chapter 3 pn junction and diode - 上海交通大学微电子学...

Prepared by Xiulan Cheng/程秀兰

Chapter 3 PN Junction and Diode

3.3 Special diode and Photodiode

Shirla Cheng

[email protected]

2012.3.28

mailto:[email protected]


Outline

Normal Application of PN Junction

Special Diode

Photodiode

3

Normal Applicaiton- Rectifiers(整流器）

V

I

Low R in forward direction:p+-n-n+ structure preferred

The p+ and n+ regions reduce the parasitic resistance.

Low I0 in reverse:Ge is worse than Si. Why?

High voltage breakdown in reverse:

p+-n-n+ structure

Higher bandgap materials preferred. Why?

Switching Diodes

Prepared by Xiulan Cheng/程秀兰4

Diodes can be used as switching devices

Need to change from conducting to non-conducting at high speed

Storage time or turn-off transients should be small

Add recombination centers to reduce minority carrier lifetimes

For example adding 1015cm–3 gold (Au) to Si reduces hole lifetime to 0.01 s from 1 s!

Use narrow-base diodesAmount of charge stored in the neutral region of the diode will be small.


Special Diodes

Zenor Diode

The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change.

The breakdown characteristics of diodes can be tailored by controlling the doping concentration

Heavily doped p+ and n+ regions result in low breakdown voltage (Zener effect)

Used as reference voltage in voltage regulators

Typically it is used for providing a stable reference voltage for use in power supplies and other equipment


.

This particular zener circuit will work to maintain 10 V across the load.

The zener diode’s breakdown characteristics are determined by the doping process. Low voltage zeners (>5V), operate in the zener breakdown range. Those designed to operate <5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to 200 V.

This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage.

Zener zone Diode zone

Avalanche

zone

5V.

Zener Diodes – Regulation Ranges

Zener LimitingZener diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to consider for a zener limiter is its zener breakdown characteristics.

See Ex.3-8


Varactor (变容二极管= Variable reactor)

Varactor Diode :Voltage-controlled capacitor

is a variable reactance device using reverse biased PN junction.

Increasing the reverse bias voltage on a P-N junction forces the charges on either side of the depletion or space charge layer further apart effectively increasing its capacitance.

Conversely reducing the bias reduces the capacitance.

Being reverse biased no current flows.

Applicationideal for use as a variable capacitor in tuned circuits.

Used in oscillators (振荡器) and detectors (e.g. FM demodulation circuits in your radios)

Response changes by tailoring doping profile:

bifor VVVC rn

rj

Varactor

Voltage-controlled capacitance of a pn junction can be used in

tuning stage of a radio or TV receiver.

CJ (VA)–n , where n = 1/2 for an abrupt pn junction. However, n

can be made higher than 1/2 by suitably changing the doping

profile.


Varactor Diodes

Varactor

Bias adjust

The varactor diode can be useful in filter circuits as the adjustable component for resonance frequency selection.

Varactor Diodes


P-I-N Diode: Charge-Storage Diode (Si 0.5~5us)

Principle

P-type --- Intrinsic --- N-type

Used as switch and attenuator

Reverse biased – off

No stored charge in the I region and the diode appears as

a capacitor, CT, shunted by a parallel resistance RP.

Forward biased - partly on to on depending on the biasholes and electrons are injected into the I region. These charges do not immediately annihilate each other; instead they stay alive for an average time, called the carrier lifetime, t.an average stored charge, Q, which lowers the effective resistance of the I region to a value RS.

ApplicationThe PIN diode is a current controlled resistor at radio and microwave frequencies. It can control large amounts of RF power with much lower levels of DC.

Wide usage in RF, UHF and microwave circuits.


Avalanche Diode and Zenor Diode

Avalanche Diode Reverse-bias impact ionization

Removing the voltage turns off the current.


Tunnel Diode (隧穿二极管)-负阻效应

A tunnel diode is a

semiconductor diode with a

negative resistance region

Allows very fast switching speeds,

up to 5 GHz.

useful in high-speed circuits

and perhaps static memories

Generately doped such that

EFp < Ev and EFn > Ec

Neaman’s book:

P224,Fig.8.31


Photodiodes


Introduction to Light Spectrum and

Cutoff wavelength

(um)length light wave -

citylight velo - c

(eV)energy sphoton'- :where

24.1

v

E

EE

hc

hv

hc

v

c

ph

phph

Semi Eg (eV) λ (um)

Ge 0.66 1.87879

Si 1.12 1.10714

GaAs 1.42 0.87324

SiC 2.83 0.43816

g

GE

24.1

h wavelengtCutoff


Semiconductor Optoelectronic Devices

Classification

Photodetector: gain and detect light signalsPN junction photodiode

P-i-n and avalanche photodiode

Solar Cell: lightelectricity

Light emitting Diode, Laser Diode: electricitylight


Optoelectronic Effect in PN junction photodiode

1ln

:VoltageCircuit -Open

)()(

)1(

0A

0A

Ln

Lp

L

LL

qkT

Ioc

NPLNPLL

LkTVq

Ldark

GVV

LLqAGLWLqAGI

IeIIII

pn

nn

p

p

np

Voc

Isc

VA

(1)

