What Are Resonant Tunneling Devices?

Resonant tunneling devices are semiconductor structures that exploit quantum mechanical tunneling through a double-barrier quantum well to produce a current-voltage characteristic with a region of negative differential resistance (NDR). In this region, increasing the applied voltage causes current to decrease, a property that enables compact oscillator, switch, and logic circuit designs not achievable with conventional transistor-based approaches. The core structure, a double-barrier quantum well (DBQW), consists of a thin low-bandgap semiconductor layer sandwiched between two thinner higher-bandgap barrier layers, all grown by molecular beam epitaxy or metal-organic chemical vapor deposition to atomic-scale precision.

The physics underlying resonant tunneling was described theoretically by Tsu and Esaki in 1973 and demonstrated experimentally in III-V semiconductor systems by Chang, Esaki, and Tsu shortly thereafter. The electron transmission probability through the double-barrier structure peaks sharply when the incident electron energy aligns with a quantized energy level inside the quantum well, a condition called resonance. This sharp energy selectivity is what distinguishes resonant tunneling from ordinary tunnel diode operation and gives the device its distinctive NDR characteristic.

Quantum Well Structure and Negative Differential Resistance

A typical DBQW resonant tunneling diode (RTD) uses InGaAs as the well material and AlAs or InAlAs as the barrier material in an InP-based heterostructure, chosen because the large conduction-band offset sharpens the resonance and raises the peak-to-valley current ratio (PVCR). The PVCR is the primary figure of merit: it compares the peak current at resonance to the valley current measured at a higher bias where the resonance condition no longer holds. InAlAs-InGaAs double quantum well devices have achieved PVCRs exceeding 144 at room temperature, as reported in early IEEE studies of resonant interband tunneling diodes. Higher PVCR improves the noise margins of logic circuits and the output power of oscillators built around the NDR region.

Device Characteristics and High-Frequency Performance

Resonant tunneling is an intrinsically fast process. Because tunneling is quantum mechanical rather than drift-diffusion in nature, the transit time through the DBQW structure is very short, on the order of picoseconds or less. This gives RTDs the ability to operate at frequencies extending into the terahertz range, where most conventional transistors cannot function. Oscillators built around RTD NDR have produced signals above 1 THz, a regime of considerable interest for imaging, spectroscopy, and communications. The switching speed also enables multi-valued logic designs, since a single RTD can exhibit multiple NDR peaks when placed in series with a second barrier, as demonstrated in IEEE research on digital circuit applications of resonant tunneling devices.

Circuit Integration and Fabrication Considerations

Integrating RTDs with high-electron-mobility transistors (HEMTs) or heterojunction bipolar transistors (HBTs) on the same wafer allows designers to combine the NDR nonlinearity of the RTD with the current gain of the transistor, producing circuits with lower power consumption and greater functionality per chip area than either device type achieves alone. Fabrication requires precise control of layer thicknesses: barrier widths in the range of 1.5 to 5 nm and well widths between 5 and 10 nm are typical. Variations of a single monolayer alter the resonance energy and the PVCR substantially. Modern epitaxial growth tools hold these tolerances reliably, making wafer-scale RTD production feasible. Broader coverage of tunneling device physics and integration approaches appears in the Tunneling Devices chapter of Springer's semiconductor device series.

Applications

Resonant tunneling devices have applications in a wide range of disciplines, including:

• Terahertz oscillators and signal sources for imaging and spectroscopy
• Multi-valued logic and memory circuits in low-power nanoelectronics
• High-speed analog-to-digital converters exploiting NDR nonlinearity
• Frequency multipliers and mixers in millimeter-wave communication systems
• Compact local oscillators for radar and wireless sensing