Amplifier design system and diode analysis

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BIOE3300-Lecture_Series_5_Diodes_SP2020.pptx

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Diodes

BIOE 3300 – Biomedical Electronics

(Chapter 10)

Applications

Signal Processing

(e.g.Rectification)

Power Supplies

AC to DC Conversion

Voltage supply regulation

Electronic Device Protection

Surge Protection (e.g. ECG/ Defibrillator)

Light Sources

Light Emitting Diodes

Laser Diodes

What are diodes?

Semiconductor based electronic components that only permit current to flow in a given direction

More specifically, diodes allow for the manipulation of normal circuit behavior

Cause currents to deviate from expected path

What Are Diodes Made Out Of?

Semiconductor material: normally silicon (Si) or Germanium (Ge) doped with select impurities

n-type material: electrons move freely

p-type material: impurities result in positive charged particles known as holes

A natural barrier exists across the pn junction which holds free electrons on n-side and holes on p-side

What Are Diodes Made Out Of?

Si and Ge are both group 4 elements, meaning they have 4 valence electrons.

Their structure allows them to grow in a shape called the diamond lattice.

In the diamond lattice, each atom shares its valence electrons with its four closest neighbors.

This sharing of electrons is what ultimately allows diodes to be built.

When dopants from groups 3 or 5 (in most cases) are added to Si or Ge. It changes the properties of the material so we are able to make the P- and N-type materials that become the diode.

The diagram above shows the 2D structure of the Si crystal. The light lines represent the electronic bonds made when the valence electrons are shared. Each Si atom shares one electron with each of its four closest neighbors so that its valence band will have a full 8 electrons.

The Periodic Table of Elements

N Type Material

N-Type Material:

When extra valence electrons are introduced into a material such as silicon, an n-type material is produced.

The extra valence electrons are introduced by putting impurities or dopants into the silicon.

The dopants used to create an n-type material are Group V elements. The most commonly used dopants from Group V are arsenic, antimony and phosphorus.

The 2D diagram to the left shows the extra electron that will be present when a Group V dopant is introduced to a material such as silicon. This extra electron is very mobile.

P Type Material

P-Type Material:

P-type material is produced when the dopant introduced is from Group III.

Group III elements have only 3 valence electrons and therefore there is an electron missing.

This creates a hole (h+), or a positive charge that can move around in the material.

Commonly used Group III dopants are aluminum, boron, and gallium.

The 2D diagram to the left shows the hole that will be present when a Group III dopant is introduced to a material such as silicon. This hole is quite mobile in the same way the extra electron is mobile in a n-type material.

Physical Fabrication

Thermal Diffusion

Ion Implantation

Diffusion

High temperature diffusion has historically been one of the most important processing steps used in the fabrication of semiconductor material

Diffusion is the redistribution of atoms from regions of high concentration to low concentration

It occurs at all temperatures, but is promoted at higher temperatures.

Ion Implantation

As of today, ion implantation has become the more dominant doping method

Very controllable process capable of producing high quality and uniform semiconductor material.

The PN Junction

Steady State

- - - - -

+ + + + +

Metallurgical Junction

Space Charge Region

ionized acceptors

ionized donors

E-Field

h+ drift

h+ diffusion

e- diffusion

e- drift

When no external source is connected to the pn junction, a natural barrier exists across the pn junction which holds free electrons on n-side and holes on p-side

Metallurgical Junction: The interface where the p- and n-type materials meet.

Space Charge Region: Also called the depletion region. This region includes the net positively and negatively charged areas. The space charge region does not have any free carriers. The width of the space charge region is denoted by W in pn junction formula’s.

(+/-): Represent the amount of negative and positive doping in number of carriers per centimeter cubed. Usually in the range of 1015 to 1020.

Ideal Diodes

The ideal diode acts as a short circuit for forward currents and as an open circuit with reverse voltage applied.

+ VD -

Circuit Analysis Using the Ideal Diode Model

Generalized Analysis of Ideal-Diode Circuits

Assume a state for each diode, either on (i.e., a short circuit) or off (i.e., an open circuit). For n diodes there are 2n possible combinations of diode states.

2. Analyze the circuit to determine the current through the diodes assumed to be on and the voltage across the diodes assumed to be off.

Generalized Analysis of Ideal-Diode Circuits

3. Check to see if the result is consistent with the assumed state for each diode. Current must flow in the forward direction (i.e., iD>0) for diodes assumed to be on. Furthermore, the voltage must be more positive at the cathode than the anode (i.e., reverse bias vD<0) for diodes assumed to be off.

4. If the results are consistent with all the assumed diode states, the analysis is finished. Otherwise, return to step 1 and choose a different combination of diode states.

A more realistic diode model ! Transconductance Curve

Ideal Actual

This is known as the transconductance curve

A more realistic diode model! Forward Bias Region

Forward Bias Voltage (Vf): In practical diodes, current will only begin to flow from the anode to cathode once a barrier potential is overcome. Therefore, as the applied voltage increases, current starts to flow across the junction once Vf has been reached. The barrier potential varies for different materials.

