- Tunnel Diode Definition: A tunnel diode (also known as an Esaki diode) is defined as a type of semiconductor diode with negative resistance due to quantum mechanical tunneling.
- Heavy Doping: The tunnel diode has a heavily doped pn junction, resulting in a very narrow depletion layer.
- Negative Resistance: Tunnel diodes exhibit a unique negative resistance region in their current-voltage characteristics.
- High-Frequency Applications: Tunnel diodes are ideal for high-frequency oscillators and microwave circuits because of their rapid transition characteristics.
- Resistance to Radiation: These diodes are resistant to magnetic fields, high temperatures, and radioactivity, making them useful in military and space applications.
What is a Tunnel Diode?
A tunnel diode, also called an Esaki diode, is a heavily doped semiconductor diode that conducts by interband quantum tunnelling at low bias. Part of its forward current-voltage curve has negative differential resistance: increasing voltage causes current to decrease between the peak and valley points. Heavy degenerate doping makes the pn junction depletion region thin enough for carriers to tunnel through the energy barrier. Junction width and band alignment depend on material, doping and bias, so 10 nm is an order-of-magnitude example rather than a universal dimension.
Transistors have frequency limits set by carrier transit time, capacitance and circuit parasitics. A tunnel diode has no stored minority-carrier charge in the same sense as an ordinary junction diode, so its tunnelling response can be fast. Circuits can use its negative differential conductance to offset resonator loss and sustain oscillation. Leo Esaki first reported the narrow-junction phenomenon in germanium semiconductors in 1958.
Representative doping concentrations are about 1024 to 1025 m-3, although the required values vary by material and design. Both sides are doped into the degenerate range, which creates a very narrow depletion barrier. The forward current-voltage characteristic of a tunnel diode therefore includes a negative-slope region between its peak and valley.
The name refers to quantum tunnelling through the junction barrier. Heavy doping moves the equilibrium Fermi level into the conduction band on the n side and into the valence band on the p side, producing degenerate semiconductor regions.
Characteristics of Tunnel Diode
Under reverse bias, occupied valence-band states on the p side overlap in energy with empty conduction-band states on the n side. Electrons can then tunnel across the narrow barrier. Reverse tunnel current starts at low voltage and increases rapidly with reverse bias.
At small forward bias, occupied conduction-band states on the n side overlap in energy with empty valence-band states on the p side, so tunnelling current rises to the peak current. Further bias reduces the number of matching states and the tunnelling contribution falls toward the valley current.
The curve reaches peak current when the supply of occupied states and available empty states has its strongest overlap. As forward bias rises beyond this point, tunnelling current decreases until ordinary diffusion current becomes dominant beyond the valley. This interval is the negative differential conductance region. At higher forward voltage, the device approaches the rising characteristic of a conventional p-n junction. The diagram shows the resulting current-voltage curve.
A resonant circuit can oscillate when the magnitude of the tunnel diode’s negative differential conductance exceeds the positive loss conductance at the operating point. Frequency and output power depend on the diode, package, bias network and resonator.
Tunnel Diode Symbol
The symbol for a tunnel diode is shown below.
Tunnel Diode Applications
A tunnel diode can provide fast switching and negative differential resistance without avalanche carrier transit. These properties support specialised oscillators, amplifiers, detectors and bistable circuits. Tunnel diodes are less common than modern transistor technologies and cannot be fabricated in every process used for integrated circuits, but integration is a manufacturing and design choice rather than a fundamental prohibition.
Forward current first rises to a peak, falls through the negative differential resistance region to a valley and then rises again as ordinary junction current dominates. A circuit must bias the diode within the intended region and remain stable against parasitic resistance, capacitance and inductance.
Oscillator Circuits:
A tunnel diode can supply negative differential electrical conductivity to compensate for loss in a tuned circuit. Achievable frequency is set by the diode’s junction capacitance, package inductance and the resonator. A fixed claim such as 5 GHz or 100 GHz requires a named device and measured circuit.
Used in Microwave Circuits:
Tunnel diodes have been used in low-power microwave oscillators, amplifiers and detectors. Modern transistors and other negative-resistance devices now cover many of these functions, so component choice depends on frequency, noise, power, availability and circuit complexity.
Operation in Harsh Environments:
Some tunnel-diode designs have been used where low voltage, fast response or radiation tolerance matters. Performance in a strong magnetic fields, at high temperature or under ionising radiation is device-specific and must be supported by qualification data. The broad claim that every tunnel diode resists all three conditions is unsafe for component selection.
Tunnel Diode Oscillator
A tunnel diode can sustain oscillation when it is biased within the negative differential resistance region and coupled to a tuned circuit or cavity. The negative conductance must exceed resonator and load losses at startup, while nonlinear operation limits steady-state amplitude.
In the example, an off-centre antenna feed probe couples the diode circuit loosely to a tunable cavity. Coupling strength affects output power, loaded Q, frequency pulling and startup margin. Output power must be taken from the specified diode and cavity design; a few hundred microwatts is only a representative scale for some low-power circuits.
Changing the tuner’s physical position changes the cavity resonance and provides mechanical tuning. Electronic tuning can instead vary a reactive element or bias-dependent circuit parameter, but the usable range is limited by stability and device capacitance.
Microwave tunnel-diode oscillators often use cavities or distributed transmission lines because ordinary lumped interconnects have important parasitics at these frequencies. Historical uses include low-power local oscillators and test sources. Required output power, phase noise and tuning range should be checked against a complete circuit specification.
