A tunnel diode is a special kind of diode that doesn’t act like a normal one. Because it is doped very heavily, its junction becomes extremely thin, so electrons can tunnel through it even at low voltage. This creates a strange region called negative differential resistance, where current can drop even while voltage rises.
Tunnel Diode Basics
A tunnel diode has two terminals, like a standard diode. The two ends must be clearly identified because the device can behave differently from a standard diode over specific voltage ranges.
Terminal names
• Anode → p-type side
• Cathode → n-type side
Terminal facts
• In forward bias, conventional current flows from anode → cathode.
• Polarity still matters, and tunnel diodes can also conduct in reverse bias because of tunneling.
• On many physical packages, the cathode is marked with a band or dot.
Structure and Quantum Tunneling in a Tunnel Diode
In a standard p–n junction, the depletion region is wide enough that carriers mainly cross the barrier by thermal injection. A tunnel diode is built differently: both the p side and the n side are very heavily doped, which squeezes the depletion region down to only a few nanometers. With such a thin barrier, electrons can pass through it by quantum tunneling, so noticeable current can appear at very low forward voltage.
What heavy doping changes (cause → effect)
• Heavy doping raises carrier concentration and narrows the depletion region.
• A thinner depletion region means a thinner energy barrier in the junction.
• When the barrier is thin enough, carriers can tunnel through it instead of going over it.
• This enables low-voltage conduction and makes the junction behavior strongly dependent on geometry and material parameters.
What tunneling means in this diode
In a normal diode, a carrier needs enough energy to go over the barrier. In a tunnel diode, even when the carrier energy is below the barrier peak, it can still pass through the barrier due to quantum mechanics, provided there are occupied states on one side aligned with empty states on the other side.
Practical design implications
• Junction capacitance is usually higher because the depletion region is extremely thin.
• Reverse blocking is limited, and reverse breakdown voltage is often lower than in standard diodes.
• Performance is more sensitive to process variation and temperature, and high-frequency behavior depends strongly on junction capacitance and package/lead inductance.
Quick comparison
| Aspect | Standard Diode | Tunnel Diode |
|---|---|---|
| Doping level (typical order) | ~10¹⁶–10¹⁸ cm⁻³ | ~10¹⁹–10²⁰ cm⁻³ |
| Depletion thickness | Wider | Very narrow |
| The main way carriers cross | Mostly over the barrier | Mostly through the barrier (tunneling) |
| Reverse blocking | Often strong | Often limited |
Energy-Band View of a Tunnel Diode
Zero or Very Small Bias
At zero bias, tunneling can occur in both directions because the barrier is thin. The net current stays near zero because tunneling from p→n is balanced by tunneling from n→p.
Small Forward Bias: Rising Toward the Peak (Ip at Vp)
With a small forward bias, the energy bands shift so that filled states on one side align with empty states on the other. The number of available tunneling paths increases, so current rises rapidly.
• The current reaches peak current Ip at peak voltage Vp when alignment is strongest.
Higher Forward Bias: Drop Toward the Valley (Iv at Vv)
As forward voltage increases beyond Vp, the band alignment becomes poorer. Fewer states line up, so tunneling paths shrink. The tunneling current decreases even though voltage increases.
• This is the NDR region, where dI/dV < 0.
• The current falls to valley current Iv at valley voltage Vv.
Even Higher Forward Bias: Normal Diode Conduction Dominates
At sufficiently higher forward bias, tunneling becomes weak because states no longer align well for tunneling. Conventional forward conduction (diffusion/injection) becomes dominant, and current rises again with voltage.
Tunnel Diode I–V Curve and Key Parameters
A tunnel diode has a distinctive forward I–V curve: current rises to a peak, then drops to a valley, then rises again. The “drop while voltage rises” is the negative differential resistance (NDR) region.
How to read the curve (high-level)
• 0 → Vp: tunneling paths increase, current rises quickly.
• Vp → Vv: tunneling paths decrease, current falls (NDR).
• V > Vv: normal diode conduction dominates, current rises again.
Key points on the curve
• Vp (Peak Voltage): voltage at the maximum tunneling current point
• Ip (Peak Current): maximum forward tunneling current
• Vv (Valley Voltage): voltage at the minimum point after the drop
• Iv (Valley Current): minimum current before normal conduction rises strongly
• Ip/Iv (Peak-to-valley ratio): indicates how pronounced the NDR behavior is
Forward Operating Regions and Bias Notes
Region A: Low-Voltage Tunneling (around 0 to Vp)
• Use when you want low-voltage conduction behavior dominated by tunneling.
