Device Physics Equations Flashcards
What is a ‘Schottky’ barrier
Metal-semiconductor junctions naturally form a barrier to current flow in one direction, known as a ‘Schottky’ barrier.
This barrier enables the formation of diode structures with one type (n or p) of semiconductor alone, and such devices are unipolar.
These diodes are faster than p-n junction diodes, but have higher leakage currents.
Hard to form ‘ohmic’ contacts to semiconductors
space charge region equation
Symbol for work function
Symbol or Schottky barrier
Electron Affinity symbol
junction capacitance equation
‘Schottky effect’
An electron in a dielectric at a distance x from the metal will create an electric field.
force on the electron due to the coulomb attraction with the image charge
potential energy of an electron due to the coulomb attraction with the image charge
Equation to find the value of the Schottky barrier lowering, ∆ϕ
position of the maximum Schottky barrier, xM,
current density taking into account image-force-lowering
reverse-saturation current of the Schottky barrier
Comparison of the Schotky barrier diode with the p-n junction diode
Two key differences
- Reverse bias leakage current
- Switching speed
- Current conduction mechanism is very different:
- Schottky diode - Unipolar, majority carrier transport dominates. Thermionic emission.
- P-n junction diode, Bipolar, minority carrier transport dominates. Diffusion of minority carriers.
For a given forward bias value, a higher current will flow in the Schottky barrier diode, as compared to the p-n junction device
MOSFET ID in the non-saturation region and saturation region
how does transconductance change
if device width increases
channel length and oxide thickness decrease
if device width increases, transconductance increases
if channel length and oxide thickness decrease, transconductance decreases
MOSFET cutt-off frequency equation(s)
for a well designed mosfet around 800MHz, in reality substantially reduced in real devices
What happens when you scale down a MOSFET and the source and drain?
The device is more susceptible to punch through breakdown.
To avoid this, substrate doping is increased to reduce the depletion widths.
Junction depths are also reduced to mitigate short channel Vth effect.
