/dV is negative, not that current flows backward.

• Both p and n sides are degenerately doped, placing the Fermi level inside the conduction band on n-side and inside the valence band on p-side.
• Depletion width narrows to approximately 10 nm due to heavy doping, enabling quantum mechanical tunneling.
• At small forward bias, conduction band electrons on n-side align with empty valence band states on p-side, maximizing tunnel current.
• As bias increases beyond Vp, band overlap reduces, tunnel current decreases while diffusion current has not yet taken over, creating negative resistance.
• Beyond valley voltage Vv, normal minority carrier diffusion dominates and current increases as in a standard diode.

Quick Revision

• Tunnel diode uses degenerately doped p-n junction (both sides above 10^19 cm^-3).
• Current mechanism: quantum tunneling through thin (~10 nm) depletion region.
• I-V curve has peak point (Ip, Vp) and valley point (Iv, Vv) with negative resistance between them.
• Negative resistance formula: rn = delta_V / delta_I gives a negative value.
• Peak-to-valley ratio Ip/Iv is the figure of merit; higher is better for oscillators.
• Applications: microwave oscillators, fast switches, low-noise amplifiers.
• Exam trap: negative resistance does NOT mean current flows in reverse; it means current decreases as voltage increases.