Introduction: In lieu of p-n junction, use of Schottky-barrier contacts for source and drain of a MOSFET can give some benefits in assembly and performance.
Schottky-Barrier Source/Drain: Figure depicts a schematic MOSFET structure with such Schottky source and drain. For Schottky contact, junction depth can be effectively made zero to abate the short-channel effects.
n-p-n bipolar-transistor action is also absent for undesirable effects such as bipolar breakdown and latch-up phenomenon in CMOS circuits. Removing high-temperature implant anneal encourage better quality in oxides and better control over geometry.
Additionally, this structure can be formed on semiconductors likeCdS where p-n junctions can’t be easily formed.
Figures b-d illustrates the working principle of Schottky source drain. At thermal equilibrium with VG = VD = 0, metal’s barrier height to the p-substrate for holes is qFBp (e.g., 0.84 eV for an ErSi-Si contact).
When gate voltage is above threshold so as to invert the surface from p to n-type, barrier height between source and the inversion layer (electrons) is qFBp= 0.28 eV.
It is to be noted that source contact is reverse biased under operating conditions (shown in Fig. d).
For a 0.28-eV barrier at room temperature, thermionic-type reverse-saturation current density is required to be of the order of 103 A/cm2
To accentuate current density, metals should be selected to yield highest majority-carrier barrier sothat minority-carrier barrier height is minimized.
Surplus current arising due to tunneling through the barrier should help in improving the channel carriers supply.
Currently, forming the structure on a p-type Si substrate for n-channel MOSFET is much moredifficult as compared to p-channel device with n-substrate, because silicidesand metals that yieldhuge barrier heights on p-type silicon are less common.
The main drawbacks of Schottkysourceldrain are high series resistance due to higher drain leakage current and finite barrier height.
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