Semiconductors Flashcards
What is a monolithic circuit
a circuit fabricated from a single silicon crystal
what is the most significant property of semiconductors
that their conductivity can be varied through the application of controlled amounts of impurity in a process called doping
what is a covalent bond
a pair of shared electrons
what is a free electron
one that can wander away from its parent atom and is able to conduct
what happens to semiconductor as temp increases
more covalent bonds are broken which increases the number of free electrons. temperature controlled conductivity
what is recombination
when electrons fills the holes
what is the band gap energy
the amount of energy needed to break a covalent bond
what is a bound charge
it does not move in the crystal it is left over after the carrier leaves the atom.
what is semiconductor mobility
the ease at which charge can move through the crystal. u_n = ~2.5 u_p. electrons can move more easily than holes
when does diffusion occur?
when carrier concentration is not uniform. diffusion of charge current leads to diffusion current. note: J=qD(dn(x)/dx)
what is drift current
due to free carriers being swept by electric field. Ix=Aqxv_drift=Aqpu_xE note Jx=Ix/A J=rE r=(q(pu_p + nu_n)
what are units of rho
ohm*cm
what is the concentration gradient
Jp = -q*Dp dp/dx
what is the Einstein relationship
Dx/u_x=v_t
how does the carrier depletion region form
diffusion uncovers bound charge which leads to a electric field which causes a junction potential. the junction potential reduces diffusion which balances drift current cause by thermal generation in a pn in equilibrium. Id=Is
how to calculate the built in potential of a pn junction
Vo=VtLn(NaNd/ni^2)
describe a forward biased pn junction
Vb=(Vo-Vf) barrier lowers which allows more diffusion across the barrier to happen. due to the applied field an excess concentration of minor carriers builds up at the barrier. minority carriers will recombine with the majority carriers and concentration will decay exponentially with distance called the diffusion length although the current at the edge gives the resulting diffusion current.
what is the diode equation for a pn junction
I=Is(Exp(Vf/Vt)-1) Is=Aqni^2(Dp/LpNd+Dn/LnNa) Is=saturation current=scale current
describe a reverse biased pn junction
Vb=(Vo+Vr) barrier raises reducing diffusion current so all that is left is drift current, Is, which is very small
what is p(x)
p(x) = p_n0+(excess concentration)e((-x-x0)/Lp)
Lp=diffusion length
excess concentration = p_n0*(e(V/Vt)-1)
