What Are Tunnel Diode Switching Circuits?

Tunnel diode switching circuits are electronic circuits that exploit the negative differential resistance characteristic of tunnel diodes to implement high-speed switching, bistable memory elements, and relaxation oscillators. A tunnel diode, also called an Esaki diode after Leo Esaki who demonstrated the effect in 1957, is a p-n junction formed between heavily doped degenerate semiconductors. Electrons cross the junction through quantum mechanical tunneling rather than by surmounting the potential barrier thermally, producing a current-voltage characteristic with a region of decreasing current for increasing voltage. This negative resistance region is the property that makes tunnel diodes useful in circuits where conventional diodes cannot provide switching action at very high frequencies.

The attraction of tunnel diodes for switching applications is speed. Because tunneling is an essentially instantaneous quantum process, tunnel diodes transition between their peak and valley current states in tens of picoseconds, far faster than the minority carrier storage that limits the switching speed of bipolar junction transistors. This made tunnel diodes the fastest switching devices available from the late 1950s through much of the 1960s, before GaAs and later silicon complementary metal-oxide-semiconductor processes approached comparable speeds with the benefit of three-terminal gain.

Negative Resistance and Bistable Operation

The negative differential resistance of a tunnel diode spans the voltage range between the peak current point and the valley current point on the I-V curve. When such a device is placed in a circuit with a suitable load line, the operating point can settle at either of two stable intersections: one on the low-voltage, high-current side of the peak, or one on the high-voltage, low-current side of the valley. These two stable states constitute a bistable pair analogous to the two states of a flip-flop, enabling single-diode memory cells with switching energies far below those of transistor-based latches of the same era. IEEE Spectrum's history of the tunnel diode and Robert Noyce's early work on tunneling recounts how the negative resistance property simultaneously attracted circuit designers and puzzled physicists who were still debating the practical implications of quantum tunneling in solid-state devices.

Circuit Topologies

Tunnel diode switching circuits were implemented in several canonical configurations. The monostable circuit, analogous to a single-shot trigger, uses the negative resistance region to produce a sharp pulse when the diode is pushed past the peak current point by an input trigger. The bistable configuration, described above, latches in either of two states and requires a pulse of sufficient amplitude in the appropriate direction to switch states. The astable, or free-running, relaxation oscillator connects the tunnel diode with a series resistance and energy-storage element so that the operating point oscillates continuously between the two sides of the negative resistance region. The ScienceDirect overview of tunnel diode characteristics documents the peak-to-valley current ratio and the negative resistance parameters that determine the speed and noise margin of these configurations.

IC Design Integration

Within integrated circuit design, tunnel diode switching circuits found application in high-speed logic families during the early 1960s. Research published by IEEE, including work on high-frequency operation of tunnel diodes, established that tunnel diodes could sustain oscillation and switching action at frequencies exceeding 5 GHz, a regime accessible to no other solid-state device of the period. Resonant tunneling diodes (RTDs), a later refinement built from quantum-well heterostructures in III-V semiconductors, revived interest in tunneling device circuits for millimeter-wave and terahertz applications in the 1980s and beyond, where their current-voltage characteristics produce multi-valued logic states that reduce circuit complexity in comparator and analog-to-digital converter designs.

Applications

Tunnel diode switching circuits have applications in a wide range of disciplines, including:

• High-speed logic families in early computer mainframes
• Microwave relaxation oscillators and pulse generators
• Bistable memory cells for cryogenic computing
• Resonant tunneling diode circuits for terahertz signal detection
• High-speed comparators and analog-to-digital converter front ends