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FUNCTIONAL MATERIALS

TITLE :

PHOTOTRANSISTORS

INSTRUCTOR’S NAME :

PROF. MADYA DR MUHD ZU AZHAN YAHYA

Prepared by:

Nor Azura bt. Che Mahmud

Nor Ashiela Nadirah bt. Jamaludin

1. HISTORY AND BACKGROUND OF PHOTOTRANSISTORS

An entirely new type of "electric eye" much smaller and sturdier than present photo-electric cells and possibly cheaper-has been invented at the Laboratories. During the past quarter century, electric eyes have found widespread use in electronics because of their ability to control electric currents by the action of light. To the layman, one type is perhaps best known for automatically opening and closing doors, but such devices have many other important uses in television, sound motion pictures, wirephotos, and still many more in industry.

One of the major advantages of the new electric eye is that it delivers very high power for a photo-electric device-in some cases enough to operate a switch directly without the preliminary amplification usually required.

Appropriately, the new device has been named the Phototransistor. The whole apparatus is housed in a tiny cylinder about as big as a 22 calibre rifle cartridge. Like the Transistor, it has no vacuum, no glass envelope, no grid, plate or hot cathode. It was invented by Dr. John N. Shive in the course of development work on Transistor-like devices.

Although the Phototransistor is still in the experimental stage, Laboratories scientists and engineers expect that, after the necessary development, it may have far-reaching significance in electronics and electrical communication. Just as the Transistor is not expected to supplant vacuum tubes, but rather to supplement them, so the Phototransistor is not expected to displace existing photo-electric cells. Because of their small size and expected long life, however, together with economies that might reasonably result from mass-production, Phototransistors should find many applications where it is not now practical to use present-day photoelectric devices.

Consideration is already being given, for example, to using them in a machine under development for toll dialing, a plan whereby a telephone operator directly dials a telephone in a distant city.

The heart of the parent device, the amplifying Transistor, is a tiny chip of germanium, a semiconductor material, against one side of which the points of two hair-thin wires are pressed, hardly two-thousands of an inch apart. The flow of very small electrical currents in one of these wires (the emitter) controls the flow of currents in the other wire (the collector) in such a way as to give signal amplification.

The Phototransistor is similar in operation to the amplifying Transistor, but it is controlled by light rather than by the electric current of the emitter. It also uses a piece of germanium but only a single collector wire. The tip of this wire rests in a small dimple ground into one side of the germanium disk. At this point the germanium disk is only three thousandths of an inch thick.

Light focussed on the opposite, un-dimpled side of the disk can control the flow of current in the wire, thus making a control device similar in function to a photo-electric cell.

The Phototransistor has a high power output for a photo-electric device and gives good response to a rapidly fluctuating light source. It is particularly sensitive to the wavelengths of light given off by ordinary incandescent light bulbs, and is well suited to operate with these easily available sources with good fidelity. Another virtue is the device’s low impedance.

1. INTRODUCTION

A phototransistor is an ordinary transistor that has been modified in two ways:

(1) there is a transparent window so that light can shine on the junctions and

(2) the structure has been modified to maximize the light capture area.

Some phototransistors have an external base lead; others do not. If there is an external base lead, it is often left floatingor connected to a high impedance bias source to bias the collector current to a specific value for the no light condition.

Base current is formed by light photons striking the junction. The phototransistor

converts received power to a collector current. The units of this transfer function are

Amperes per Watt. A more common unit that does not require that the user know the

effective surface area is milliamperes per watt per square centimeter. This unit of

sensitivity is much easier to use since all one has to know is the power density. Typical

values for sensitivity (measured at the wavelength of peak response) range from about 0.4

to 2.0 mA /(mW / cm2) for ordinary phototransistors up to about 20 mA /(mW / cm2) for

Darlington connected phototransistors.

Common phototransistors can operate with a collector current up to about 20 mA. This

means that the typical maximum light level that can be handled is about 20 mW/cm2 (this

seemingly small number is the same as 200 Watts/meter2 which is very intense) assuming

a transfer function of 1 mA / (mW / cm2). Note that these are all rough numbers and vary

from device to device.

To put things into perspective, we should confirm the following relations. Note

that the parentheses are not technically required but greatly assist in clarity.

