Specific contact resistivity

The input characteristics in Figure 6.18 show that a relatively small of 3.2V is required to drive a base current of about 5A. This is attributed to a relatively better specific contact resistance of the p contact ( 1 x 10"" ohm-cm ). 

The operating voltage of the conductive buffer chip shown in FIGURE 4(a) is 3.66 V at 20 mA. This voltage is well within the range of voltages obtained for devices on sapphire (3.5 - 4.0 V). From the forward bias I-V characteristics, the total resistance of the chip is 15 Cl. Most of this resistance is produced by the p-type ohmic contact. If all of the device resistance is attributed to the p ohmic contact, the specific contact resistivity (pe) for this contact would be less than 6 x 10 3 Cl cm2. 

It is usual to state " that molybdenum disulphide is a p type semiconductor, while niobium diselenide is a conductor, However Mikhailov has shown that pure molybdenum disulphide is a conductor and that only specimens having a developed film of oxidised material on the surface of the lamellae show semiconductor properties. Correspondingly a composite containing 15% was found to have a specific contact resistance of only 0.4 m.ohm.cm. compared with 0.7 m.ohm.cm ... 

As mentioned earlier, channel dimensions are also critical in determining the importance of contact effects in OFETs. Figure 2.4.5 shows the top view of a typical OFET with channel length (L) and channel width (W). It is useful for comparison purposes to report the normalized or specific contact resistance (RJ) as a raw resistance value multiplied by the width (W) of the channel. Thus, the units of R are Qcm. Physically, the reciprocal of this value is the contact conductance per unit width of contact. 

In terms of the channel sheet resistance (Rch d, units of Q/square, and Vq dependent) and the specific contact resistance (RJ, units of Q cm and also Vq dependent), this equation becomes ... 

FIGURE 2.4.9 Example of a transmission line Rj-ar vs. L) plot at a given Vq value. Extrapolation of the data to a channel length of zero yields the specific contact resistance R/ as the y-intercept. 

Type of contact Specific contact resistance rc (Q cm2) Annealing conditions Comments Ref... 

For a given process and bias the contact resistance scales with the inverse width of the transistor. This specific contact resistance is specified by some authors in ohms/micron (f2/p,m). 

Reeves, G. and Harrison, H., Obtaining the specific contact resistance from transmission line model measurements , IEEE Electron Device Lett., 1982, 3, 111-13. 

A common way to find out the nature of ohmic contact to semiconductor is to measure the value of the specific contact resistance, pc (G cm ). Zimin etal. (1995), Zimin and Komarov (1998) measured the transition resistivity of the PS surfaces prepared from both p-type and n-type sihcon wafers with Al contact. Specific contact resistance of Al and Ni to p-type PS (55 % porosity) was reported by Kanungo and co-workers (Kanungo et al. 2009a Kanungo et al. 2006). Maji et al. reported the same for Al contact to macroporous silicon (2010). P. Vinod (2005,2009,2013) studied silver ohmic contact to p-type porous silicon by quantitative measurements of the specific contact resistance. Table 2 gives the summary of the reported work on specific contact resistance/transition-specific resistivity measurements. A separate chapter in this handbook Electrical Transport in Porous Silicon reviews the various factors that influence the resistivity of the porous silicon layer itself. 

Table 2 Specific contact resistance/transition-specific resistivity of the metal contacts to porous silicon made by different methods ...

Type of PS Metallization technique Metal used Type of contact Specific contact resistance/transition-specific resistivity (Q cm ) References... 

P Evaporation Al Ohmic Specific contact resistance 1.51 x 10 Kanungo et al. 2009a... 

P Screen printing Ag Ohmic Specific contact resistance 0.0736 Vinod2005... 

P Screen printing firing at 800 °C in air anneahng at 450 °C in N2 Ag electroplating Ag Ohmic Specific contact resistance 1.01 x 10 Vinod2009... 

