Field-effect threshold

For example, the localized electronic states within the HOMO-LUMO gap impair the performance of the field-effect transistors by increasing the field-effect threshold voltage and reducing the effective mobility of charge carriers [100]. The surface density of electronic defects in high-quality single-crystal OFETs can be less than 10 ° cm ° [38], which corresponds to interdefect distances of 0.1 jm. 

Photoconductivity and SCLC experiments have been used to obtain information on localized states in the bulk of organic crystals. However, information about the surface states that are relevant to the OFET operation is stiU very limited. Fortunately, the calculation of several basic OFET parameters does not require a detailed knowledge of the trap spectrum. For example, the field-effect threshold voltage is determined by the total density of deep traps... 

Nanocrystals are receiving significant attention for nano-electronics application for the development of future nonvolatile, high density and low power memory devices [1-3]. In nanocrystal complementary metal oxide semiconductor (CMOS) memories, an isolated semiconductor island of nanometer size is coupled to the channel of a MOS field effect transistor (MOSFET) so that the charge trapped in the island modulates the threshold voltage of the transistor (Fig. 1). 

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]... 

TFT field-effect mobility compensation OLED threshold voltage compensation... 

Figure 9.7a shows an example of a-Si H TFT transfer characteristics in linear regime with different W/L ratios and an example of a-Si H TFT characteristics in saturation regime with W/L = 170 pm/6 kpm. A threshold voltage (Vth) of 10 = 11 V, a field-effect mobility (p) of 0.2 0.3 cm2/(V s), a subthreshold swing slope of 0.8 dec/V, and a current ON/OFF ratio of larger than 106 for VGS from —10 to 30 V were obtained from these curves for a-Si H TFTs. These devices were used in 4-a-Si H TFTs AM-PLEDs. The electrical properties of a-Si H TFTs used in 3-a-Si H TFTs AM-PLEDs are described in Ref. [18]. 

The transistor performance is generally characterized by field-effect transistor (FET) mobility (j i), current on/off ratio (f n/f0ff), and threshold voltage (Vj)-... 

Digital lithographically processed TFT arrays having 128 x 128 pixel with 75 dpi resolution were fabricated directly on the PEN. Figure 11.15 shows a photograph of the completed flexible array. The I-V transfer and output curves for a typical TFT pixel (W/L x 1) within the array are shown in Fig. 11.16. The device has on/ofF ratios of >108, sub-threshold slopes of 0.5 V decade-1, and field-effect mobility of... 

Figure 12.9 shows the output and transfer characteristics of a state-of-the-art, polymer FET fabricated using the Plastic Logic direct-write manufacturing process (L = 10 pm). No encapsulation of the TFT is present other than what is naturally provided by the presence of the PET substrate on the bottom and an inkjet printed silver gate electrode on the top. The device exhibits a field-effect mobility of 0.04 cm2 V 1 s 1 and ON-OFF current ratio of 5 x 10s. The threshold potential is Vt = 5-6 V. These basic TFT performance values can be achieved consistently in a manufacturing environment, and are sufficient to drive a 100 dpi electronic paper display with A5 size. 

A 0.5-pm film of parylene-C was deposited on pentacene TFTs at room temperature. TFT characteristics were measured and compared before and after the deposition without patterning of the parylene. The bottom-contact TFT (W/L = 400/80 pm) characteristics before and after parylene passivation are compared in Fig. 15.9. Field-effect mobility decreased by 50% from 1.6 cm2 V-1 s-1 to 0.80 cm2 V 1 s 1 whereas the threshold voltage stays the same. 

A uniformity test array consisting of 105 TFTs (W/L = 200/20 pm) was included on the edge of the backplane design the most important properties, field-effect mobilities and threshold voltages, are plotted in Fig. 15.13. The statistics on threshold voltage show the mean value was 13.7 V, the standard deviation 0.78 V, and... 

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-... 

Rashid, and G. Schitter, Threshold voltage shift in organic field effect transistors by dipole monolayers on the gate insulator , Journal of Applied Physics 96, 6431 (2004). 

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