1 mW / cm2 = 10 W / m2

1 mA / (mW / cm2) = 0.1 mA / (W / m2)

Note that the aperture of a typical 5 mm diameter phototransistor is roughly 0.2 cm2 (this

is a bit optimistic since not all of the area is useful for reception). Thus, a device with a

sensitivity of 1 mA / (mW / cm2) can also be expressed as having a sensitivity of 1 mA /

(0.2 mW) or 5 mA / mW. Depending on how a problem is structured it is either simpler to work with power density or absolute power.

2. PHOTOTRANSISTOR SYMBOL

The phototransistor symbol for use in electronic circuit diagrams is very straightforward. It is formed from the basic transistor symbol with arrows point in to it to indicate that it is light sensitive.

The phototransistor symbol often has two arrows pointing towards it, but other phototransistor symbols show a jagged arrow. Both versions of the phototransistor symbol are acceptable and understood.

Figure 1: Phototransistor symbol

The circuit symbol also has the convention arrow and directions on the emitter connection. It points inwards on a PNP phototransistor circuit symbol and outwards on an NPN phototransistor symbol.

It can be seen that the phototransistor symbol shown does not give a base connection. Often the base is left disconnected as the light is used to enable the current flow through the phototransistor. In some instances the base may be biased to set the required operating point. In this case the base will be shown in the normal way on the phototransistor symbol.

3. PHOTOTRANSISTOR CIRCUIT CONFIGURATIONS

The phototransistor can be used in a variety of different circuit configurations. Like more conventional transistors, the phototransistor can be used in common emitter and common collector circuits. Common base circuits are not normally used because the base connection is often left floating.

The choice of common emitter or common collector phototransistor circuit configuration depends upon the requirements for the circuit. The two phototransistor circuit configurations have slightly different operating characteristics and these may determine the circuit used.

3.1.Common emitter phototransistor circuit

The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.

The circuit actually acts as an amplifier. The current generated by the light affects the base region. This is amplified by the current gain of the transistor in the normal way.

Figure 2: Common emitter phototransistor circuit

3.2.Common collector phototransistor circuit

The common collector, or emitter follower phototransistor circuit configuration has effectively the same topology as the normal common emitter transistor circuit - the emitter is taken to ground via a load resistor, and the output for the circuit being taken from the emitter connection of the device.

The circuit generates an output that moves from the low state to a high state when light is detected.

Figure 3: Common collector / emitter follower phototransistor circuit

3.3.Use of base connection in phototransistor circuits

On some phototransistors, the base connection is available. Access to the base connection allows the phototransistor circuit conditions to be set more appropriately for some applications.

Figure 4: Phototransistor circuit with base resistor

High values of base resistor R_b prevent low levels of light from raising the current levels in the collector emitter circuit and in this way ensuring a more reliable digital output. All other aspects of the circuit function remain the same.

4. HOW IT WORKS

The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance, if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation.

Now, if the PN junction is the collector-base diode of a bipolar transistor, the light-induced current effectively replaces the base current. The physical base lead of the transistor can be left as an open terminal, or it can be used to bias up to a steady state level. It is the nature of transistors that a change in base current can cause a significant change (increase) in collor current. Thus, light stimulation causes a change in base current, which in turn causes a bigger increase in collector current and, considering the current gain (h_fe), a rather large increase at that.

5. PHOTOTRANSISTOR FREQUENCY RESPONSE

All silicon photosensors (phototransistors, etc.) respond to the entire visible radiation range as well as to infrared. In fact, all diodes, transistors, Darlingtons, triacs, etc. have the same basic radiation frequency response. This response peaks in the infrared range.

This is why manufacturers offer infrared-emitting diodes. Their goal is to maximize the signal-to-noise ratio, by using an emitter with the best match to the phototransistor response. However, note the response is very broad and virtually any light source will work.

Basically, a phototransistor can be any bipolar transistor with a transparent case. There are some variations provide advantages. For example, a focusing lens can be built into the case for directional sensitivity. Coatings can be applied to block some higher or lower wavelengths. The transistor itself may provide higher gain, or higher frequency, or lower capacitance, etc.

The diagram above illustrates the frequency response of silicon phototransistor junctions, along with the spectral output of an infrared LED.

