1. President UniversityErwin SitompulSDP 8/1 Lecture 8 Semiconductor Device Physics Dr.-Ing. Erwin Sitompul President University http://zitompul.wordpress.com 2013

2. President UniversityErwin SitompulSDP 8/2 Chapter 7 pn Junction Diodes: Small-Signal Admittance Semiconductor Device Physics

3. President UniversityErwin SitompulSDP 8/3 V 0 << V A R S : serial resistance C: capacitance G: conductance Y : admittance Small-Signal Diode Biasing Chapter 7pn Junction Diodes: Small-Signal Admittance When reversed-biased, a pn junction diode becomes functionally equivalent to a capacitor, whose capacitance decreases as the reverse bias increases. Biasing additional a.c. signal v a can be viewed as a small oscillation of the depletion width about the steady state value.

4. President UniversityErwin SitompulSDP 8/4 Junction / depletion capacitance, due to variation of depletion charges Diffusion capacitance, due to variation of stored minority charges in the quasineutral regions Minority carrier lifetime Total pn Junction Capacitance Chapter 7pn Junction Diodes: Small-Signal Admittance C J dominates at low forward biases, reverse biases. C D dominates at moderate to high forward biases.

5. President UniversityErwin SitompulSDP 8/5 N B : bulk semiconductor doping, N A or N D as appropriate. Relation Between C J and V A Chapter 7pn Junction Diodes: Small-Signal Admittance For asymmetrical step junction, Therefore, A plot of 1/C J 2 versus V A is linear. The slope is inversely proportional to N B. An extrapolated 1/C J 2 = 0 intercept is equal to V bi.

6. President UniversityErwin SitompulSDP 8/6 Chapter 8 pn Junction Diodes: Transient Response Semiconductor Device Physics

7. President UniversityErwin SitompulSDP 8/7 t r : recovery time t s : storage delay time t rr : reverse recovery time Diode switching circuit Turn-Off Transient Chapter 8pn Junction Diodes: Transient Response In order to turn the diode off, the excess minority carriers must be removed through net carrier flow out of the quasi- neutral regions and recombination. Carrier flow is limited by the switching circuit.

8. President UniversityErwin SitompulSDP 8/8 The junction remains forward biased for 0 < t < t s v A (t)  0 at t  t s Turn-Off Transient Chapter 8pn Junction Diodes: Transient Response Voltage-time transient

9. President UniversityErwin SitompulSDP 8/9 Transient Response of pn Diode Chapter 8pn Junction Diodes: Transient Response Suppose a pn diode is forward biased, then suddenly turned off at time t = 0. The excess minority carrier will be removed through recombination and reverse current flow. Because of C D, the voltage across the pn junction depletion region cannot be changed instantaneously. The delay in switching between the ON and OFF states is due to the time required to change the amount of excess minority carriers stored in the quasi-neutral regions.

10. President UniversityErwin SitompulSDP 8/10 t vA(t)vA(t) tsts pn(x)pn(x) Decrease due to recombination and reverse current flow t i(t)i(t) tsts x p n0 xnxn The current is reversed but the diode remains forward biased during 0 < t < t s Decay of Stored Charge Chapter 8pn Junction Diodes: Transient Response Consider a p + n diode: For t > 0:

11. President UniversityErwin SitompulSDP 8/11 t i(t)i(t) tsts Increase I F Increase I R Decrease  p t i(t)i(t) tsts t i(t)i(t) tsts Examples i-t transient Chapter 8pn Junction Diodes: Transient Response

12. President UniversityErwin SitompulSDP 8/12 Q P : excess hole charge Storage Delay Time ts Chapter 8pn Junction Diodes: Transient Response t s is the primary quantity used to characterize the transient response of pn junction diodes By separation of variables and integration from t = 0 + to t = t s, noting that And making the approximation of We may conclude that

13. President UniversityErwin SitompulSDP 8/13 pn(x)pn(x) t i(t)i(t) vA(t)vA(t) xnxn x t p n0 A positive current I F is forced to flow through the diode beginning at t = 0 Turn-On Transient Chapter 8pn Junction Diodes: Transient Response Again, consider a p + n diode: For t > 0:

14. President UniversityErwin SitompulSDP 8/14 Steady state Turn-On Transient Chapter 8pn Junction Diodes: Transient Response Rewriting for turn-on characteristics, By separation of variables and integration, we have The stored hole charge in an ideal diode is given by Finally, by assuming that the build-up of stored charge occurs quasistatically, V A  v A

15. President UniversityErwin SitompulSDP 8/15 Chapter 9 Optoelectronic Diodes Semiconductor Device Physics

16. President UniversityErwin SitompulSDP 8/16 Electron-hole pair generation due to light Reverse current due to carriers swept by the E -field Photodiodes Chapter 9Optoelectronic Diodes

17. President UniversityErwin SitompulSDP 8/17 Open circuit voltage v oc Short circuit current i sc Upper limit ~ highest wavelength ~ lowest frequency ~ lowest energy I–V Characteristics and Spectral Response Chapter 9Optoelectronic Diodes

18. President UniversityErwin SitompulSDP 8/18 W ≈ W i-region Most carriers are generated in the depletion Faster response time (~10 GHz operation) current arises mostly in the totally depleted i-region, not in quasineutral region as in pn diode generated carriers do not need to diffuse into the depletion region before they are swept by the E -field enhanced frequency response p-i-n : positive–intrinsic– negativeReverse biased p-i-n Photodiodes Chapter 9Optoelectronic Diodes

19. President UniversityErwin SitompulSDP 8/19 Increasing E G Forward bias Light Emitting Diodes (LEDs) Chapter 9Optoelectronic Diodes LEDs are typically made of compound semiconductors (direct semiconductors with band-to-band recombination). It releases energy by dissipating light / emitting photon.

20. President UniversityErwin SitompulSDP 8/20 Homework 6 Due: 18.11.2013. 1. (9.50) Consider a diode with a constant junction capacitance of 18 pF at forward bias and 4.2 pF at a reverse bias. The minority carrier lifetimes are 10 –7 s. The diode is switched from a forward bias with a current of 2 mA to a reverse bias voltage of 10 V applied through a 10 kΩ resistor. Estimate the reverse recovery time (t rr ). Hint: t rr is reached when the magnitude of the reverse current stay inside the vicinity of 10% of I R. 2. (7.2) Problem 8.2, Pierret’s “Semiconductor Device Fundamentals”. Chapter 9Optoelectronic Diodes