Contact charging metal-insulator

Interface states played a key role in the development of transistors. The initial experiments at Bell Laboratories were on metal/insulator/semiconductor (MIS) stmctures in which the intent was to modulate the conductance of a germanium layer by applying a voltage to the metal plate. However, only - 10% of the induced charges were effective in charging the conductance (3). It was proposed (2) that the ineffective induced charges were trapped in surface states. Subsequent experiments on surface states led to the discovery of the point-contact transistor in 1948 (4). 

It seems fortuitous that the same order of maximum charge levels has been reported for metal-metal contacts as for insulator-insulator contacts. This might indicate that back discharge could be a controlling factor in establishing final charge levels. 

Wl. Wagner, P. E., Electrostatic charge separation at metal-insulator contacts, J. Appl. Phys. 27, 1300 (1956). 

The detection principle of field-effect sensors with catalytic metal contacts is based on tbe change of the electric charge at the insulator surface caused by dissociation of the gas molecules by the catalytic material. Adsorbed gas molecules and reaction products form a polarized layer at the metal-insulator interface (Figure 2.1). This gives rise to an electric field in the insulator, which causes the concentration of mobile carriers in the semiconductor underneath the insulator to change. 

The band diagram of the metal-insulator contact is shown in Fig. 3.7(a) in thermal equilibrium and in Fig. 3.7(b) under an applied bias. The charge carrier density N0 at the boundary (i.e. at the contact) in the insulator is given by [36],... 

Since Coulomb forces decay much more slowly than van der Waals forces, an EFM experiment can be performed even without any feedback, if the sample roughness is low compared to the tip-sample distance. Terris et al. used this technique for the first time to investigate a contact electrification of a metal-insulator system [355-357]. Later, further improved experiments on contact electrification with single charge sensitivity were performed by... 

Internal photoemission of charge carriers from metal electrodes takes place when a metal, placed in contact with an insulator, absorbs light so that it promotes electrons from its Fermi level to the conduction band of the insulator. This happens only if the incident photon energy is equal to or greater than the barrier height between the insulator and the metal electrode. On the basis of Fowler s theory [9], the measured photocurrent is given by the expression... 

What may we then expect to happen in the case of a metal-insulator contact, which is a case of prime importance in the context of contact charging... 

This formula predicts that the surface charge density imparted by metal contact to the insulator surface is directly proportional to the work function of the metal. Also, the charge is not sensitive to any critical separation at which electrical contact is lost this is a consequence of the effective capacity of a surface with a... 

We have described how a molecule which has an internal electronic asymmetry should exhibit rectifying properties. However, since we are interested in current flow rather than simply charge separation within the molecule, we must characterize the molecule as part of a circuit. It is important to realize that the measurements being made are of the junction as a whole rather than just the molecular properties. That is to say, the electrical response is a convolution of the molecular properties, the external circuit and, in particular, the contacts between the molecule and the electrodes. The interactions at the interfaces are responsible for many of the effeets that are observed in metal-insulator-metal devices. 

Lowell J. Charge accumulation by rq>eated contacts of metals to insul ois. J Phys D 1984 17 1859-70. 

Contact electrification of insulative materials, predominantly in film form, has been studied in many laboratories. In this paper, electric field dependent charging of polymeric and polymer-carbon black powders in contact with a metallic electrode has been studied. Results show the charging behavior to be strongly dependent on the composition of the powder surface. Carbon black loading, type of carbon black and degree of dispersion are methods used to alter the powder surface. The field dependent contribution to the charge exchange dominates over the zero field values. 

In the usual space-charge limited theory, electrons are injected into the insulator conduction band, and some of these electrons are immobilized in localized defect states. We have considered an alternate mechanism more appropriate to the polymer structure. Contact charge transfer studies in Polyethylene Terephthalate (PET) and other polymers (15-16) suggest that the electronic states accessible from metal contacts are localized molecular-ion states located deep in the forbidden energy gap. Charge transport is by hopping between localized states. 

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. 

Fig. 5. Metal-insulated semiconductor field effect transistor with n-channel (n-MISFET). (A) the state with uncharged G-I-Su capacitor, (B) open transistor with charged G-I-Su capacitor. Abbreviations S, source electrode D, drain electrode G, gate electrode Su, substrate Al, aluminium contact I, insulator (dielectric) n-Si, p-Si, n- or p-type of silicone...

In the presence of an applied electric field, the barrier profiles ate altered. If the applied field is high enough and the electrode makes ohmic contact with the dielectric, then the charge carriers are bjected bto the insulator by lowering the barrier of metal-insulator bterface. This effect is known as Schottky-Richardson (S-R) cmissbo. The expression for the current due to S-R emission is given by... 

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