A tunnel diode and a resonant-tunneling diode both utilize the same physics principle of quantum tunneling. In what important way are they different?

Quantum tunneling is a phenomenon where particles can pass through barriers that classically they should not be able to overcome, due to their energy being lower than the energy of the barrier. This happens because, on a quantum level, particles have some probability of being found on the other side of the barrier, even if there is no classical path for them to take. Both tunnel diodes and resonant-tunneling diodes rely on this phenomenon to function.

A tunnel diode is a semiconductor diode that has a heavily doped p-n junction, which creates a very narrow depletion region. As such, it has a negative resistance region within its current-voltage characteristics, meaning that the current decreases as the voltage increases in that region. This is due to the tunneling effect, which allows charge carriers (electrons or holes) to tunnel through the narrow depletion region, effectively bypassing the energy barrier. Tunnel diodes have been widely used for high-frequency and low-noise applications like oscillators, amplifiers, and switches.

A resonant-tunneling diode (RTD) consists of a thin, double-barrier quantum well structure sandwiched between two electrodes, often using a combination of n-type and p-type semiconductors. Due to quantum confinement, discrete energy levels form within the quantum well. When the energy levels of the incoming electrons align with the energy levels in the quantum well, resonant tunneling occurs. This results in very high current peaks at specific voltages, known as resonance voltages. The RTD has unique characteristics such as high speed, low power, and negative differential resistance, making it suitable for various high-speed and low-energy electronics applications.

The key differences between a tunnel diode and an RTD can be summarized as follows: 1. Structure: A tunnel diode is based on a heavily doped p-n junction, while the RTD has a double-barrier quantum well structure. 2. Negative resistance: Both devices exhibit negative differential resistance, but the RTDs show pronounced resonant peaks in their current-voltage characteristics in certain voltage ranges due to quantum well energy levels. 3. Applications: Tunnel diodes are used mainly for high-frequency, low-noise applications, while RTDs are used for high-speed, low-energy electronics. In conclusion, although both tunnel diodes and resonant-tunneling diodes rely on quantum tunneling, they have distinct structures and application areas due to their unique characteristics and properties.