Optoelectronic Semiconductor Devices Principals and Characteristics

Table 6.: Some typical metals used for optoelectronic devices. 
[4]
Group A 
Group B 
. . . . .Ti . . . . Cr . . . . . . . . . . . . Ni 
Cu . . . . . . . . . . 
. . . . . . . . . . Mo . . . . . . . . . . . . . . 
Ag . . . . . . . . . . 
. . . . . . . . . .(W) 
. . . . . . . . . . . . Pt 
Au . . . . . . . . . . 
Early transition 
metals 
Near-noble metals 
(electronegative) 
Noble metals (very 
electronegative) 
Schottky type 
(inert interface) 
Alloy type 
Table 
6.
 shows some typical metals used for the ohmic contacts. 
Metals from the Group A for an inert interface between the metal and the semiconductor and thus form 
Schottky-type electrodes. If the metal is selected from this group and the semiconductor is heavily doped 
(≥10
19
cm
-3
), an inert interface with a thin depletion layer is formed between the deposited metal and the 
semiconductor. This type of contact shows ohmic characteristics and is called the Schottky-type ohmic 
contact. 
Typical Schottky-type electrodes are Ti/Pt/Au and Cr/Au, in which Pt is the barrier metal that prevents 
the diffusion of Au toward the semiconductor under operation and during bonding of the device chip. The 
thicker Au layer is required for easy bonding of wire. 
Metals from Group B easily interact with semiconductors having band-gap energies less than 2.5 eV at 
room temperature and thus form alloy-type electrodes. When we choose the metal from this group, and 
alloying is carried out under a relatively high temperature (around 400ºC), the width of the depletion layer 
decreases enough to be comparable to the extent of the diffuse conductor-semiconductor interface and the 

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barrier decreases. Then, when bias is applied, current can easily flow over the reduced barrier by the 
thermionic emission. 
Typical alloy-type electrodes for p-type and n-type semiconductors are AuZnNi and AuGeNi, 
respectively.