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11mdt1sol

# 11mdt1sol - ECE 432 MIDTERM 1 Fall 2011 Pledge No aid given...

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Unformatted text preview: ECE 432 MIDTERM 1 Fall 2011 Pledge: No aid given, received, nor observed. Name: Signature: Question # 1 / 8 Diode symbol (2 points) Consider the diode shown in the Figure below. o—|<]—o .--. k I Check the correct answer(s). Note that the ordering matters. l]; O a P—N junction V; 9 a N-P junction V 1, O a P-metal Schottky diode \ I z Q a N-metal Schottky diode \ A Q a metal-P Schottky diode , \ /5 O a metal-N Schottky diode Question # 2 / 8 Diode Band diagram (6 points) Consider the 3 band diagrams shown below. E,2 l o Specify below each band diagram whether the diode is in (E)quilibrium, (F)orward biased or (R)everse biased. 0 Indicate on each band diagram the polarity required if the diode is biased with a battery of voltage V3 > 0. Question # 3 / 8 Radiative Process (12 points) 5n=5p O Vziz/illision with lattice Sop! T Consider the experiment shown above. The electron and hole distributions are given by 6n(x) = An exp (1m ) for I512] > L, and with An = gown/2 n 6p(:r) = Apexp (_l\$|) for |x| > LI) and with Ap = gown/2 LP 6 The diffusion current is given by: Judi” = an% and Jp'diff = —qu% Specify the direction ((L)eft or (R)ight) of the following currents and ﬂuxes: Electron diffusion ﬂux: at :1: = —d/ 2: L“ ‘ Electron diffusion current: at a: = —d/ 2: Q Hole diffusion ﬂux: at a: = —d/ 2 : L Hole diffusion current: at x = —d/ 2 : L o If the electron and hole diffusion lengths are NOT equal: Ln = \/DnTn 75 Lp = “DPT!” is the assumption 6n(:r) = 612(2)) correct? NO 0 If 6n(:1:) aé 6p(a:) is there a builtin space charge? YE; o If there is a builtin space charge are there drift currents arising beside the diffusion currents? V”: S For the radiative process involved (see above) we have Twin = 2 eV, Ec — E't = 0.5 eV and E9 = 1.5 eV. 0 What is the name of this radiative process? P H05 F 710 £55 (E V6 I: o What is the energy hwm of photons emitted? I CV, o What is the fraction of the incident photon energy converted to light? ———~ 0 What is the fraction of the incident photon energy converted to heat? I 0 Could the semicOnchictor used have an indirect bandgap? V ES \ Question # 4 / 8 Diode in Forward Bias (15 points) Consider the pn junction shown below. L1 L2 L3 P Flux of holes attracted by the excess electron Qn and recombining with them: V L Flux of holes overcoming the junction potential and injected on the n—side: \f Flux of excess holes (62,) diffusing on the n—side 86 recombining with electrons: 31 "f Flux of holes drifting I/(qA) = (1,, + Ip)/(qA): V l Hole diffusion length L} Electron diffusion length L“ Depletion Width Fermi level drop due to: _ Ohmic loss on the p side (—qR,, x 1) Eq- Ohmic loss on the n side (_ER x I) i; Ll Fermi energy drop associated with the intrinsic diode voltage (—qVDi) ”a Fermi energy drop associated with the extrinsic diode voltage applied (—~qV) E l Sketch on the band diagram the electron and holeistributions versus energy (Sailboat) on both the N and P side (4 sailboats with the right size of the sail) which are at the origin for the exponentional dependence of the current over the bias voltage. Question # 5 / 8 Depletion Approximation & Capacitances in a Diode (10 points) space charge ~ —\ N Consider the band diagram in equilibrium with 8 donors and 8 acceptors. Q 0 Indicate the charge of the ionized donors and acceptors. (L 0 Locate in the band diagram the electrons and holes ass ociated with the donors and acceptors. 0 Show in the top PN diode the resultin space charge region. Consider a N P+ g charges raised in the P and N sides of the diode. The storage capacitance Csm associated with excess carriers Qp dQ (11 q . Cstm. = (1—; 2 TpGD = TPW = TPEI With 1 '2 Ioqu/(kT) . . . . . . . . de 6A The Junction capamtance C June assoc1ated w1th the depletlon region 1s Cjunc — [W — W (V) q 0 Which capacitance (Csm or CJWC) dominates under reverse bias? Cd “M \ 0 Which capacitance (05,30r or C June) dominates under forward bias? C§ *6 I \ o If such a diode is placed in shunt with an LC resonator should it be reverse or forward biased? PC U‘U’SQ { o What is the name of the diode used as voltage tunable capacitor? V0 J‘aC‘lDV Question # 6 / 8 Breakdown in a diode (10 points) 0 Show the total energy trajectory of electron(s) (and possibly holes) for the Zener or Avalanche breakdown process in the band diagrams below 0 What is the required kinetic energy for breaking a Silicon bond? 0 Why is that energy larger than the bandgap? Avalanche Breakdown Punchthrough through voltage VpT of a short N+P. 0 Calculate the punch-throu gh voltage VPT (neglect the N side depletion) in terms of arm. LE ‘Xno ’2." 1:32 Jim“ 9"- L l) M M ..——I } he - Koo ~ ’- / Question # 7 / 8 Schottky or Ohmic Contacts Semiconduc s (10 points) ”—3 > Complete the two metal semiconductor band diagrams shown below assu 'ng the junctions are in equilibrium. 2 conductor and metal for the IMPURETIES SHOWN. o Specify whether the junctions form a ohmic or schottky comma X {L o Specify whether we have an accum lation or depletio 0 Show the charges associated with the electron, holes acceptors Etono s in the semi- Vacuum Le—_vel —_Ec ---------- FR 0 o o o o o __E" Semiconductor Metal Vacuum Level 0 o o o o o 0 EC W —————————— Fm Metal ———Ev Semiconductor Question # 8/8 Biased Schottky Diode ( A Schottky barrier is formed between a metal h doping is such that EC — E178 a) Draw the equilibrium band b) Draw the band diagram wit 10 points) aving (Pm = 4.6V and N—type (X = 4 V). The = 0.2eV. The bandgap is E9 = 1.1eV at room temperature. diagram, showing a numerical value for qu. {)7 h 0.2 V forward bias and —O.3 V reverse bias. . .. "'Vacﬁuum'léi/él ................................................................................ . .. if,.f.'..'ff].' fff_,1':wgrd.jgiggﬁ[,T'.Mf.'ff.' ................................................... ReversewBias. 3 2 ...
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11mdt1sol - ECE 432 MIDTERM 1 Fall 2011 Pledge No aid given...

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