Unipolar FETs

Another important feature of the transfer curves is the Ion/ W ratio that is defined as the ratio of the drain current in the on-state at a given gate voltage and the drain current in the off state. To obtain excellent switching properties, this ratio has to be maximized. 

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Figure 16.1 Operating regimes of a unipolar FET schematic of a typical FET and graph representation of the linear regime (a), the pinch off point (b), and the saturation regime (c). (Reprinted with permission from Ref [18]).

For integrated circuits using / -channel diamond, bipolar logics and the complementary MOS (CMOS) logic are not available and the circuits are limited to the logics of unipolar FETs, which are familiar with compound semiconductor devices. The typical unipolar FET logics are summarized in Table 3. 

In an HBT the charge carriers from an emitter layer are transported across a thin base layer and coUected by a third layer called the coUector. A small base current is present which iacludes the carriers that did not successfully cross the base layer from the emitter to the coUector. The FET is a unipolar device making use of a single charge carrier in each device, either electrons or holes. The HBT is a bipolar device, using both electrons and holes in each device. The emitter and coUector layers are doped the same polarity n- or -type), with the base being the opposite polarity (p- or n-ty- e). An HBT with a n-ty e emitter is referred to as a n—p—n device ap—n—p device has a -type emitter. The n—p—n transistors are typicaUy faster and have been the focus of more research. For the sake of simplicity, the foUowing discussion wiU focus on n—p—n transistors. 

F. Capasso, F. Beltram, S. Sen, A. Pahlevi, and A. Y Cho, Quantum Electron Devices Physics and Applications P. Solomon, D. J. Frank, S. L. Wright and F. Canora, GaAs-Gate Semiconductor-Insulator- Semiconductor FET M. H. Hasherrd and U. K. Mishra, Unipolar InP-Based Transistors... 

The surface FET was proposed by Lilienfeld42 [23] and by Heil43 [24]. The junction unipolar or field-effect transistor (JFET) was proposed by... 

Here I, represents the drain current and ju, jUp the respective electron and hole mobility. C defines the area capacitance of the insulator. The channel geometry is defined by the channel width W and length L. The ambipolar range, described by Eq. (3), is only valid as long as both electrons and holes can be injected and further transported in the active layer of the transistor. However, in most cases the injection and/or the transport in the transistor channel are suppressed for one charge carrier type. In that case, the FET operates only in the unipolar and saturation range as described by Eqs. (1) and (2). 

Figure 16.3 Typical current-voltage characteristics of a unipolar n-type FET ...

Field effect transistors In the fabrication of some potentiometric and gas-sensitive biochemical sensors an important role is played by unipolar transistors controlled by an electrical field (field effect transistors, FET) with a conducting channel isolated from the control electrode (gate) by a thin layer of insulator (MISFET-metal insulated semiconductor FET) made of Si02 (MOSFET-metal oxide semicon-... 

Field effect transistors (FETs) work on an entirely different principle than junction transistors. These devices are sometimes called unipolar devices since only one type of carrier is involved. Although they are much simpler and faster than junction transistors, their development did not begin as early because specialized methods and controls had to be invent for their manufacture. 

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