Increasing Field Effect Mobility - It’s All About Charge Scattering
Reducing charge scattering in the semiconductor channel maximizes switching speed and field effect mobility
Electron mobility relates the average velocity of electrons in a material to the local electrical field. The standard assumption is that this velocity will be parallel to the electrical field, and be directly proportional to the field. The constant of proportionality is called the "mobility."
Mobility is most generally defined as the ratio of drift velocity to gate electric field energy. Velocity is a vector quantity; it is dependent upon both distance traveled and time required to travel the distance. However, in any semiconductor, the distance a charge travels is three-dimensional as the semiconductor is a volumetric solid. This means that drift velocity is defined more by mean free path - the average of the distance each charge travels from drain to source - than by the physical space between drain and source.
If one knows the voltages, the threshold, the geometry, and capacitance of a transistor, one can calculate the mobility. The result from this calculation is called the "field effect mobility" because it was calculated based on the behavior of a field effect transistor.
Amorphyx’s Dr. Andre Zeumault presented a paper summarizing the importance of gate metal roughness for maximizing IGZO TFT field effect mobility at the 2024 Electronic Materials Conference. Click the image at right to view the presentation.
Drift velocity (v(d)) is defined by the distance traveled between drain and source. That distance is dominated not by the physical separation of the drain and source, but by the number and intensity of scattering events a charge experiences as it transitions the conduction channel.
Mimimizing scattering events is the path to maximizing field effect mobility.
Typical display industry metal oxide TFTs operating in bulk conduction mode, operating at an accumulation thickness of perhaps 10% of IGZO bulk thickness. This not only limits drain-source current, but also leaves oxygen in the IGZO bulk to trap scattered charges.
The AMeTFT operates in bulk accumulation mode, with an accumulation thickness equal to the bulk semiconductor thickness. This maximizes drain-source current while minimizing charge trapping by activating all of the IGZO bulk.
The IGZO Amorphous Metal TFT Technology maximizes amorphous metal oxide TFT field effect mobility through three mechanisms unique to AMeTFT.
An order-of-magnitude increase in gate electric field energy over traditional display industry IGZO TFTs, achieving bulk accumulation of the IGZO to maximize drain-source current.
The use of a high-k oxide gate insulator with high activation energy to minimize leakage current.
The use of an amorphous gate metal, leveraging its ultra-smooth surface in minimizing both surface and remote surface charge scattering.
The chart at right shows how an order-of-magnitude increase in gate electric field impacts carrier concentration in the conduction channel. In the 5nm of accumulated bulk (sampled at IGZO cross section A-A’), the gate electric field controls carrier concentration over 8 orders of magnitude. The remaining 35nm of bulk (B-B’) remains at a carrier concentration of about 10^13 charges/cubic centimeter.
The single-gate AMeTFT achieves accumulation throughout 100% of the IGZO bulk, expanding full dynamic control of charge concentration to all of the bulk and thus increasing drain-source current over bulk conduction mode.
But dramatically increasing the surface potential induced across the semiconductor bulk through increasing the gate electric field energy isn’t sufficient for achieving a stable high-mobility amorphous metal oxide TFT. To achieve that, we need to minimize charge scattering in semiconductor’s conduction band.
Charge scattering in an amorphous metal oxide semiconductor is dominated by two mechanisms: surface roughness and remote surface roughness. Both mechanisms create means for deviating charges from a straight-line path through the conduction channel between drain and source. Increasing gate electric field energy to increase mobility also increases the impact of both scattering mechanisms.
Surface roughness scattering defines the impact of the roughness of the gate insulator-semiconductor interface on charge transport in the “front” of the conduction channel. Peaks and valleys in the interface create physical traps for charges, removing them from conduction. Remote surface roughness defines the impact of the gate metal surface roughness on the uniformity of the gate electric field. Nonuniformities in gate electric field manifest themselves similarly to how nonuniformities in a medium’s index of refraction affect photons - they create angles of deviation from straight-line conduction. The less uniform the gate electric field - and the variations in field strength - combine to increase mean free path of charges in the channel.
As gate electric field strength is increased - through increasing gate oxide dielectric constant and/or reducing gate oxide thickness - mean free path increases…unless something is done to minimize gate metal surface roughness.
The display industry is quite familiar with the negative impact of surface roughness on field effect mobility. Amorphyx’s work incorporating ultra-smooth amorphous gate metal into metal oxide TFTs reveals remote surface roughness scattering as increasingly the dominant negative impact on field effect mobility as gate electric field strength is increased.
Multiple published papers from TFT researchers in Japan and China have shown increasing gate electric field strength without mitigating the impact of gate metal roughness results in a net zero benefit to field effect mobility. In these research efforts, increasing gate electric field energy to increase mobility was completely counterbalanced by remote surface roughness resulting from the use of a crystalline gate metal.
How does an amorphous gate metal minimize charge scattering in the semiconductor channel?
The roughness of the gate insulator-semiconductor interface is dominated by the roughness of the gate metal transferred to the insulator surface at the semiconductor interface. The chart at right shows Amorphyx data showing the surface roughness of aluminum oxide at 5 thicknesses from 5 to 75nm deposited on 3 crystalline gate metals - titanium, aluminum neodymium, and molybdenum - and amorphous titanium aluminide.
The data shows the RMS surface roughness of each metal (at insulator thickness = 0nm) along with the roughness of the 5 different thicknesses of Al2O3 film. In the case of each insulator thickness - and consistent across all 4 metals - the gate metal roughness is replicated at the insulator-semiconductor interface. Thus, gate metal roughness defines surface roughness charge scattering.
Roughness of the gate insulator-semiconductor surface interface defines the amount of charge scattering in the conduction channel due to interface surface roughness. Amorphyx’s patented use of an amorphous gate metal in thin film transistors thus minimizes surface roughness charge scattering.
The dramatic impact of remote surface charge scattering is documented in the Amorphyx data shown at left. Identically structured IGZO AMeTFTs were fabricated using two different gate insulators - aluminum oxide and silicon dioxide. Each insulator was deposited onto a crystalline (titanium) and amorphous (titanium aluminide) gate metal. The gate insulators were deposited at various thicknesses between 10 and 100nm.
First, the impact of gate oxide dielectric constant (SiO2 ≈ 3, Al2O3 ≈ 9) is evident from the two solid-line plots (crystalline gate metals). Increasing gate electric field strength through increasing gate oxide dielectric constant increases field effect mobility.
More impressively, the benefit of an amorphous metal gate - independent of gate oxide material - is the dominant factor in both increasing field effect mobility and maintaining that increase as gate electric field energy increases (gate insulator thickness decreases).
Also notice the difference in slope of the two solid lines as gate insulator thickness is reduced. This difference further emphasizes the benefit of an amorphous gate metal on achieving high field effect mobility in metal oxide TFTs.