+ -

A more realistic diode model ! Forward Bias Region

Another difference between an ideal diode and actual diode is how current flows in the forward bias direction

i=-Vf/Ra

A more realistic diode model! Reverse Bias Region

Under reverse bias in the ideal case, the diode acts as an “open circuit” model when VD<0.

In an actual sense, the ideal model is close to the actual behavior

The only difference is reverse bias occurs when VBR<VD < VF and there is a small leakage current, Is (saturation current) which flows under reverse bias conditions.

This can often be ignored as it is in the nA to pA range

This saturation current is made up of electron-hole pairs being produced in the depletion region. Saturation current is sometimes referred to as scale current because of it’s relationship to junction temperature.

Ideal

Actual

VBR VF

A more realistic diode model! Reverse-Breakdown Region

In the ideal model, the Reverse-Breakdown Region is ignored

Ideal Actual

A more realistic diode model! Reverse-Breakdown Region

VBR

|VBR|

A more realistic diode model! Reverse-Breakdown Region

Diodes normally enter the reverse-breakdown region when VD << 0

At this point, the diode current reverses (i.e. the normal or ideal diode characteristics breakdown.)

For most applications that use standard/normal diodes, the reverse breakdown region does not need to be considered as the reverse-breakdown voltage is on the order of -100 to -1000V

However, there are some diodes meant to operate in this region

Zener Diodes

Practical Diode Models

VBR

|Is|

|VBR|

Example: Diode Modeling

Generalized Analysis of Practical-Diode Circuits

1. Assume a state for each diode, either on Forward Bias Model), off (i.e., Reverse Bias Circuit Model), or Reverse-Breakdown Model. For n diodes there are 3n possible combinations of diode states.

2. Analyze the circuit to determine the current through and/or voltage across the diodes for the assumed state

Generalized Analysis of Practical-Diode Circuits

3. Check to see if the result is consistent with the assumed state for each diode.

- Forward Bias: Current must flow in the forward direction (+) for diodes assumed to be in this region (ID>0) and VD>VF

-Reverse Bias (diode off): VBR< VD <VF

-Reverse Breakdown: Current must flow in the reverse direction (-) for diodes assumed to be in this region (ID<Is) and VD<VBR

4. If the results are consistent with all the assumed diode states, the analysis is finished. Otherwise, return to step 1 and choose a different combination of diode states.

Generalized Analysis of Practical-Diode Circuits

For example, in a circuit that contains 2 diodes, there are 9 possible scenarios with only 1 being correct (static)

Forward Bias (FB)

Reverse Bias (RB)

Reverse Breakdown (RBR)

Diode 1	Diode 2
FB	FB
RB	FB
RBR	FB
FB	RB
RB	RB
RBR	RB
FB	RBR
RB	RBR
RBR	RBR

Diodes from a Physics Standpoint

The transconductance curve is characterized by the following equation:

iD is the current through the diode

IS is the saturation current

vD is the applied biasing voltage

VT is the thermal voltage and is approximately 26 mV at room temperature.

n is the emission coefficient (takes a value between 1 and 2 depending on device structure)

For a silicon diode, n is around 2 for low currents and goes down to about 1 at higher currents

Shockley Equation

Transconductance curve

Types of Diodes

Normal/Standard PN-Junction Diodes

Zener Diodes

Schottky Diodes

Shockley Diodes

Light Emitting Diodes (LEDs)

Laser Diodes (LDs)

Photodiodes

Normal/Standard PN-Junction Diodes

Used to allow current to flow in one direction while blocking current flow in the opposite direction. The pn junction diode is the typical diode used in many applications

Schematic Symbol for a PN Junction Diode

Representative Structure for a PN Junction Diode

Normal/Standard PN-Junction Diodes

Transconductance Curve

Will vary depending on diode model

Sample Specification Sheet

Component

Illustration

(2)

(3)

(4)

(5)

Zener Diodes

Are specifically designed to operate under reverse breakdown conditions. These diodes have a very accurate and specific reverse breakdown voltage.

Often more positive than standard PN diodes (but still negative)

Zener diodes are available for a range of breakdown voltages

Useful in applications for which a constant voltage in the breakdown region is desired

Application(s): Voltage Regulators, Clipper and Clamp circuits

Schematic Symbol for a Zener Diode

Common Zener Diode Transconductance Response Curve

Zener Diode Specification Sheet

Schottky Diodes:

These diodes are designed to have a very fast switching time which makes them a great diode for digital circuit applications. They are very common in computers because of their ability to be switched on and off so quickly.

Schematic Symbol for a Schottky Diode

Shockley Diodes:

The Shockley diode is a four-layer diode while other diodes are normally made with only two layers. These types of diodes are generally used to control the average power delivered to a load.

Schematic Symbol for a four-layer Shockley Diode

Light-emitting diodes are designed with a very large electronic bandgap so movement of carriers across their depletion region emits photons of light energy.

(Ultraviolet (UV) – Visible - Near Infrared (NIR)

This band gap is an energy range in a solid where no electron states exist. In a graph of the electronic band structure of a solid, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band.