6. USE OF PHOTOTRANSISTORS

Phototransistors are sensitive only to a certain range of wavelengths of light. The spectral

response factor is normalized to 1.0 at the wavelength of peak response. The sensitivity

of the phototransistor is measured at the wavelength of peak response. The response at

other wavelengths is then relative to this. There are three types of phototransistor spectral

responses.

Visible block: This phototransistor is specially made to not respond to visible

light. These types have a response factor practically zero for wavelengths shorter

than 700 nm. The response factor peaks at about 850 nm with the -3dB

wavelengths at about 780 and 940 nm. The response factor drops to less than 0.1

for wavelengths longer than about 1100 nm.

Visible: This phototransistor is made to have useful response from infrared

through the visible spectrum although the response factor in the visible spectrum

is not very high. The response factor peaks at about 800 nm with the -3 dB

wavelengths at about 680 and 920 nm. The response factor at 500 nm is down to

about 0.1 to 0.2 and the response factor at 400 nm is practically zero. The infrared

response extends to about 1100 nm where the response factor is about 0.05 to

0.15.

Blue enhanced: This phototransistor is specially made to have significantly more

response at shorter wavelengths thus making it more useful for visible light

applications. The response factor peaks at about 800 nm with -3 dB wavelengths

at about 710 and 920 nm. The infrared response extends out to about 1100 nm

where the response is less than 0.1. The response factor at 660 nm is in the

neighborhood of 0.45 and the response at 400 nm is around 0.3. This type is

ideally suited for visible light applications.

Phototransistors typically have a built-in lens made of a transparent epoxy. Depending on

how the lens is made, the phototransistor will have an angular response that ranges from

about +-6 degrees for a narrow acceptance device up to about +-50 degrees for a wide

acceptance device. Narrow angle devices are ideal for rejecting potential off axis

interference sources. Wide angle devices are ideal when the phototransistor must be

capable of detecting light from a source that is significantly off the central axis.

The collector current will be the product of the received power density, sensitivity,

spectral response factor, and angle response.

Phototransistors also have a time domain response. Typical response times of an ordinary

phototransistor with a low impedance load range from about 5 to 50 microseconds. The

response time increases as load impedance is increased. Although a Darlington

connected phototransistor is much more sensitive, its response time is much slower –

typically around 400 microseconds.

For all the circuits shown, it is very important to shield the phototransistor both optically

and electrically from interference sources. Optical shielding consists of any opaque

material covering all sides of the phototransistor except the desired light direction.

Electrical shielding consists of a metal enclosure (could be closely wrapped metal foil on

the outside of the optical shield) located as close to the phototransistor as possible and

connected to the electrical ground of the phototransistor circuit.

A key fact to remember in using phototransistors is that all phototransistors are most

sensitive in the infrared region and thus work best with infrared emitting diodes (IRED).

There is no requirement that the wavelength of the IRED exactly coincide with the

phototransistor peak response wavelength but the response factor for any deviation should

be known and accounted for in the design. Generally, any spectral response factor greater

than about 0.1 is considered useful.

7. TYPES OF PHOTOTRANSISTORS

7.1. 3.0mm Round Type Phototransistor

Features:

Low power consumption.
High efficiency.
Versatile mounting on P.C. Board or panel.
Low current requirement.
This product don’t contained restriction.

7.2. Phototransistor (infrared led,led lamp)

Features:

Normal tower type package.
Low forward voltage.
Reliable and rugged.
The product itself will remain within RoHS compliant Version.

7.3. Optocoupler, Phototransistor Output,SFH6186-3T

FEATURES
• Good CTR linearity depending on forward
current
• Low CTR degradation
• High collector emitter voltage, VCEO = 55 V
• Isolation test voltage, 5300 VRMS
• Low coupling capacitance
• End stackable, 0.100" (2.54 mm) spacing
• High common mode transient immunity
• Lead (Pb)-free component

7.4. 1206 Package Phototransistor With Inner Lens

Features :
Fast response time
High photo sensitivity
Small junction capacitance
Package in 8mm tape on 7 diameter reel
Pb free
The product itself will remain within RoHS compliant version.

8. CHARACTERISTICS OF PHOTOTRANSISTORS

8.1. Spectral Response

The output of a phototransistor is dependent upon the wavelength of incident light. These devices respond to light over a broad range of wavelengths from the near UV, through the visible and into the near IR part of the spectrum. Unless optical filters are used, the peak spectral response is in the near IR at approximately 840 nm. The peak response is at a somewhat shorter wavelength than that of a typical photodiode. This is because the diffused junctions of a phototransistor are formed in epitaxial rather than crystal grown silicon wafers.

