Semiconductors

Question 1

What is a monolithic circuit

Answer 1

a circuit fabricated from a single silicon crystal

Question 2

what is the most significant property of semiconductors

Answer 2

that their conductivity can be varied through the application of controlled amounts of impurity in a process called doping

Question 3

what is a covalent bond

Answer 3

a pair of shared electrons

Question 4

what is a free electron

Answer 4

one that can wander away from its parent atom and is able to conduct

Question 5

what happens to semiconductor as temp increases

Answer 5

more covalent bonds are broken which increases the number of free electrons. temperature controlled conductivity

Question 6

what is recombination

Answer 6

when electrons fills the holes

Question 7

what is the band gap energy

Answer 7

the amount of energy needed to break a covalent bond

Question 8

what is a bound charge

Answer 8

it does not move in the crystal it is left over after the carrier leaves the atom.

Question 9

what is semiconductor mobility

Answer 9

the ease at which charge can move through the crystal. u_n = ~2.5 u_p. electrons can move more easily than holes

Question 10

when does diffusion occur?

Answer 10

when carrier concentration is not uniform. diffusion of charge current leads to diffusion current. note: J=qD(dn(x)/dx)

Question 11

what is drift current

Answer 11

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)

Question 12

what are units of rho

Answer 12

ohm*cm

Question 13

what is the concentration gradient

Answer 13

Jp = -q*Dp dp/dx

Question 14

what is the Einstein relationship

Answer 14

Dx/u_x=v_t

Question 15

how does the carrier depletion region form

Answer 15

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

Question 16

how to calculate the built in potential of a pn junction

Answer 16

Vo=VtLn(NaNd/ni^2)

Question 17

describe a forward biased pn junction

Answer 17

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.

Question 18

what is the diode equation for a pn junction

Answer 18

I=Is(Exp(Vf/Vt)-1) Is=Aqni^2(Dp/LpNd+Dn/LnNa) Is=saturation current=scale current

Question 19

describe a reverse biased pn junction

Answer 19

Vb=(Vo+Vr) barrier raises reducing diffusion current so all that is left is drift current, Is, which is very small

Question 20

what is p(x)

Answer 20

p(x) = p_n0+(excess concentration)e((-x-x0)/Lp)
Lp=diffusion length
excess concentration = p_n0*(e(V/Vt)-1)