Source-drain voltage drop

Experiments were also conducted to study the effect of the nature of the contact on the respective part of the voltage drop at source and drain. This point will be dealt with in the next section. 

Here we see that the resistance is independent of the electrode separation. The physical reason is that the major part of the voltage drop occurs in the immediate vicinities of the tips of the electrodes, and we may think of the resistance as the sum of a spreading resistance from the source electrode and a convergence resistance to the drain electrode. From this we conclude that 2-point probe measurements will not be very reliable or reproducible, because they will depend so sensitively on the small sample of material at the electrode tips. On a pliable material the form of indentation and contact area will be especially difficult to control with precision, and, in addition, we recognise that the material will be most distorted by the electrode pressure in just that region to which the resistance measurement is most sensitive. Therefore 2-point volume-resistivity measurements are not recommended. 

One should also invoke Fermi statistics. A typical tunnel curve is shown in Fig. 12 for SET model with D = 14 a.u., a = 1 a.u., the work function of electrodes W = 0.4 a.u., the Fermi energy Ee = 0.2 a.u., and the polarizability a = 200 a.u. (of Na atom). The potential drops near the interface of the source-drain electrodes, as it should for the ballistic regime. The tunnel curve has a single shallow well at a small bias voltage. When the latter increases, the well becomes deeper, and the dot is attracted to the inter-electrode gap center... 

The three key areas examined were fallout after packaging and electrical testing of packaged devices, where Vsd and Rdson were the key electrical parameters evaluated. Vsd is a measure of voltage drop across a P-N junction, (source to drain) or the body diode of the device. This measurement is effected by the epitaxial layer, substrate,... 

The previously described four-probe technique [43,44] allows a separate determination of the source and drain contact resistances. If contacts would behave as Schottky barriers, one would expect the voltage drop at source to be substantially higher than that at drain. This is what is indeed observed with bad contacts. However, good contacts show comparable drops at both electrodes. A possible origin of this behavior has been recently put forward [46]. The model assumes that the regions immediately adjacent to the electrodes are made of organic material of quality different from that of the rest of the conducting channel, with very low mobility. 

In contrast in the case of Cr contacts or generally for systems with Schottky barriers exceeding 0.3 eV, the voltage drops across the contacts become very significant and the source resistance is found to be larger than the drain resistance, as... 

As depicted in Figure 2.4.10(b), contact resistance at the source and drain electrodes results in a smaller than expected slope of the potential versus channel position profile. The profile is estimated by linear extrapolation between Vi and V2. Individual source and drain contact resistances are calculated by dividing the voltage drops AVs and AV by the source-drain current, respectively. By isolating the source and drain contact contributions to the total contact resistance, the gated four-probe technique provides more information than the transmission line technique, and it is possible to determine in one device (vs. several). An important caveat for the gated four-probe technique is that the extrapolated channel potential profile wiU only be valid for strict linear regime OFET operation (Vq V, ), where the channel potential profile can be expected to be linear and uniform. 

FIGURE 2.4.11 (a) Application of KFM to characterize an OITiT. (b) Hypothetical channel potential profile measured by the KFM technique voltage drops at the source and drain contacts (AVs, AFp) are measured directly. The dashed line is the ideal (no contact resistance) linear potential profile for an applied drain voltage of 15 V and a gate voltage of 75 V. 

Equation (4) corresponds to the so-called linear regime, where Vb < Vb - Vr-As Vd increases, the voltage at the drain electrode gradually decreases, up to a point where it falls to zero. This occurs at the so-called pinch off point, when Vd = Vg - Vt. Beyond pinch off, a narrow depletion zone forms next to the drain because the local potential there drops below threshold. Further increase of Vd leads to a slight extension of the depletion zone and a subsequent shift of the pinch off point towards the source. Because the potential at the pinch off point remains equal to Vg - Vr, the drain current becomes independent of the drain voltage this is the saturation regime. Here, the current is obtained by equating Vd to Vg — Vt in Eq. (4) ... 

Additionally, the respective voltage drops at the source and drain electrode can be calculated by... 

The corresponding channel, source, and drain contact resistances are obtained by dividing the respective voltage drop by the drain current. 

Vds and Vth are the drain-source voltage drop and the threshold voltage, respectively. However, for a layer thickness d > Zdep (the depletion length Zdep = v 2eo j2 [Pg.159]

Alternatively, a local non-contact potentiometry of the transistor channel allows one to measure the potential distribution V (jc) across the entire OFET [70], unveiling thereby possible contact resistances. Due to the local determination of the surface potential, the voltage drops at the source and drain electrodes are accessible. By... 

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