Semiconductor field-effect mobility

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

Figure 12.3. Benchmark of peer-reviewed academic reports of organic semiconductor device field-effect mobility versus time of report. All data points are for spin-coated organic semiconducting transistors. Solid points are derived from the benchmark study completed in 2002 by Brazis and Dyrc at Motorola (unpublished). The curve is a calculated estimation, based on these data, of what the expected mobility values will be in the future. The open points are data derived in 2005 from the public journals for verification of the 2002 prediction.6 38... 

MOS metal oxide sensor, MOSFET metal oxide semiconductor field-effect transistor, IR infrared, CP conducting polymer, QMS quartz crystal microbalance, IMS ion mobility spectrometry, BAW bulk acoustic wave, MS mass spectrometry, SAW siuface acoustic wave, REMPI-TOFMS resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry... 

There is a very short list of organic semiconductors with reported thin-film field-effect mobilities greater than 1 cm2 V-1 s h These include pentacene, sexithio-phene [5a], and anthradithiophene [17]. If we extend this list to include single crystal and n-type materials, we can add perylene [18], rubrene [19], copper phtha-locyanine (CuPc) [20], tetracyanoquinodimethane (TCNQ) [21], and dithiophene-tetrathiofulvalene (DT-TTF) [22] - still a short list. 

An alternative route to solution-processible organic semiconductors is to use precursors to small molecule semiconductors, such as pentacene [59] or tetrabenzo-porphyrin [60], which can be converted into their fully conjugated, insoluble form by thermal or irradiative [61] treatment on the substrate. Pentacene precursors have been shown to yield field-effect mobilities of 0.01-0.1 cm2 V-1 s 1 [62], and 0.1-0.8 cm2 V-1 s 1 [63] after thermal conversion at 150-200 °C. Small molecule organic semiconductors can also be rendered solution processible by attachment of flexible side chains [64-66], Due to the relatively low solubility of these molecules the growth of uniform thin films of these molecules remains challenging, however. 

Using naphthalenetetracarboxylic dianhydride as the semiconductor, changes in bulk conductivity, field-effect mobility, and threshold voltage were separately observed in response to exposure to water and oxygen [35, 36], Another more elaborate kind of pattern was produced by a virtual array of eleven different semiconductor OFET monitoring on-current in response to polar and nonpolar organic vapors [37]. Responses (0.8-0.3-fold reductions and 1.5-2-fold increases) were dis-... 

FET mobility measurements thus constitute a sound method for investigating changes in the mobility of an organic semiconductor due to morphology variations. On the basis of the FET characteristics of MDMO-PPV films spin-cast from different solvents, we will discuss the influence of interchain polymer aggregates on the hole field-effect mobility and further consequences for the short-circuiting of solar cells. 

Aromatic hydrocarbon pentacene is important for application in OTFTs as it has superior field effect mobility, good semiconducting behavior, and stability [153], The chemical structure is shown in Fig. 6.10. Pentacene semiconductor films can be fabricated by sublimation in a vacuum deposition system. Optimization of the fabrication parameters, such as the substrate temperature and deposition rate, can yield a highly ordered pentacene film with improved device performance. Oriented films have optical and electrical anisotropies. 

Fig. 21. Diagram of the experimental tube used for measurement of the surface conductance and field effect mobility. From P. Handler, Semiconductor Surface Physics Ed. R. H. Kingston, (1957)...

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