Quantum Tunneling
Integrated Circuits…without Semiconductors
Semiconductor-based transistors conduct current based on the concept of free charges being transported through the semiconductor material.
Fowler–Nordheim tunneling is the wave-mechanical tunneling of electrons through a barrier created at the surface of an electron conductor through applying a very high electric field. Individual electrons can escape by Fowler–Nordheim tunneling from many materials in various circumstances.
Cold field electron emission (CFE) is the name given to a particular statistical emission regime, in which the electrons in the emitter are initially in internal thermodynamic equilibrium, and in which most emitted electrons escape by Fowler–Nordheim tunneling from electron states close to the emitter Fermi level.
Quantum tunneling operates on the fundamental principle of quantum mechanics - in the presence of sufficient motive force, a wave packet of charge will pass through a barrier with a probability proportional to the motive force.
In thin film transistors, the motive force is an electric field formed across an insulator between two electrodes.
The Fowler-Nordheim tunneling equation was derived in 1928, resulting from early work in field electron emission going back to Thomson’s identification of the electron, Lilliefield’s work on medical x-rays, and Millikan’s work at the California Institute of Technology. A few decades later, Carver Mead continued Millikan’s work in the 1960s. In the early 2000s, post-docs at University of Colorado formed a startup company (Phiar) that collaborated with Motorola in producing a 5µm x 5µm quantum tunneling diode capable of amplitude demodulation of a 67 GHz radio signal.
The dominant issue with Fowler-Nordheim devices through over 60 years of research: mean time to failure in the tens of minutes. F-N devices require thin (tens of nanometers) insulators and very high electric field strengths (~10MV/cm) to achieve levels of charge tunneling capable of supporting electric currents in the 1-100µA range. The use of silicon dioxide (k≈3) insulators and crystalline electrode metals (RMS surface roughness = 1-3nm) combined in creating tunneling current density “hot spots” that burned the insulator, causing device failure.
Amorphyx’s groundbreaking work with the incorporation of amorphous electrode metals (RMS surface roughness <0.25nm) and aluminum oxide (k≈9) gate insulator supports insulator breakdown field strengths sufficient for achieving long-term stable Fowler-Nordheim tunneling. Maintaining a uniform current density across the tunnel junction results in a thin film device exceeding the 50,000 hr MTBF requirement for consumer electronics displays.
The single largest limitation on the performance of today’s thin film transistors is their reliance upon semiconductor materials for producing electric current. Free charges travel along grain boundaries in crystalline semiconductor materials. Increasing the switching speed of a transistor relies on manipulating the semiconductor material to either reduce the charge conduction path from drain to source or increase the number of free charges available for conduction.
But…what if semiconductor materials could be eliminated from thin film electronic devices? How would that benefit performance and manufacturability? And what physics enables such a radical idea?
AMNR implemented as row select switch in IPS LCDs fabricated in collaboration with BOE’s Beijing R&D team in 2017. The row select switch was constructed of 4 tunneling junctions per AMNR. One of the interesting features of increasing the number of tunneling junctions in an AMNR is that it decreases device capacitance.
The “no leakage current” feature of Fowler-Nordheim quantum tunneling was featured in these IPS LCDs. After turning off the power supply to the display, the AMNR IPS LCD held brightness and color accuracy to 8 bits for more than 5 minutes. This feature of Fowler-Nordheim tunneling delivers unique value to the variable image refresh rate display in pulsewidth modulation approaches to AMOLED and microLED displays..
AMNR is a 2-terminal device that can be constructed of multiple tunneling junctions to reduce capacitance while increasing current. The images above show the programmability of the I-V function with varying insulator thickness (“Device 1, 2 and 3”), the symmetry of the I-V function around 0V/0A, and how well the I-V curves comply with the linearized and non-linearized versions of the Fowler-Nordheim tunneling equation.
Key to note in the equations: no tunneling current sensitivity to temperature or illumination.
Amorphous Metal Nonlinear Resistor (AMNR)
Amorphyx AMNR (Amorphous Metal Nonlinear Resistor) technology combines the benefits of quantum tunneling - fastest switching speed, extremely low leakage current, and a simple thin film device structure - in creating a high-performance backplane pixel circuit for premium LCD TVs and gaming monitors. The AMNR is a 2-terminal diode-based device with perfect I-V symmetry about 0V/0A. Its current-handling capability and threshold voltage scale with its physical dimensions.
The AMNR replaces silicon and metal oxide switching TFTs in AMOLED and microLED pixel circuits. Combining with the IGZO AMeTFT technology, AMNR enables replacing LTPO with higher image resolution, from sub-Hz to >240Hz image refresh rates, and dramatically simplifies backplane manufacturing processes on glass and flexible substrates.
AMNR operates on the principle of Fowler-Nordheim quantum tunneling - a conduction mechanism that does not require semiconductor metals as electron carriers. Fowler-Nordheim tunneling is the mechanism on which all non-volatile “flash” memory operates. AMNR extends Fowler-Nordheim into analog integrated circuit applications.
AMNR technology delivers these key features to thin film electronics
a strong yet flexible materials stack optimized for flexible displays, fabricated at room temperature
the combination of low leakage current and very fast switching speed required for enabling 0.1Hz-240Hz variable image refresh rate
a simple structure that minimizes the importance of vertical alignment in photolithography - ideal for simplifying flexible display fabrication
a smaller physical footprint than TFTs, enabling increased display resolution and aperture ratio
AMNR Fabrication Process Flow
The process uses no multi-tone masks and supports wide registration tolerances.
4 single-tone metal masks
all-PVD processing; wet or dry etch
Lower Electrode: amorphous TiAlx
Insulator: Al2O3
Upper electrode: choice of amorphous or crystalline metals
<200°C anneals