3. Diodos de unión p-n 4. El MOSFETyolotli.inaoep.mx/portalfiles/AplicacionesDiodo.pdf · Curso...

Curso propedéutico de Electrónica INAOE 2009 Dr. Pedro Rosales Quintero 95

3. Diodos de unión p-n

3.1 Diodo de unión p-n

3.2 Diodos túnel

3.3 Fotodiodos

3.4 LEDs y láseres

3.5 Transistor Bipolar de unión

4. El MOSFET

41 El capacitor MOS ideal

4.2 Efectos en superficies reales

4.3 Voltaje de encendido

4.4 Operación básica del transistor MOS

4.5 Control del voltaje de encendido

4.6 Fabricación del MOS


p-n Junction DiodesThe diodes are designed to exploit specific junctions properties.

Rectifiers

The most obvious property of a p-n junction is its unilateral nature; that this, to a good approximation its conducts current in only one direction.


From the theory derived in chapter 5 we can easily list the various requirements for a good rectifier diodes.

1. Band gap. Since ni is small for large band gap materials, then Irdecreases with increasing Eg. On the other hand, the contact potential and E0 generally increases with increasing Eg.

2. The doping concentration on the each side of the junction influences the avalanche breakdown, the contact potential and the series resistance of diode.


Switching diodesA common technique for improving the switching speed of a diode is adding efficient recombination centers to the bulk material.

Thus, for example, a p+-n silicon diode may have τp = 1µs and reverse recovery time of 0.1 µs before Au doping. If the addition of 1x1014 Au atoms reduces the lifetime to 0.1 µ s and tsd to 0.01 ns.

The second approach to improving the diode switching time is to make the lightly doped neutral region shorter than a minority carrier diffusion length.


The breakdown DiodeWhen a diode is deigned for a specific breakdown voltage, it is called breakdown diode. Such diodes are also called Zenerdiodes.

Breakdown diodes can be used as voltage regulators in circuits with varying inputs.


The Varactor DiodeTerm varactor is shortened form of variable reactor, referring to the voltage-variable capacitance of a reverse biased p-njunction. The equations in the previous section indicate that the junction capacitance depends on the applied voltage breakdown diode. Such diodes are also called Zener diodes. If the p-n junction is abrupt, the capacitance varies as the square root of the reverse bias. In a graded junction, however, the capacitance can be usually be written in the form:

0VVforVC rn

rj >>−α

When such a capacitor is used with an inductor L in a resonant circuit, the resonant frequency varies linearly with the voltageapplied to the varactor.

rr VLC

αω 1=




3.2 Diodos túnel

3.3 Fotodiodos

3.4 LEDs y láseres


4. El MOSFET








Degenerated SemiconductorsIf the conduction band electron concentrations n exceeds the effective density of states Nc, the Fermi level is no longer within the band gap but lies within the conduction band. When this occurs, the material is called degenerate n-typeThe analog case of degenerate p-type material occurs when the acceptor concentration is very high and the Fermi level lies in the valence band.


Tunnel Diode OperationIn semiconductors, a forbidden energy gap exists between the valence energy band (with maximum energy Ev) and the conduction energy band (with minimum energy Ec). In most devices, current flows when electrons are excited from the valence band to the conduction band. In tunnel diodes, electrons can tunnel through a potential barrier without changing energy. This results in complex current-voltage curves that can be explained by energy gap diagrams






3.2 Diodos túnel

3.3 Fotodiodos

3.4 LEDs y láseres


4. El MOSFET








Photodiodes Two-terminal devices designed to respond to photon absorption are called photodiodes.

Current and voltage in an illuminated Junction.If the junction is uniformly illuminated by photons with hν > Eg, and added generation rate gop (EH/cm3-s) participates in this current.The resulting current due to collection of these optically generated carries by the junction is: Iop=qAgop(Lp+Ln+W)


If we call the thermally generated current described Ith, we can add the optical generation to find the total reverse current with illumination.

I= Ith(eqV/kT -1) - Iop

I = qA(Lppn/τp+Lpnp/τn)(eqV/kT -1) - qAgop(Lp+Ln+W)

When the device is shortcircuit (V=0), the current is Iop.

When there is an open circuit across the device, and the voltage V0 = Voc is:

V0C = kT/q ln[Iop/ Ith+1]

V0C ≈ (kT/q)ln[gop/ gth] for gop >> gth

The appearance of forward voltage across an illuminated junction is known as the photovoltaic effect.