P Screen printing baking at 240 °C in inert ambient firing at 725 °C in air annealing at 450 °C in N2 —> Ag electroless Ag Ohmic Specific contact resistance 1.025 x lO " (before electroless Ag deposition) 3.25 X 10 (After electroless Ag deposition) Vinod2013... 

Ohmic and rectifying contacts to porous silicon are important and challenging for the commercial applications of PS-based electronic, optoelectronic, and sensor devices. The choice of contact materials and the nature of PS surface (including porosity) are the prime factors for the successful achievement in the contact formation with the desired specific contact resistance. Surface modification of porous silicon by Pd improves both ohmic and rectifying contacts along with the stability as it was verified by intermittent I-V studies. Verification of specific contact resistance at regular intervals can be an alternative method to study the junction stability. 

Vikulov VA, Strikha VI, Skryshevsky VA (2000) Electrical features of the metal-thin porous silicon-silicon structure. J Phys D Appl Phys 33 1957-1964 Vinod PN (2005) Specific contact resistance of the porous silicon and silver metal Ohmic contact structure. Semicond Sci Technol 20 966-971... 

Vinod PN (2009) Specific contact resistance and carrier tuimeling properties of the silver metal/ porous silicon/p-Si ohmic contact structure. J Alloys Compd 470 393-396 Vinod PN (2013) The fire-through processed screen-printed Ag thick film metal contacts formed on an electrochemically etched porous silicon antireflection coating of silicon solar cells. RSC Adv 3 3618-3622... 

Electrical conductivity respectively the resistivity is often used for material characterization (composite) and is given volume specific in S cm, respectively in Cl cm . As metals show unlikely higher bulk conductivity but a high contact resistance for the characterization of metallic bipolar plates the area specific contact resistance in mO cm is used (see Section 6.2.1). 

Consider a 100 cm PEMFC with anode and cathode catalyst layers each of thickness 20 pm and conductivity of 4 S/m, on either side of the Nation electrolyte of thickness 100 pm and conductivity of 10 S/m. The total cell specific contact resistance is 50 mii cm. Calculate (i) the cell total resistance and (ii) the net ohmic loss if the current density is 1 A/cm. ... 

Kim et al. [23] have investigated nonaUoyed Al and Al/Pt ohmic contacts on n-type ZnO and found that the I-Vcharacteristics of as-deposited Al and Al/Pt contacts on n-type ZnO reveal a linear behavior with a specific contact resistivity of 8.0 x 10 and 1.2 X 10 Dcm, respectively. Pt overlayer on Al contact, compared to the case without the overlayer, has resulted in a large reduction in the specific contact resistivity on n-type ZnO. This reduction has been attributed to the prevention of surface oxide layer (Al-O) by the Pt metal, which is similar to the case of GaN. A nonaUoyed In-based ohmic contact, formed on a hydrothermally grown n-type ZnO substrate by KrF excimer laser irradiation with 0.3 J cm and metal deposition... 

Ip et al. [30] could obtain a minimum specific contact resistance of 6 x 10 Q cm for Ti/Al/Pt/Au contacts on undoped (n = 10 cm ) bulk ZnO. The contacts did not show ohmic behavior in the as-deposited state and reached their minimum resistance after 250 °C annealing. This value was essentially independent of the surface cleaning procedure employed, including sequential solvent cleaning or H2 plasma... 

Figure 8.6 Experimental specific contact resistances as a function of temperature for unintentionally doped (open squares) and heavily Ca-doped (solid squares) ZnO. (After Ref. [19].)...

Some of the various ohmic contact metallization approaches on n-type ZnO are summarized in Table 8.1 together with carrier concentration, specific contact resistance, and method used to measure the specific contact resistivity. A1 [33-35],... 

Figure 8.7 Specific contact resistance versus carrier concentration ofthe as-deposited ohmic contact measured at 30°C and at 30 and 200°C after annealing at 200 C for 1 min (After Ref. [31].)...

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