Electrical- Extrinsic Semiconductors Flashcards
What does extrinsic conduction result from?
Chemical doping of the pure material
Two types of extrinsic semiconduction
n-type: in which electron charge carriers (electrons) dominate
p-type: in which positive charge carriers (electron holes) dominate
What types of dopants are Al and P for silicon?
Al is p-type as it has only 3 valence electrons so is an acceptor. A donor has a higher valence level than the host.
P is n-type as it has 5 valence electrons so is a donor. An acceptor has a lower valence level than the host
What do dopants physically do?
Replace the host atoms in its crystal structure. One host atom is replaced by the dopant atom and a hole or extra electron such that the doped solid remains electrically neutral.
When is the overall appearance of the lattice in a doped material almost unchanged?
If less than 1 dopant in 10^6 atoms
How does n-type doping with P in Si work?
4 of the 5 valence electrons in P are needed for bonding to 4 adjacent Si atoms in the crystal structure. The extra electron no needed is relatively unstable and produces a donor level ED near the CB. This means the energy barrier to forming a conduction electron (Eg-ED) is substantially less than in the intrinsic material (Eg). Due to the extra electrons from the doping process, Ef is shifted upwards. Promote electrons from the donor levels, ED, that are single occupied into the CB (exhaustion). P distributed randomly in lattice so donor energy levels are discrete. σ proportional to [P] as n=ND and conductivity not thermally activated.
How does p-type doping of Al in Si work?
Al distributed randomly in the lattice so acceptor energy levels, EAcc, are discrete. Promote electrons from the filled VB into the empty EAcc levels. Creates mobile holes in the VB (saturation). Much higher conductivity at 300K as n =NAcc and conductivity not thermally activated. σ proportional to [Al]. EAcc<0.1eV above VB
Temperature dependence of σ for extrinsic semiconductors
At low T, e or h are thermally activated into CB or acceptor levels. At medium temperatures all donors have promoted their e to the CB (exhaustion) or all acceptors are filled with electrons from VB (saturation). Level of conductivity depends on [D] or [A] and conductivity decreases slightly with increasing T due to reduction in μ. At high T, intrinsic conductivity dominates as is sufficient thermal energy to promote e across band gap.
Regions of temperature dependence graph
lnσ vs 1/T. For high T (left) negative gradient of -Eg/2 where intrinsic conductivity dominates. Lower T is roughly horizontal during saturation/exhaustion. Even lower T shallow negative gradient of -EAcc/k (p-type) or -(Eg-ED)/k (n-type) when some e trapped on donor states or e retained in VB
Temperature dependence of Ef
For n-type: low temperature starts between CB and ED (very high) then curves down as T increases to constant Eg/2.
For p-type: low temperature starts between VB and EAcc (very low) then curves up as T increases to constant Eg/2.
What is useful about the exhaustion/saturation range?
The level of extrinsic conductivity can be controlled by the level of dopant. It minimises the need for temperature compensation in electrical circuits.
Equation relating concentrations of electrons and holes at thermal equilibrium
[n][p]=constant
Equation relating concentrations of electrons and holes for intrinsic semiconductors
[n][p]=constant=ni^2=pi^2
ni is intrinsic electrons conc in CB
pi is intrinsic hole conc in VB
What happens if a large number of donors are added to that [n] increases?
The number of holes must decrease accordingly to keep
[n][p]=constant true
Also is added at conc ND and become ionised and donate their electrons to CB, these extra electrons swamp the intrinsic population
So [n]=[ND]+[ni] roughly=[ND]
Means [p]=ni^2/[n] roughly=ni^2/ND (reverse argument for p-type)
For mixed dopants what is equation for lattice neutrality?
ND+p=NAcc+n
Positive defects=negative defects
