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
