Depletion mode device

The usual TFT structure is shown in Fig. 10.7 and comprises the a-Si H channel, a gate dielectric, and source, drain, and gate contacts. N-channel accumulation mode operation using an undoped a-Si H channel is the only structure widely used. Depletion mode devices are prevented by the high defect density of doped material, which makes it difficult to deplete the channel. The much lower mobility of holes compared to electrons gives p channel devices a lower current by about a factor 100, which is undesirable. 

Fig. 5.10. A schematic cross-section of a reduced mask process, which patterns the source, drain, and gate all in one step. The source and drain are exposed during the patterning of the gate dielectric. This process produces a region of ungated semiconductor, and is therefore only applicable to processes which produce depletion mode devices.

FETs can operate in a number of modes. Enhancement mode devices, such as in Figure 3.25, are normally off There is no channel under the gate and current does not flow unless a bias voltage is applied to turn conduction on. Depletion-mode devices are normally on, having a pre-existing channel. In these devices a voltage... 

The first SiC MOSFETs were fabricated by Suzuki et al [1] in 3C-SiC. At the present time, SiC MOSFET research is carried out by several groups, including Cree Research [2-4] and Westinghouse [5]. Enhancement mode and depletion mode (3-SiC MOSFETs have been fabricated by Palmour et al [3]. Enhancement mode devices with 5 pm gate lengths had a maximum transconductance (gm) of 0.46 mS mm 1 at room temperature (see FIGURES 1 and 2). The devices were operational up to 823 K. 

The depletion mode 6H-SiC MOSFETs fabricated by the General Electric group [8] exhibited a fairly low leakage current of 5 x 10 " A at 23 °C at -10 V gate bias and drain bias of 8 V with the on-current as high as 1.6 mA. However, the subthreshold was quite low. These devices operated up to 350 °C. 

Since extrinsic silicon photoconductor material has high resistivity at cryogenic temperatures it can be used to form the substrate of an accumulation mode CCD as shown in Fig. 6.11. With an accumulation mode MIS structure the gates are biased so that majority carriers are stored and transferred down the insulator semiconductor interface. Local potential wells are formed under the gates however the dynamics of the charge transfer process will be very different from those for an inversion mode device since with an accumulation mode device the transverse electric fields will extend all the way to the back contact instead of being confined to the depletion region of an inversion mode structure. 

Analogous to these field-effect devices is the buried-channel, depletion-mode, or normally on MOSFET, which contains a surface layer of the same doping type as the source and drain (opposite type to the semiconductor body of the device). As a result, it has a built-in or normally on channel from source to drain with a conductance that is reduced when the gate depletes the majority carriers. 

Fig. 18.2 Cross section of a JFET, with the symbol shown at the right, device is N-channel, is normally on, and works in depletion mode.

Figure 21 shows the transistor characteristics of a typical printed device fabricated with regioregular PHT [89]. All the transistors are p-channel devices and can operate both in accumulation-mode and depletion-mode. The field-effect mobility was found to be between 0.01 and 0.03cm s . This is one of the... 

If the half-width a of the MESFET channel is designed to be larger than the zero bias depletion width of the MESFET gate structure, the MESFET behaves as a depletion-mode (or normally on) device structure. When an increasing positive bias is applied to the drain (Db) of the Baliga pair with gate (Gb) shorted to the source (Sb), the voltage is initially supported by... 

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