Solar CellsSince the power can be deliberated to an external circuit by an illuminated junction, it is possible to convert solar energy into electrical.The voltage is restricted to values less than the contact

potential, which in turn is generally less than the bandgapvoltage Eg/q. For silicon the voltage V0C is less than about 1 V.To utilize a maximum amount of available optical energy, it is necessary to design a solar cell with a large area junction located near to surface of the device.


PhotodetectorWhen a photodiode is operated in the third quadrant of its I-V characteristic, the current is essentially independent of voltage but is proportional to the optical generation rate. Such a device provides a useful means of measuring illumination levels or converting time-varying optical signals into electrical signals.In the most optical detection applications the detector's speedof response is critical.When the carries are generated primarily within the depletion layer W, the detector is called a depletion layer photodiode.




3.2 Diodos túnel

3.3 Fotodiodos

3.4 LEDs y láseres


4. El MOSFET








Light Emitting MaterialsAn LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a p-n junction.

When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon as it does so.


The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect bandgap materials.

The materials used for an LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.


Laser A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. When an electron and a hole are present in the same region, they may recombine by spontaneous emission—that is, the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron and hole states involved.

These injected electrons and holes represent the injection current of the diode, and spontaneous emission gives the laser diode below lasing threshold similar properties to a LED.

Spontaneous emission is necessary to initiate laser oscillation,but it is a source of inefficiency once the laser is oscillating. Under suitable conditions, the electron and the hole may coexistin the same area for quite some time (on the order of microseconds) before they recombine.



Then a nearby photon with energy equal to the recombination energy can cause recombination by stimulated

emission.





3.2 Diodos túnel

3.3 Fotodiodos

3.4 LEDs y láseres


4. El MOSFET







120Curso propedéutico de Electrónica INAOE 2009 Dr. Pedro Rosales Quintero

This tiny triangle of plastic, gold and germanium was the first solid-state amplifier

The bipolar transistor was invented in 1947 at Bell labs by Bardeen & Brattain consisting of metal points contacting a germanium substrate. It was later explained by Shockley in 1949 and the three shared the Nobel prize for their work.

Base

Collector

Emitter


Modern devices

Modern bipolar transistors (so called as both holes and electrons participate in the conduction) are based on silicon substrates with two closely coupled p-n junctions.


Introduction

The goal of a transistor is to use a small input to control a large output e.g. a small input signal to be amplified.

A bipolar transistor controls the flow of current through the device by using the base current to modify the potential profile in the channel…like water flowing over a bump in the ground…

Water flowing freely over flat ground

Water flow stopped by a bump in the ground

Water flow controlled by the height of the bump


BJTs

Semiconductor Devices, 2/E by S. M. Sze Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.

(a) Idealized one-dimensional schematic of a p-n-p bipolar transistor and (b) its circuit symbol. (c) Idealized one-dimensional schematic of an n-p-n bipolar transistor and (d) its circuit symbol.


Modes of operation



Thermal equilibrium


The device consists of a heavily doped p+ emitter, a doped n base region and a lightly doped p collector region.

All regions are grounded.

Note the widths of the depletion regions, and the position of the valence and conduction bands in relation to the Fermi level.

The results obtained for the p-n junction are equally applicable to the emitter-base and base-collector junctions here.

Note the ‘bump’ in the band diagram.


Active mode

Sem

icon

duct

or D

evic

es, 2

/E b

y S

. M. S

zeC

opyr

ight

©20

02 J

ohn

Wil

ey &

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s. I

nc. A

ll r

ight

s re

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ed.

The transistor is biased in the common-base configuration (base lead is common to both input & output circuits).

Note the widths of the depletion regions now under bias.

This is because the EB junction is forward biased and the BC junction is reverse biased.

Since the EB junction is forward biased, holes are injected or emitted from the p+

emitter into the base and electrons are injected from the n base region into the emitter.

Under ideal conditions there is no generation-recombination in the depletion region, so these two current components constitute the total emitter current.


Since the BC junction is reverse biased a small reverse saturation current will flow across the junction (as seen for p-njunctions).

If the base is narrow compared to the diffusion length, holes injected from the emitter can diffuse through the base to the BC depletion region edge, where they are swept into the collector by the reverse bias.

