Chapter 7 pn Junction Diodes: Small-Signal Admittance

2. Semiconductor Device Physics Chapter 7 pn Junction Diodes: Small-Signal Admittance

3. Chapter 7 pn Junction Diodes: Small-Signal Admittance Small-Signal Diode Biasing • 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 va can be viewed as a small oscillation of the depletion width about the steady state value. V0 << VA RS : serial resistance C : capacitance G : conductance Y : admittance

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

5. Chapter 7 pn Junction Diodes: Small-Signal Admittance Relation Between CJ and VA • For asymmetrical step junction, NB : bulk semiconductor doping, NA or ND as appropriate. • Therefore, • A plot of 1/CJ2 versus VA is linear. • The slope is inversely proportional to NB. • An extrapolated 1/CJ2 = 0 intercept is equal to Vbi.

6. Semiconductor Device Physics Chapter 8 pn Junction Diodes: Transient Response

7. Chapter 8 pn Junction Diodes: Transient Response Turn-Off Transient • 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. tr : recovery time ts : storage delay time trr : reverse recovery time Diode switching circuit

8. Chapter 8 pn Junction Diodes: Transient Response Turn-Off Transient • Voltage-time transient The junction remains forward biased for 0 < t < ts vA(t) = 0 at t =ts

9. Chapter 8 pn Junction Diodes: Transient Response Transient Response of pn Diode • 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 CD, 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. Chapter 8 pn Junction Diodes: Transient Response i(t) ts t vA(t) t ts Decay of Stored Charge • Consider a p+n diode: Dpn(x) Decrease due to recombination and reverse current flow pn0 x xn • For t > 0: • The current is reversed but the diode remains forward biased during0 < t < ts

11. Chapter 8 pn Junction Diodes: Transient Response i(t) i(t) i(t) ts ts ts t t t Examples i-t transient Decrease tp Increase IF Increase IR

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

13. Chapter 8 pn Junction Diodes: Transient Response Turn-On Transient • Again, consider a p+n diode: Dpn(x) i(t) A positive current IF is forced to flow through the diode beginning at t = 0 t vA(t) pn0 x xn • For t > 0: t

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

15. Semiconductor Device Physics Chapter 9 Optoelectronic Diodes

16. Chapter 9 Optoelectronic Diodes Photodiodes Reverse current due to carriersswept by the E-field Electron-hole pair generation due to light

17. Chapter 9 Optoelectronic Diodes I–V Characteristics and Spectral Response Open circuit voltage voc Upper limit ~ highest wavelength ~ lowest frequency ~ lowest energy Short circuit current isc

18. W ≈Wi-region Most carriers are generated in the depletion Faster response time (~10 GHz operation) Chapter 9 Optoelectronic Diodes p-i-n Photodiodes p-i-n : positive–intrinsic– negative Reverse biased • 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

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

20. Chapter 9 Optoelectronic Diodes Homework 6 • 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 (trr). Hint: trr is reached when the magnitude of the reverse current stay inside the vicinity of 10% of IR. • 2. (7.2) • Problem 8.2, Pierret’s “Semiconductor Device Fundamentals”. • Due: 18.11.2013.