Chapter 7 pn Junction Diodes: Small-Signal Admittance

President University Erwin Sitompul SDP 8/1

Dr.-Ing. Erwin SitompulPresident University

Lecture 8Semiconductor Device Physics

http://zitompul.wordpress.com


Chapter 7pn Junction Diodes: Small-Signal Admittance


V0 << VA

RS : serial resistance

C : capacitance

G : conductance

Y : admittance

Small-Signal Diode BiasingChapter 7 pn 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 va can be viewed as a small oscillation of the depletion width about the steady state value.

Y G j C


DCD

ICkT q

sJC A

W

Junction / depletion capacitance,due to variation of depletion charges

i

J DC C C av

1R G

Diffusion capacitance,due to variation of stored minority charges in the quasineutral regions

Minority carrier lifetime

Total pn Junction CapacitanceChapter 7 pn Junction Diodes: Small-Signal Admittance

• CJ dominates at low forward biases, reverse biases.• CD dominates at moderate to high forward biases.


2

bi A2 2 2 2J s B S

1 2 ( )W V VC A qN A

sbi A

B

2W V VqN

NB : bulk semiconductor doping, NA or ND as appropriate.

Relation Between CJ and VA

Chapter 7 pn Junction Diodes: Small-Signal Admittance

For asymmetrical step junction,

Therefore,


Chapter 8pn Junction Diodes: Transient Response


R

R

VR

F

F

VR

tr : recovery timets : storage delay time

trr : reverse recovery timeDiode switching circuit

Turn-Off TransientChapter 8 pn 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.


The junction remains forward biased for 0 < t < ts

vA(t) 0 at t ts

Turn-Off TransientChapter 8 pn Junction Diodes: Transient Response

Voltage-time transient


Transient Response of pn DiodeChapter 8 pn Junction Diodes: Transient Response

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.


n

n

P

0x x

dp idx qAD

t

vA(t)

ts

Dpn(x)

Decrease due to recombination and

reverse current flow t

i(t)

tsFI

RI

xpn0

xn

• The current is reversed but the diode remains forward biased during 0 < t < ts

Decay of Stored ChargeChapter 8 pn Junction Diodes: Transient Response

Consider a p+n diode:

For t > 0:


t

i(t)

ts

Increase IF Increase IR Decrease p

t

i(t)

ts

t

i(t)

ts

Examples i-t transientChapter 8 pn Junction Diodes: Transient Response


P P PR s

p p

0dQ Q Qi I t tdt

P PF

p p

(0 ) (0 )Q QI

P s( ) 0Q t

s p F Rln(1 )t I I

QP : excess hole charge

Storage Delay Time tsChapter 8 pn Junction Diodes: Transient Response

ts 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 = ts, noting that

And making the approximation of

We may conclude that


Dpn(x)

t

i(t)

vA(t)

xn

n

n

P

0x x

dp idx qAD

x

t

AF 0 ( 1)qV kTI I e

ON F 0ln(1 )kTV I Iq

pn0

A positive current IF is forced to flow through the

diode beginning at t = 0

Turn-On TransientChapter 8 pn Junction Diodes: Transient Response

Again, consider a p+n diode:

For t > 0:


P P PF

p p

for 0dQ Q Qi I tdt

p/P F p( ) (1 )tQ t I e

A /P DIFF p 0 p( ) ( 1)qV kTQ t I I e

p/FA

0

( ) ln 1 (1 )tIkTv t eq I

Steady state

Turn-On TransientChapter 8 pn 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, VA vA


Chapter 9Optoelectronic Diodes


dark LI I I

Electron-hole pair generation due to light

Reverse current due to carriers swept by the E-field

L N P L( )I qA L W L G

PhotodiodesChapter 9 Optoelectronic Diodes


L LI G

Open circuit voltage voc

Short circuit current isc

Upper limit ~ highest wavelength

~ lowest frequency ~ lowest energy

I–V Characteristics and Spectral Response Chapter 9 Optoelectronic Diodes


W ≈ Wi-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– negative Reverse biased

p-i-n PhotodiodesChapter 9 Optoelectronic Diodes


Incr

easi

ng E

G

Forward bias

Light Emitting Diodes (LEDs)Chapter 9 Optoelectronic Diodes

LEDs are typically made of compound semiconductors (direct semiconductors with band-to-band recombination).

It releases energy by dissipating light / emitting photon.


Homework 6

Deadline: 14.06.2012, at 08:00.

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”.

Chapter 9 Optoelectronic Diodes

Chapter 7 pn Junction Diodes: Small-Signal Admittance

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