Week 9 Quiz Answers ECE 606: Solid State Devices Mark Lundstrom Purdue University, Spring 2013 (Revised 3/25/13) Quiz 1: Answer the four multiple choice questions below by choosing the one, best answer. Then ask a question about the lecture. 1) When the “n-­‐factor of a diode I D = I 0 eqVA /nk BT − 1 is 2 at low forward bias voltages, 2)
3)
(
what do we suspect? )
a) That the current is dominated by recombination in the lightly doped, quasi-­‐
neutral region. b) That the current is dominated by high level injection in the lightly doped, quasi-­‐
neutral region. c) That the current is dominated Zener tunneling d) That the current is dominated by recombination in the space charge region. e) That the current is dominated by generation in the space charge region. The diode equation, I D = I 0 eqVA /nk BT − 1 predicts that the current saturates at − I 0 in (
)
reverse bias. If we find that the reverse bias current increases as the square root of reverse bias voltage, what do we conclude? a) That the current is dominated by recombination in the lightly doped, quasi-­‐
neutral region. b) That the current is dominated by high-­‐level injection in the lightly doped, quasi-­‐
neutral region. c) That the current is dominated Zener tunneling d) That the current is dominated by recombination in the space charge region. e) That the current is dominated by generation in the space charge region. What effect does an intrinsic layer have on a reverse-­‐biased PN junction? a) It changes the ideality factor from n = 1 to n = 2. b) It changes the ideality factor from n = 2 to n = 1. c) It reduces the peak electric field thereby reducing the breakdown voltage due to impact ionization. d) It reduces the peak electric field thereby increasing the breakdown voltage due to impact ionization. e) It increases the peak electric field thereby increasing the breakdown voltage due to impact ionization. 1 Continued on next page 4) Which of the following statements is (are) true about reverse bias breakdown due to impact ionization and Zener tunneling? a) Zener tunneling breakdown generally occurs at lower voltages than impact ionization. b) Zener tunneling generally has a weaker temperature dependence that impact ionization. c) Significant Zener tunneling only occurs when the junction is heavily doped. d) All of the above. e) Statements (a) and (b) Quiz 2: 1) What is the small signal circuit model of a forward-­‐biased diode? a) A resistor in series with the parallel combination of two capacitors. b) A resistor in parallel with two capacitors and then this parallel connection of three elements is in series with another resistor. c) An inductor in parallel with two capacitors and then this parallel connection of three elements is in series with another resistor. d) A resistor in parallel with an inductor and capacitor and then this parallel connection of three elements is in series with another resistor. e) A resistor in series with a capacitor. 2) What can we determine from a plot of one over junction capacitance squared vs. reverse bias voltage? a) The minority carrier lifetime. b) The minority carrier diffusion coefficient. c) The semiconductor bandgap. d) The doping density. e) The series resistance. 3) The dielectric relaxation time describes the temporal response of majority carriers. Which of the expressions below is the correct dielectric relaxation time? a) τ r = σ κ S ε 0 b) τ r = κ Sσ ε 0 . c) τ r = σ Ln d) τ r = κ S ε 0 Dn e) τ r = κ S ε 0 σ 2 4)
Continued on next page When is the diffusion capacitance important? a) For low DC forward bias and for frequencies that are well below 1 ( 2πτ n ) . b) For low DC forward bias and for frequencies that are well above 1 ( 2πτ n ) . c) For high DC forward bias and for frequencies that are well below 1 ( 2πτ n ) d) For high DC forward bias and for frequencies that are above 1 ( 2πτ n ) . e) For low DC forward bias and for frequencies that are well below 1 ( 2πτ n ) . Quiz 3: 1) When I abruptly switch a forward biased diode to reverse bias, there is a “storage time.” This storage time is used to measure what important parameter? a) The minority carrier diffusion coefficient. b) The minority carrier lifetime. c) The minority carrier diffusion time. d) The dielectric relaxation time. e) The diode lifetime. 2) Physically, what is happening inside the diode during the storage time? a) The depletion region is collapsing. b) The excess minority carrier concentration is decreasing. c) The excess minority carrier concentration is increasing. d) The QFL splitting is increasing. e) The minority carrier lifetime is increasing. 3) Roughly how long does it take to turn on a “long” forward biased diode? a) Much less than the minority carrier lifetime. b) Approximately the minority carrier lifetime. c) Much less than the dielectric relaxation time. d) Approximately the dielectric relaxation time. e) The majority carrier lifetime. 3 4)
Continued on next page How long does it take for minority carrier electrons to diffuse across a P-­‐type region of width, WP ? a) WP ( µ nE ) b) W 2 2D p c) W 2 2Dn d) W 2 2 ( Dn + DP ) e) W 2 ( Dn + DP ) ( 2Dn DP ) 4