4.10. PROBLEMS 209
Figure 4.37: Diode for problem 4.28.
Problem 4.26Consider a GaAS p-n junction withNA=ND=10^17 cm−^3. Assume a
mid-gap trap in the material that causes the minority carrier lifetime to be 0. 1 ns. Calculate
and plot the electron and hole currents (including the recombination current) in the
depletion region. Explain the features on the graph.
Problem 4.27Consider a Sip-njunction biased as a solar cell. Light falls on this solar
cell leading to optical generationGOP=10^20 /s. What is the optically generated
current(IOP) for the diode? What is the open-circuit voltage(VOC)? Plot the minority
carrier profiles when the voltage across the junction isVOC,VOC/ 2 and 0. Consider
generation in the depletion region. Useτp=τn=10−^6 s, μn=μp= 1000cm^2 /V.sand
ND=NA=10^17.
Problem 4.28Consider the diode in figure 4.37. A sheet of acceptors of areal density QA
is placed in an intrinsic region of GaAs such that it is at a distance L 1 from one n+region
and L 2 from another.
(a) Calculate an expression for the potential across the structure in terms of QA,L 1 ,L 2 ,
NDand other material parameters of GaAs.
(b) Sketch the band diagram for the case where L 1 =0.1μm, L 2 =0.2μm, QA=
5 × 1011 cm−^2 and ND=NC. What will the turn-on voltage of the diode be in each
direction?
(c) Calculate the maximum value of QA,QA,M AXthat gives the highest turn-on voltage in
each direction/polarity.
(d)IfInowsetQA=2QA,M AX, what will the turn-on voltage of the diode be? Explain
what happens.
Problem 4.29Consider ap-i-njunction in AlInAs (Eg=1.4eV,ni=10^7 cm−^2 )thatis
grown by MOCVD. To prevent the acceptor atoms from diffusing, the temperature of
