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Chapter 7 pn Junction Diodes: Small-Signal Admittance. Chapter 7. pn Junction Diodes: Small-Signal Admittance. Small-Signal Diode Biasing.

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slide2

Chapter 7

pn Junction Diodes: Small-Signal Admittance

small signal diode biasing

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

total pn junction capacitance

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.
relation between c j and v a

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,
slide6

Chapter 8

pn Junction Diodes: Transient Response

turn off transient

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

turn off transient1

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

transient response of pn diode

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.
  • 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.
decay of stored charge

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
examples i t transient

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

storage delay time t s

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
turn on transient

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

turn on transient1

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
slide15

Chapter 9

Optoelectronic Diodes

photodiodes

Chapter 9

Optoelectronic Diodes

Photodiodes

Reverse current due to carriersswept by the E-field

Electron-hole pair generation due to light

i v characteristics and spectral response

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

p i n photodiodes
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
light emitting diodes leds

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.
homework 6

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”.
  • Deadline: 14.06.2012, at 08:00.