Photodetector

(2)

Solar Cell

(1) photo detector: VA<0 I due to light;

(2) Solar cell: VA>0, I<0=VA*I<0 (power

is generated)


Light spectrum (Wavelength) response of PN

PhotodetectorAll PN optoelectronic detectors only detect limited spectrum range

Only upper limit of obsorbing wavelength depends on the Eg

If photonic energy of incident light is higher than the forbidden band width (Eph >= Eg)of semiconductorsemiconductor obsorbs photon’s energygenerate hole-electron pairs.

Cutoff wavelength G=1.24/Eg

If Eph < Egsemiconductor seems transparent to the light

Spectral response Measures how the photocurrent, IL varies with the wavelength of incident light. IL / Imax - (Si, G=1.1um)

spectrum response reduces at much shorter wavelength range

Smaller lager photon’s energy (but total light power is constant)means the number of photons reduces

Lower frequency responseLight-generated minority diffuse to depletion layerthen the current can be detected.

Only few tens MHz.

Higher frequency response (a few GHz) can be achieved using p-i-n diodes.


p-i-n PhotodetectorW Wi-region, so most carriers are generated in

the depletion region

i region can look as the depletion regions in a pn

diode.

Most of the light is absorbed in the i-region.

Under small reverse bias, the i-region is depleted,

and the carriers generated in the i-region are

collected rapidly due to the strong electric field.

Faster response time (~10 GHz operation than pn

photodetector

because generated carrier do not need to diffuse

into n zone before they are collected Operate near avalanche enhance the

obsorbing rate of minority generated by lightget higher response frequency.

satiimax

1

acrosstimetransitcarrier

1

v/WWf

p-i-n Photodetector


p-i-n photodiodes operating at 1.55 m are made on In0.53Ga0.47As

deposited on InP substrate.


Solar Cell (1)

Si: 80% light absorbtion( including 40% transfer to thermal engergy )GaAs 65% absorbtion (including 30% transfer to thermal engergy )

Solar Cell (2)


Solar cells are large area pn-junction diodes designed

specifically to avoid energy losses.

Voc= the open circuit voltage

Isc = current when device is

short circuited

= power conversion efficiency = (Im Vm)/Pin

second.per energy photons'incident -inP


Solar Cell(3)

Voc

Isc

VA

(1)

Photodetector

(2)

Solar Cell

second.per energy photons'incident - :where

.

:effeciencynsmit Energy tra

1 FF:ratioDuty

power.output largest generatingpoint operation theis -m:I ,V

currentcircuit short – I

lightsolar by the provided ltagelargest vo the:ltagecircuit voopen - V

max

max

mm

sc

oc

in

in

ocsc

in

mm

in

ocsc

mm

ocsc

P

P

VIFF

P

VI

P

P

VI

VI

VI

PFF

Isc

Voc

Im

Vm


Light Emitting Diodes (LEDs)

Light emission is due to

recombination- direct band gap

is required (for direct

semiconducto)

LEDs preparation conditionsDirect semiconductor: typically compound semiconductors

Eg:1.77~3.10eV for visible light.

Easy to form PN junction.

Increase

Eg

Visible spectrumi:

Red:~ 650nm

Green:~550nm

Blue:~ 450nm

White LED: blue LED +Yellow LED

Forward-bias


Material

Direct /

Indirect

Bandgap

Band Gap

Energy at

300 K (eV)

C

(diamond)

Indirect 5.47

Ge Indirect 0.66

Si Indirect 1.12

Sn (grey) Direct 0.08

GaAs Direct 1.42

InAs Direct 0.36

InSb Direct 0.17

GaP Indirect 2.26

GaN Direct 3.36

InN Direct 0.7

Gr-IV

compounds

α-SiC Indirect 2.99

ZnO Direct 3.35

CdSe Direct 1.7

ZnS Direct 3.68

Groups III-V

compounds

Groups II-VI

compounds

Elements


Organic LEDsSome organic materials exhibit semiconducting properties

OLEDs are attractive for low-cost, high-quality flat-panel displays

http://www.pconline.com.cn/images/html/viewpic_pconline.htm?http://img3.pconline.com.cn/pconline/0704/15/999829_sony-oled3.jpg&namecode=diy&subnamecode=home

OLED原理

色彩输出原理

红绿蓝三色法：将光的红绿蓝三原色按不同比例调配。


常见空穴传输材料常见电子传输材料

发光材料

chapter 3 pn junction and diode - 上海交通大学 微电子学...

Documents

chapter 3 pn junction and diode - 上海交通大学微电子学...