So holes, injected from a nearby emitter junction can result in a large current flow in a reverse-biased collector junction –transistor action.


emitter (p+) base (n) collector (p)

IE IC

IB

Current gain

IEp

IEP -ICP

ICp

ICnIEn IBB

Hole current and hole flow

Electron currentElectron flow

EnEpE III +=

CnCpC III +=

CEB III −=


Current gain in the common-base configuration

An important parameter in the characterisation of the bipolar transistor is the common-base current gain,

γEmitter efficiency (a measure of the injected hole current compared with the total emitter current)

Base transport factor (a measure of how much of the hole current injected from the emitter reaches the collector)

Tγαα =0

For a well-designed transistor the common-base current gain is close to unity (typically 0.99)

Tα

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

+=

+==

Ep

Cp

EnEp

Ep

EnEp

Cp

E

Cp

I

I

II

I

II

I

I

I0α


Current gain in the common-base configuration

We can express the collector current in terms of the common-base current gain,

CnECnEp

TCnEpTCnCpC IIII

IIIII +=+⎟⎟⎠

⎞⎜⎜⎝

⎛=+=+= 0αγγαα

The collector-base current flowing with the emitter open-circuited (IE = 0)

CBOEC III += 0α

IC is the collector current for the common-base configurationICBO is the current between the collector and the base with the emitter-base junction open


Current gain in the common-emitter configuration

The collector current for the common-emitter configuration is given by,

Since, CEB III −=

( ) CBOCBCBOEC IIIIII ++=+= 00 αα

00

0

11 ααα

−+

−= CBO

BC

III

CEOBC III += 0β

0

00 1 α

αβ−

=

The common-emitter current gain, (typically high ~99).

So a small change in the base current gives rise to a much larger change in the collector current – amplification.

BBE

EEB

NWD

NWD

min

min0 =β


Current gain in the common-emitter configuration

Since, 00 ⇒BCI

B

C

I

I=0β

t

p

B

C

I

I

ττ

β ==0

In a BJT Wb << Lp. Thus, the average excess hole spend a time τt, defined as the transit time from the emitter to the collector. This transit time is much less than the average hole lifetime in the base.


Static characteristics of the bipolar transistor

Since the bipolar transistor is simply two p-n junctions back-to-back we can apply to it the same quantitative analysis (with very similar simplifying assumptions).

We also assume that holes are injected from the forward-biased emitter into the base and that these holes then diffuse across the base region to the collector junction.

Once we can determine the minority-carrier distribution we can obtain the current from the minority-carrier gradient…


Carrier distribution in the base region

The minority-carrier distribution (holes) in the neutral n-type base region is described by the field-free steady-state continuity equation.

02

2

=−

−p

nonnp

pp

dx

pdD

τ

The solution of this for pn(x) where the width of the base region (W) is much smaller than the diffusion length (Lp) is

( ) ( ) ⎟⎠⎞

⎜⎝⎛ −=⎟

⎠⎞

⎜⎝⎛ −=

W

xp

W

xepxp n

kTqVnon

EB 101

Linear decay to zero from one boundary to the next


Emitter & Collector regions

The emitter and collector regions can be considered as semi-infinite compared with the diffusion length of the injected electrons so the minority-carrier distributions decay exponentially into the bulk of the regions to their respective equilibrium concentrations…

( ) ( ) EEEB LxxkTqVEoEoE eennxn +−+= 1

In the forward biased p+ emitter region,

( ) ( )CC LxxCoCoC ennxn −−−=

In the reverse biased p collector region,


Ideal transistor currents

If we know the minority-carrier distributions, we can calculate the various current components flowing through the device…

kTqVnop

x

npEp

EBeW

pqAD

dx

dpqDAI ≅⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

=0

The hole current injected from the emitter at x=0 in the active mode of operation e.g. is proportional to the gradient of the minority carrier concentration at that point,

If we work through all of the current components and net current flows we obtain a set of general expressions applicable to all modes of operation of the bipolar transistor…

see Sze for the full derivation


Ideal transistor currents

( ) ( )( ) ( )

( )( ) ( )( )11

11

11

22122111

2221

1211

−−−−−=

−−−=

−−−=

kTeVkTeVB

kTeVkTeVC

kTeVkTeVE

CBEB

CBEB

CBEB

eaaeaaI

eaeaI

eaeaI

Where,

⎟⎟⎠

⎞⎜⎜⎝

⎛+≡⎟⎟

⎠

⎞⎜⎜⎝

⎛≡≡⎟⎟

⎠

⎞⎜⎜⎝

⎛+≡

W

pD

W

pDeAa

W

pDeAaa

L

nD

W

pDeAa CoCnopnop

E

EoEnop22211211 ,,

The currents in the three terminals of the transistor are mainly determined by the minority-carrier concentration in the base region.

We can use these general equations to obtain the I-V characteristics of the transistor in the common-base configuration and the more often used common-emitter configuration…


3. Diodos de unión p-n 4. El MOSFETyolotli.inaoep.mx/portalfiles/AplicacionesDiodo.pdf · Curso...

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