Enabling thin-film transistor technologies and the device metrics that matter

The field-effect transistor kickstarted the digital revolution that propelled our society into the information age. One member of the now large family of field-effect devices is the thin-film transistor (TFT), best known for its enabling role in modern flat-panel displays. TFTs can be used in all sorts of innovative applications because of the broad variety of materials they can be made from, which give them diverse electrical and mechanical characteristics. To successfully utilize TFT technologies in a variety of rapidly emerging applications, such as flexible, stretchable and transparent large-area microelectronics, there are a number of metrics that matter.

world away from their inorganic counterparts. But the same peculiar chemical nature that is responsible for their low mobilities also gives organic semiconductors extreme processing versatility, as well as unique mechanical properties, such as flexibility and stretchability. The latter qualities spawned the idea of a foldable, flexible electronics market that captured the imagination of people throughout the world. This enthusiasm for flexible, printed electronics has driven huge research efforts to improve the critically low mobilities of organic semiconductors, measured in organic TFTs (OTFTs).
Nevertheless, it is the high charge carrier mobility values for OTFTs that have been in the spotlight in recent yearsfor all the wrong reasons. In a haze of glory, excitement and misinterpretation, overestimated OTFT mobility values gradually started creeping into the literature circa 2004, until they were being published frequently at the peak of the mobility hype (2012-2017) 2 . The quest for higher mobility numbers has been driven by the desire to use OTFTs in as many applications as possible, including emerging foldable, stretchable and flexible opto/electronics. However, understanding the fundamental nature of charge transport in the myriads of organic semiconductors has, in many ways, proven to be a challengeparticularly when it comes to field-effect carrier mobility interpretation. With the mobility hype only recently going from hot potato to hot topic, all are in agreement that proper mobility analysis from considered data is critical. And despite this brief interruption, OTFT mobility values have made remarkable improvements, with reliable values going from 0.00001 to over 20 cm 2 /Vs in the past 30 years 3 . But how much does mobility matter? Has mobility been hogging the limelight for so long that is has detracted attention from other important metrics?

Channel miniaturisation
The ultimate goal at the end of the TFT research journey is their successful deployment in optical displays and integrated circuits (ICs) for a broad range of emerging applications. For the former, brightness, glare and colour blending can be improved in high definition (HD) displays by improving the so-called 'fill factor', or reducing the size of the black area of a pixel, where the black area is the non-emissive part of the pixel that contains the driving circuitry (capacitor & TFTs), the pixel interconnects and interdot spacing 4 . One approach for increasing the pixel fill factor is to reduce the size of the TFTs. This is done either by shortening the channel length (L) and/or increasing the mobility of the semiconductor, which in turn allows reduction of the channel width (W) (Fig. 1a) without reducing the current-driving capabilities of the device. Reducing L and improving µ can also be used to increase the operating frequency of ICs, since the maximum switching speed (f C ) of the individual TFTs is related to both L where V D is the applied source-drain potential 5,6 . Channel miniaturization has therefore long been a popular topic for all types of transistor technologies, and formed the basis of Moore's famous law.
In the context of channel miniaturization, the general transistor mobility ethos of the bigger the number, the better the device, starts to become untrue. In long-channel TFTs (>5 µm), charge carrier mobility is a good metric for gauging the suitability of a given material for a specific application, because L is long enough for the semiconductor to dominate charge transport across the device. However, as L reduces, transistor operation starts to deviate from its ideal behaviour and becomes dominated by potential barriers present at the injecting contacts and the associated contact resistances, R C . In situations where R C exceeds that of the gate-field dependent channel resistance, R Ch (V G ), analysis of the carrier transport using the standard transistor  c Conceptual illustration of the transfer curve shift due to continuous bias application. d Approximate plot of the circuit yield as a function of the number of concatenated unipolar NOT gates using σV T as the only variable parameter. The plot shows that the less V T fluctuates (i.e. σV T becomes smaller), the larger the number of logic gates/ TFTs that can be integrated onto the same circuit model 7 is no longer applicable, and the measured mobility is often considerably lower than in long-channel devices 6,8,9 . The impact of injection limited operation is known to be particularly pronounced in TFT families where intentional doping of the contacts is technologically challenging, with organic transistors being a representative example 2 . It turns out that for many emerging TFT technologies, parasitic contact resistance effects tend to dictate device operation, resulting in μ often being significantly lower than the semiconductor's intrinsic value. Consequently, for the majority of envisioned TFT-based applications, L and related R C are more critical metrics than µ. Indeed, this resonates with the story behind the metal-oxide-semiconductor field-effect transistor (MOSFET), for which it was initially believed that higher μ was the answer for achieving better performing ICs 8 . However, it turned out that channel miniaturization-not higher µ-gave the desired improvements in device speed and transistor integration density 10 .

