MIS capacitors

Metal inert gas (MIG) welding, 27 369 Metal-insulator-semiconductor (MIS) capacitor, 29 140-143 Metal-insulator-semiconductor devices, 22 191, 192... 

Field-effect transistors (Appendix C) are miniature cousins of the Kelvin probe. The most common is the insulated gate field-effect transistor. The heart of the insulated gate field-effect transistor is the Metal-Insulator-Semiconductor (MIS) capacitor. Let us form this capacitor from palladium (to be modulated by hydrogen), silicon dioxide (insulator), and p-type silicon (semiconductor), and examine the energy levels in this structure (Fig. 6.32). 

This condition is naturally also satisfied in the Kelvin probe measurement, where the selective layer is interfaced to the air-gap which is an insulator. However, now we see a subtle but important difference between the Kelvin probe and a MIS-type measurement in the Kelvin probe, it is the bulk and the surface of the selective layer that contributes the WF modulation and the Ceii In the MIS capacitor (such as in the IGFET), it is the bulk and interface between the solid insulator (e.g., silicon dioxide) and the selective layer that contributes to the overall signal. 

Hysteresis effects usually occur in organic field-effect transistors (OFET) or MIS capacitors [1-4], A detailed literature survey will be given in Section 16.2. In spite of the large number of publications, systematic investigations of the hysteresis effects are rare. Examples are discussed e.g. in Ref [5]. 

In Section 16.4 we come back to hysteresis effects by trap recharging. We have shown in Ref [10] that this mechanism can lead to hysteresis in quasistatic CF-curves of organic MIS capacitors. However, the form of the curves was qualitatively different from the observed ones. This analysis is extended... 

In the following we discuss measured hysteresis effects occurring in OFETs and in the corresponding MIS capacitors. For this purpose measurements are presented which are representative of the different devices prepared by us. 

Sinee the hysteresis is predominantly determined only by the gate-voltage sweep, i.e. aeeumulation and depletion at the interface to the gate oxide, it should be possible to study the effeet mainly in the corresponding MIS capacitors. Results of such investigations will be discussed in the next section. 

Figure 16.3 Measured quasi static CV curves of a MIS capacitor for different V g sweep directions at given ramp rate and different temperatures (a) and at room temperature for different ramp rates (b). The organic semiconductor is a 54 nm thick arylamino-PPV layer, the silicon dioxide insulator is 40 nm. Data from [2],...

Figure 16.4 Dynamic capacitance-voltage curves of an organic MIS capacitor measured at 1 Hz for different gate-bulk voltage (V(3b) sweep directions at different temperatures. The organic semiconductor is a 48 nm thick poly(3-octylthiophene)...

Figure 16.5 Dynamic CV curves of organic MIS capacitors measured at 10 Hz for different gate-bulk voltage (Fob) sweep directions at room temperature. The organic semiconductors are P30T purified P3HT, and pentacene prepared by a precursor route.

Figure 16.6 Simulated CV characteristics of a MIS capacitor with an exponential distrihution of donor-like traps with different maximum concentration. Data taken from Ref. [10].

Figure 16.7 Simulated CV curves of a MIS capacitor with an exponential distribution of donor-like traps for different combinations of maximum DOS and ramp rates. Ef, = 0,...

Numerical simulations on the trap recharging mechanism in MIS capacitors and transistors indicate that energetically distributed traps in particular can lead to hysteresis in these devices. On the other hand, the form of the simulated hysteresis deviates from the observed one and extreme parameter values are needed either for the total trap concentration or for the product from capture cross section and thermal velocity. We conclude that it is more likely that trap recharging can modify a hysteresis caused by another mechanism than being the main origin of the hysteresis. 

Figure 21.15 Comparison of ferroelectric MIS capacitors (MFIS) and ferroelectric OFET, as flatband shift and shift of threshold voltage, respectively, versus voltage amplitude (F2PVDF Upvdf) = AFiij. For example,...

The ferroelectric hysteresis of P(VDF-TrFE) is directly investigated by MIS capacitors and OFETs. By using MIS capacitors, a systematic shift of flatband voltage is observed, after applieation of different voltage scan windows. The MIS structures are built up as Al/P(VDF-TrFE)/Si02/Si sandwieh strueture. The dependence of the remanent polarisation on thickness of the eopolymer shows an elevated polarisation voltage for a copolymer film thiekness below 100 nm, obviously due to the above mentioned interface reaction between the eopolymer and aluminium. 

There are two groups of semiconductors oxide and non-oxide (typically, silicon). Non-oxide semiconductors cannot work as a receptor because they are coated with a protective insulation layer, but they can provide a transducer in the form of MIS FETs and MIS capacitors. In contrast, oxide semiconductors can work as both a receptor and a transducer (mostly in the form of a resistor) owing to their chemical and physical stability in hostile environments at elevated temperatures. 

MIS capacitor, (a) Structure of MIS capacitor, (b) capacitance vs applied voltage characteristic obtained. 

The MIS capacitor represents the heart of most field effect sensor devices, and the physics of MIS capacitors is of importance and is treated in semiconductor physics and other sensor books (Sze, 1981 Lundstrom, 1995 Dimitrijev, 2000). Here, we will only give the basic physical principles regarding the metal insulator semiconductor field effect transistor (MISFET), since this is the ultimate transducer for commercial sensor devices. 

In field-effect transistors (FET) a potential is applied via metal contacts between two -type semiconductor areas - called source and drain - in a bulk of otherwise /7-type semiconductor material. A metal layer - called the gate - in contact with a thin insulating layer placed on top of the semiconductor (between source and drain) forms a metal/insulator/semiconductor (MIS) capacitor. If the gate is charged, the semiconductor region below the insulator is influenced by the electric field. The electric field thus affects the current flowing between source and drain. In ion-sensitive FETs (iSFETs) the metal layer on top of the insulating layer is replaced by an ion-sensitive material. This ion-sensitive layer is in contact with the analyte solution, and a reference electrode is placed close to it. 

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