• Keep layout parasitics small if the signal is fast or RF.
Region B: NDR Window (Vp to Vv)
• This is the region used for oscillators and negative-resistance RF circuits.
• Bias at a stable operating point inside the NDR window, not right on the edges.
• Use a bias network that prevents runaway or unwanted jumps between operating points.
• Minimize added series resistance where you need strong NDR behavior, since series resistance reduces the effective negative resistance.
Region C: Normal Forward Conduction (above Vv)
• Treat it more like a conventional diode region (current rises with voltage).
• NDR effects are no longer dominant, so it is not the region for negative-resistance operation.
Quick bias checks (fast sanity list)
• Verify the intended bias point against the device I–V data (Ip, Vp, Iv, Vv).
• Check temperature drift: Vp/Ip/Iv shift can move the operating point.
• Check parasitics: Co and package inductance can reshape the apparent I–V at high frequency.
• Confirm stability with the surrounding network (especially in NDR operation).
Reverse Bias and Backward-Diode Mode
A tunnel diode can conduct noticeable current even in reverse bias because its depletion region is fragile. When a small reverse voltage is applied, the energy levels can line up, allowing carriers to tunnel in the reverse direction. This reverse conduction at low voltage is often called the backward diode mode.
What reverse tunneling looks like
• A small reverse voltage shifts the energy alignment so tunneling happens in the reverse direction.
• Reverse tunneling can support: Low-level RF detection. Mixing or frequency conversion (in some circuit setups)
Why is it not used as a power rectifier
• Reverse conduction can begin at low reverse voltage, so reverse blocking is limited.
• Reverse voltage handling is usually much lower than in many power diodes.
Tunnel Diode Materials and Ip/Iv
| Material | Bandgap (approx.) | Tunneling tendency |
|---|---|---|
| Ge (Germanium) | ~0.66 eV | Strong at low voltage |
| GaAs (Gallium Arsenide) | ~1.42 eV | Strong with good control |
| Si (Silicon) | ~1.12 eV | Usually weaker |
Tunnel Diode Equivalent Circuit
| Element | Symbol | Represents | Main effect |
|---|---|---|---|
| Negative resistance | −Ro | NDR slope near the bias point | Allows gain or oscillation in the right conditions |
| Junction capacitance | Co | Junction (depletion) capacitance | Limits high-frequency response and affects resonance |
| Series resistance | Rs | Internal losses | Reduces sharpness and lowers effective performance |
| Series inductance | Ls | Lead/package inductance | Shifts in resonance can affect stability |
Tunnel Diode Applications
Microwave Oscillators and RF Signal Generation
With bias in the NDR region and a resonant network, a tunnel diode can generate RF and microwave oscillations.
Reflection Amplifiers and RF Front-End Circuits
Its negative resistance can be combined with an impedance network to produce RF gain in low-power front-end circuits.
Relaxation Oscillators and Pulse Circuits
The NDR region supports fast switching between operating points, which can create pulse and timing waveforms.
Radar and Legacy Hardware
Tunnel diodes still appear in some older equipment, where the device behavior has already been proven and well documented.
Detection and Frequency Conversion
In backward-diode mode, a tunnel diode can detect low-level RF signals at low voltage and can also support frequency conversion.
Conclusion
Tunnel diodes work because heavy doping makes the junction so thin that quantum tunneling becomes a major path for current. This leads to the well-known peak-and-valley I–V curve and the negative differential resistance region. Those features make tunnel diodes useful for RF and microwave oscillators, small-signal detection, and fast pulse circuits. They also have limits, like low voltage and power handling and weak reverse blocking.
Frequently Asked Questions [FAQ]
What controls the Ip/Iv (peak-to-valley) ratio?
Doping level, junction quality (defects), material bandgap, and temperature.
How does temperature change tunnel diode behavior?
It shifts Vp, Ip, and Iv and weakens the NDR region (often lowering Ip/Iv), which can move the operating point and reduce stability.
What limits a tunnel diode’s highest practical frequency?
Junction capacitance (Co), series resistance (Rs), and package/lead inductance (Ls).
Can a tunnel diode be damaged by improper biasing?
Yes. Excess forward current or reverse voltage can overheat or permanently damage the junction and alter the I–V characteristics.
Why aren’t tunnel diodes common in modern designs?
High-frequency transistors and RF ICs provide better control, higher gain, improved scalability, and greater power handling.
How is a tunnel diode different from a backward diode?
A backward diode is optimized for strong reverse-bias tunneling (often for zero-bias detection), while a tunnel diode is used for forward NDR operation.