Transistor parameter variation
Having said that, it will not matter how small L and associated R C are, if the operating parameters fluctuate between individual TFTs. Importantly, for any ICs, if TFT parameter variability is too great then it won't be possible to integrate thousands or even millions of devices onto one functional circuit. In digital circuits, the gate voltage applied to make the TFT channel conduct electricity (the so-called threshold voltage, V T ) is critical, with its variability between individual devices, i.e. standard deviation, σV T , being the most important metric out of all other influential parameters 10,11 . Figure 1b, c illustrates how the transfer curves for a large set of identical TFTs may deviate (shaded area) from the ideal conceptual characteristic (solid line) due to V T fluctuations. The origin of V T -variation can, in most cases, be traced to either manufacturing process (Fig. 1b) or the operational stability of the device (Fig. 1c), and in the case of flexible devices and circuits, can arise from mechanical bending. If one excludes the latter, then the degree of V T fluctuation depends heavily on the semiconductor used. For example, high mobility poly-Si TFTs suffer from manufacturing associated σV T (Fig. 1b), but they are well-known for their excellent operational stability. On the other hand, a-Si TFTs are infamous for their operational instability (Fig. 1c) and low carrier mobility, but renowned for their high parameter uniformity (small σV T ) even when manufactured over large area substrates (Table 1).
In TFT backplanes for displays, V T variations affect the brightness of the individual pixels and cause luminance variation across the display. In ICs, a large σV T not only causes delays in circuitry 12 , but in some cases, such as unipolar ICs, the magnitude of σV T actually dictates the maximum number of TFTs that can be integrated onto a circuit. Therefore, the more complex an IC becomes, the more important σV T gets 13 . Figure 1d illustrates this by showing the impact of σV T as a single parameter on circuit yield, as a function of the number of concatenated unipolar logic NOT gates. To overcome this rather serious integration bottleneck, pseudo-complementary or complementary circuitry are often employed, but unfortunately at the expense of manufacturing complexity and cost 14 . Similarly, in displays, V T shift in a-Si TFT-based pixels is managed with additional transistors in the pixel circuit, although this adversely affects the fillfactor 4,10,15 . On the whole, considering multiple TFT parameters together-not only µ-builds a picture of how suitable the various emerging TFT technologies are for the different applications.
Transistor metrics that matter It is evident that channel miniaturisation and large-scale integration are important for the future of TFTs. Achieving the highest possible carrier mobility is often considered an important goal for this particular transistor technology. However, if TFTs are put into real-life context of optical displays and other emerging electronic products, then channel length, contact resistance and threshold voltage fluctuations become equally important-if not more important-metrics than carrier mobility. For OTFTs in particular, so much focus has been placed on achieving the highest µ that it may have detracted attention from other important metrics, such as operating instability, large R C and manufacturing uniformity. Although µ and V T are reported as standard in the vast majority of the relevant TFT literature (for organic and inorganic semiconductors), high mobility values that are reported out of context run the risk of becoming abstract numbers. If emerging TFT technologies are to make it into commercial applications, then putting performance metrics into context becomes critical.