Schematic Symbol for a Light-Emitting Diode

The arrows in the LED representation indicate emitted light.

Light-Emitting Diodes:

Wavelength (nm)

Atomic Structure Electron Orbitals

Lower bandgap LEDs (Light-Emitting Diodes) emit infrared radiation, while LEDs with higher bandgap energy emit visible light.

E: bandgap in J

h: Planck’s Constant 6.626e-34 Js

c: Speed of Light 3e8 m/s

: wavelength (m)

1 J=6.24e18 eV

Reduced Equation

E: bandgap in eV

: wavelength (nm)

Many stop lights and illumination lights are now starting to use LEDs because they are extremely bright, have very long lifetimes, and are more energy efficient.

Medical LED and Laser Diode Applications

Chemical Analysis

Spectrometry

Blood Gas Sensors

pulse oximeters

Therapeutic Devices (Infrared)

pain management/blood flow

Tissue Ablation/Cauterization

Bloodless surgery

Noninvasive Optical Biopsy

Optical Coherence Tomography (OCT)

Tattoo Removal

Photodiodes:

While LEDs emit light, Photodiodes are sensitive to received light. They are constructed so their pn junction can be exposed to the outside through a clear window or lens.

In Photoconductive mode the saturation current increases in proportion to the intensity of the received light. This type of diode is used in CD players.

In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light, the diode generates a voltage/current and can be used as an energy source and/or light detector. This type of configuration is used in the production of solar power and many optical biosensors.

Schematic Symbols for Photodiodes



Diode Applications

Rectifiers

Half-Wave (+/-): For use in selectively clipping or removing either positive or negative components from a signal

Full-Wave (+/-): For use in inverting the polarity of either the positive (-) or negative (+) components in a signal.

Positive Half-Wave

vin(t)

Realistic Diode Half-Wave Rectifier

Applications: Power Supplies, Battery Chargers, Signal Processing

How would a negative half wave rectifier be constructed?

Rectifiers: Power Supplies, Battery Charging Circuits, Demodulation, etc….

Rectifiers Circuits and Batteries

A battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy.

There are two main types of batteries: primary batteries (disposable batteries), which are designed to be used once and discarded, and secondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times

Rechargeable batteries are very common in portable instrumentation and medical devices

Prefer to use a convenient power source to recharge (e.g. AC)

Rectifiers Circuits and Batteries

At tR (previous slide), we want to recharge the battery.

What would happen if we connect an AC source to the battery using a resistor to control the current?

The battery would eventually drain to 0V, as t2 > t1

Obviously, we do not want this to happen. How can it be prevented?

Half-Wave Rectifier Application: Battery Recharger

Current flows into the battery whenever the instantaneous ac source voltage is higher

than the battery voltage.

The resistor limits the current magnitude so not to damage the rechargeable battery

On the negative cycles (or when vs(t)<Vb), the diode is off (open circuit) and current is zero.

Therefore, current only flows in the direction that will charge the battery and not cause a drain.

Current flows whenever the instantaneous ac source voltage is higher than the battery voltage.

A resistor is added to limit the current magnitude so not to damage the rechargeable battery

On the negative cycles, the diode is off and current is zero. Therefore current only flows in direction that will charge the battery and not cause a drain.

AC to DC Adapters/Converter

An AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in a case similar to an AC plug.

These adapters are used with electrical devices that require DC power but do not contain internal components to derive the required voltage and power from the main AC power delivered to our homes or businesses.

Use of an external power supply allows portability similar to battery-powered equipment without the added bulk of internal power components

Half-Wave Rectifier with Smoothing Capacitor (ACDC Conversion)

Full Wave Rectifier (Positive)

EXAMPLE

Smoothing Capacitor

Full Wave Rectifier (Concept)

POSITIVE CYCLE OF vin(t) (i.e., vin(t) >0)

Diode C: “on” or “off”?

“off” because VC+ < VC-

Diode A: “on” or “off”?

“on” because iA>0

Diode D: “on” or “off”?

“off”? because VD+ < VD-

Therefore, current will

flow through RL toward ground

Diode B: “on” or “off”?

“on” because iB>0

During the positive cycle of the input (i.e., vin(t) > 0), the current flows from vin(t)  A  RL  B  vin(t), where vo(t) > 0 (i.e., vo(t) tracks vin(t) for vin(t) > 0)

Full Wave Rectifier (Concept)

NEGATIVE CYCLE OF vin(t) (i.e., vin(t) < 0)

Diode B: “on” or “off”?

“off” because VB+ < VB-

Diode D: “on” or “off”?

“on” because iD>0

Diode A: “on” or “off”?

“off”? because VA+ < VA-

Therefore, current will

flow through RL toward ground

Diode C: “on” or “off”?

“on” because iC>0

During the negative cycle of the input (i.e., vin(t) < 0), the current flows from vin(t)  D  RL  C  vin(t), where vo(t) > 0 (i.e., vo(t) tracks -vin(t) for vin(t) < 0)