What does cMOS stand for?
Complementary Metal Oxide Semiconductor
What is cMOS?
cMOS is a type of technology used in the manufacturing of computer processors, memory chips, and other digital devices
Why is cMOS so widely used?
- High levels of integration
- Small devices
- Large numbers on chip
- → High functionality
- → Low cost/function
- Very low static power
- Only consumes power when switching state
- Portable appliances
What is planar processing?
- Mass production of devices on wafer
- Diffuse dopants into surface
- Grow layers on surface
- Pattern and etch layers
- All devices formed simultaneously
- Complex multi-layer printing process
State suitable materials for an n-type dopant for silicon.
- Phosphine
- Arsine
Group 5
5 valence electrons
Four outer electrons combine with ever one silicon atom, while the fifth electron is free to move and serves as charge carrier.
State a suitable material for an n-type dopant for GaAs.
The n-type dopant must be electron donating, therefore group 6.
- Selenium
- Sulfur
State a suitable material for a p-type dopant for GaAs.
The p-type dopant must be electron accepting, therefore group 2.
- Zinc
- Cadmium
What is GaAs?
Gallium Arsenide
What is the work function for an n-type semiconductor?
WF = IE - E(g)
IE: Ionisation Energy
E(g): Energy Band Gap
What is the work function for a p-type semiconductor?
WF = IE
IE: Ionisation Energy
How do you find the potential difference for a p-n junction?
V(b) = (k(B)T/q) * ln [N(D) * N(A) / n(i)^2]
Where:
* k(B)T/q = 0.026
* N(D) is Donor impurity concentration
* N(A) is Acceptor impurity concentration
* n(i) is intrinsic concentration
How do you find the depletion width of a p-n junction?
d = d(n) + d(p) = ( 2ε(r)ε(0)V(T) / q ) ^(1/2) * ( (1 / N(D)) + (1 / N(A)) ) ^(1/2)
Where:
* d(n) is n-type region
* d(p) is p-type region
- ε(r) is relative permittivity
- ε(0) is vacuum permittivity
- V(T) is total potential difference across the junction
- q is the charge of an electron
What is capacitance per unit area equal to?
C / A = ε(r) * ε(0) / t(0)
Where:
* ε(r) is relative permittivity
* ε(0) is vacuum permittivity
* t(0) is thickness
Describe the diffusion of electrons from the n-type region into the p-type region of a p-n junction at thermal equilibrium.
- Charges flow in each direction due to the concentration gradients.
- The uncompensated donor (acceptor) ions generate a potential difference
- that moves free charges in the opposite direction to the diffusion.
- At equilibrium, there is a region of width, d, that is depleted of free charges.
Suggest dopant materials for n-type lnP.
- Sulfur (S)
- Selenium (Se)
Suggest a dopant material for p-type lnP.
- Zinc (Zn)
Suggest a dopant material for n-type ZnSe.
- Gold (Au)
- Copper (Cu)
Give the equation that describes the chemical reaction that occurs during wet oxidation of silicon.
Si + 2H(2)O -> SiO(2) + 2H(2)
Give the equation that describes the chemical reaction that occurs during dry oxidation of silicon.
Si + O(2) -> SiO(2)
Summarise wet oxidation.
- Temperature 600°C - 1400°C
- Rate varies with temperature and crystal face
- Fast, low quality oxide
Summarise dry oxidation.
- Temperature 600°C - 1400°C
- Slow, high quality oxide
List 4 types of doping methods.
- During crystal growth & deposition
- Diffusion
- Ion implantation
- Transmutation doping
How do you build a cMOS device?
- Put nMOS and pMOS devices on a single substrate
- Use doped well to put pMOS device in
- Use polysilicon gates
- common to both nMOS and pMOS
Why is the channel of a pMOS device typically made 3x wider than the channel width of the corresponding nMOS device when they are used together to form an inverter?
- Mobility of electrons > Mobility of holes
- -> Make p-type devices wider than that of n-type devices
