Electronic- Extrinsic Semiconductors Flashcards
Principle of doped semiconductors
Deliberate introduction of impurities in small quantities (e.g 1 in 10^5) used to control conductivities. The impurity has a different valence to the host semiconductor
What happens when impurity levels below 1 in 10^6?
Structure is basically unaltered and the wave functions are similar to those for host semiconductor
What happens when a group 5 element is added to Si?
Each group 5 element contributes 4 valence electrons in the VB but has one extra electron. This enters a state similar to those in the CB. Because of excess positive charge on the nucleus this level is slightly below CB (0.01eV lower). It is a weakly bound state.
Where do electrons sit at 0K when group 5 added to Si?
Full VB. Empty CB. Extra electron in donor level Ed just below CB
Where do electrons sit above 0K when group 5 added to Si?
Extra electron in Ed easily promoted to conduction band since energy gap to bottom of CB is of the order kT. A positive donor ion remains from where the extra electron was
What happens when a group 3 element is added to Si?
Each group 3 element only has 3 valence electrons to contribute to VB so there is one missing electron. Creates a state available for VB electrons to occupy. Because of excess negative charge on the nucleus this level is about 0.01eV above the VB. It is a weakly bound state
Where do electrons sit at 0K when group 3 added to Si?
VB filled and CB empty. An acceptor Ea state just above the VB has no electron in it
Where do electrons sit above 0K when group 3 added to Si?
An electron from the VB is easily promoted to the acceptor state since the energy gap is of the order kT. Leaves a mobile hole in the VB
Names for semiconductors with excess of donor or acceptor states
Excess donor states is n-type (negative)
Excess acceptor states is p-type (positive)
Formula for total carrier concentration
nc=ni=rt(nenh)
ni is concentration if intrinsic (ne=nh)
ne is number of electrons
nh is number of holes
What happens if ne increases in a material?
nh decreases proportionally
Majority and minority charge carriers for n and p-type
n-type: majority electrons and minority holes
p-type: majority holes and minority electrons
How does carrier concentration of extrinsic semiconductors vary with temperature?
Starts at 0. Rapid increase at low T in freeze-out region. Reaches constant after 100K into extrinsic region. Decreases slightly due to phonons and scattering. Until 400-500K where intrinsic behaviour takes over and more rapid rise
Where is the Fermi level for p-type semiconductors near 0K?
Half way between top of VB and Ea
Where is the Fermi level for n-type semiconductors near 0K?
Half way between Ed and bottom of CB
Where is the Fermi level for extrinsic semiconductors at high temperatures?
Tends towards the band gap midpoint as intrinsic effects take over
Graph of where Ef is for extrinsic semiconductors vs temperature
Mirror image above and below half way up band gap. Starts half way between Ed and CB. Curves down then less so and levels off near mirror line. Opposite for p-type. Increasing impurity composition means curves down slower and levels off later
