Unipolar transistors

The first unipolar transistor was a JFET the layout of an n-channel JFET is given in Fig. 9.31. The mechanism of function is similar to the MOSFET The n-channel is narrowed by the electric field applied to the gate and the body. The operational characteristics, mutatis mutandis, resemble those of the MOSFET discussed above. 

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-... 

Unipolar Transistor Uses electrons only or holes only field effect transistor. 

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... 

W. Shockley, Unipolar Field-effect transistor, Proc. IRE 40 1365-1376 (Nov 1952). 

Figure 17.1 Sketch for the unipolar and ambipolar operation regimes of an organic field-effect transistor.

Figure 17.3 Output and transfer characteristics of unipolar field-effect transistors with neat Cso (a, c) and neat CuPc (b, d) films. The substrates were treated with 02-plasma and the films evaporated at 375 K substrate temperature. The direction of the hysteresis is indicated by arrows. (Figure adopted from Ref. [27].)...

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). 

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

The demonstrated change in OFET polarity can be utilized to integrate unipolar p- and n-type pentaeene transistors on a single substrate, without altering the organic semiconductor, or even the device cross section. 

The differenf operating regimes of an ambipolar tiansisfor are shown in Figure 13.8a. Under cerfain biasing conditions, fhe charmel current in an ambipolar transistor can be approximated by an equivalent circuit comprising of two unipolar, i.e., a p-charmel and an n-channel, transistors connected in parallel as depicted in Figure 13.8b. 

Smits et al. have derived analytical equations that describe transistor operation under ambipolar and unipolar regimes [110]. The model has been successfully employed for fhe sfudy of ambipolar organic transistors based on a narrow bandgap conjugated molecule, and more recently for the description of light-emitting organic transistors [117]. 

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