What Are Tunnel Diodes?

Tunnel diodes are semiconductor p-n junctions in which both the p-type and n-type regions are doped so heavily that both sides are degenerate, meaning the Fermi level lies within the conduction band of the n-type material and within the valence band of the p-type material. This extreme doping produces a very thin depletion layer, typically one to ten nanometers wide, through which charge carriers can pass by quantum mechanical tunneling rather than by thermally overcoming the potential barrier. The result is a current-voltage characteristic containing a region of negative differential resistance, which sets tunnel diodes apart from every other two-terminal device and makes them useful for oscillators, amplifiers, and fast switches at frequencies unattainable by conventional junction diodes.

Leo Esaki demonstrated the first working tunnel diode in 1957 while at Sony in Japan, providing the first experimental confirmation of band-to-band tunneling in solid-state materials and earning the 1973 Nobel Prize in Physics. IEEE Spectrum's account of Robert Noyce's early engagement with the tunnel diode documents that Noyce had theoretically predicted tunneling in germanium junctions in 1956 but did not fabricate a working device, illustrating how the invention occupied the intersection of quantum physics and semiconductor engineering before either discipline had fully worked out its implications.

Quantum Tunneling Mechanism

In a conventional p-n diode, current flows only when carriers acquire enough thermal energy to surmount the built-in potential barrier, which produces the exponential I-V relationship described by the Shockley equation. In a tunnel diode, the depletion layer is thin enough that the quantum mechanical wave function of an electron extends across it with non-negligible amplitude, giving a finite probability of the electron appearing on the other side without absorbing any energy. This process, band-to-band tunneling, is essentially instantaneous compared to thermal carrier injection, which gives tunnel diodes their extraordinary speed: switching times of tens of picoseconds have been documented at room temperature. The probability of tunneling decreases exponentially with barrier width and height, so the heavy doping that narrows the depletion layer is the critical enabling parameter.

Current-Voltage Characteristics

The I-V curve of a tunnel diode has three distinct regions. As forward voltage increases from zero, current rises rapidly due to tunneling from the n-side conduction band directly into the p-side valence band; this is the tunneling current peak. Beyond the peak, the overlap between occupied n-side states and empty p-side states decreases as the bands shift, and current falls despite increasing voltage, producing the negative differential resistance region. At higher voltages, conventional minority carrier diffusion current dominates and the I-V characteristic recovers the normal diode shape. The ratio of peak current to valley current, typically three to ten for germanium devices and higher for III-V compound semiconductors, determines the available signal swing in switching and oscillator circuits. The ScienceDirect reference on tunnel diode characteristics describes the role of dopant concentration and material band structure in setting these parameters.

Device Construction and Materials

Early tunnel diodes were fabricated from germanium, which offered a relatively narrow bandgap that facilitated tunneling. Gallium arsenide and gallium antimonide devices followed, providing higher peak-to-valley ratios and better performance at microwave frequencies. A later development, the resonant tunneling diode (RTD), uses semiconductor heterostructure quantum wells between two potential barriers; carriers tunnel resonantly through the well when their energy matches a quasi-bound state, producing sharper and more controllable negative resistance characteristics. RTDs fabricated in InGaAs/AlAs and InAs/AlSb material systems have demonstrated operation as high-frequency oscillators and as high-power tunnel diodes at frequencies extending into the terahertz range.

Applications

Tunnel diodes have applications in a wide range of disciplines, including:

• Microwave and millimeter-wave oscillators requiring low phase noise
• High-speed relaxation oscillators and pulse generators
• Bistable memory cells and threshold detectors in cryogenic electronics
• Resonant tunneling diode terahertz sources and detectors
• Negative resistance amplifiers for satellite and radar receiver front ends