Phototransistors will respond to fluorescent or incandescent light sources but display better optical coupling efficiencies when matched with IR LEDs. Standard IR LEDs are GaAs (940 nm) and GaAlAs (880 nm).

8.2. Sensitivity

For a given light source illumination level, the output of a phototransistor is defined by the area of the exposed collector-base junction and the dc current gain of the transistor. The collector-base junction of the phototransistor functions as a photodiode generating a photocurrent which is fed into the base of the transistor section. Thus, like the case for a photodiode, doubling the size of the base region doubles the amount of generated base photocurrent. This photocurrent (I_P) then gets amplified by the dc current gain of the transistor. For the case where no external base drive current is applied:

I_C = h_FE (I_P)

where:

I_C = collector current

h_FE = DC current gain

I_P = photocurrent

As in the case with signal transistors, h_FE is not a constant but varies with base drive, bias voltage and temperature. At low light levels the gain starts out small but increases with increasing light (or base drive) until a peak is reached. As the light level is further increased the gain of the phototransistor starts to decrease.

Figure 5 : Transistor Gain vs Light Intensity

H_FE will also increase with increasing values for V_CE. The current -voltage characteristics of a typical transistor will demonstrate this effect.

Figure 6 : Current vs Voltage Curves

For a constant base drive the curve shows a positive slope with increasing voltage. It is clear that the current gain at collector-emitter voltage V_CE2 is greater than the current gain at V_CE1. The current gain will also increase with increasing temperature.

8.3.Linearity

Unlike a photodiode whose output is linear with respect to incident light over 7 to 9 decades of light intensity, the collector current (I_C) of a phototransistor is linear for only 3 to 4 decades of illumination. The prime reason for this limitation is that the dc gain (h_FE) of the phototransistor is a function of collector current (I_C) which in turn is determined by the base drive. The base drive may be in the form of a base drive current of incident light.

Figure 7 : Photodetector Relative Linearity

While photodiodes are the detector of choice when linear output versus light intensity is extremely important, as in light intensity measuring equipment, the phototransistor comes into its own when the application requires a photodetector to act like a switch. When light is present, a phototransistor or photodarlington can be considered "on", a condition during which they are capable of sinking a fair amount of current. When the light is removed these photodetectors enter an "off" state and function electrically as open switches. How well phototransistors function as switches are covered in the next few sections.

8.4.Collector-Emitter Saturation Voltage - V_CE(SAT)

By definition, saturation is the condition in which both the emitter-base and the collector-base junctions of a phototransistor become forward biased. From a practical standpoint the collector-emitter saturation voltage, V_CE(SAT), is the parameter which indicates how closely the photodetector approximates a closed switch. This is because V_CE(SAT) is the voltage dropped across the detector when it is in its "on" state.

V_CE(SAT) is usually given as the maximum collector-emitter voltage allowed at a given light intensity and for a specified value of collector current. EG&G Optoelectronics tests their detectors for V_CE(SAT) at a light level of 400 fc and with 1 mA of collector current flowing through the device. Stock phototransistors are selected according to a set of specifications where V_CE(SAT) can range from 0.25 V (max) to 0.55 V (max) depending on the device.

8.5.Dark Current - (I_D)

When the phototransistor is placed in the dark and a voltage is applied from collector to emitter, a certain amount of current will flow. This current is called the dark current (I_D). This current consists of the leakage current of the collector-base junction multiplied by the dc current gain of the transistor. The presence of this current prevents the phototransistor from being considered completely "off", or being an ideal "open" switch.

The dark current is specified as the maximum collector current permitted to flow at a given collector-emitter test voltage. The dark current is a function of the value of the applied collector-emitter voltage and ambient temperature.

EG&G Optoelectronics stock phototransistors and photodarlingtons are tested at a V_CE applied voltage of either 5 V, 10 V or 20 V depending on the device. Phototransistors are tested to dark current limits which range from 10 nA to 100 nA.

Dark current is temperature dependent, increasing with increasing temperature. It is usually specified at 25°C.

9. APPLICATION OF PHOTOTRANSISTOR

There are some application of phototransistor such as:-

9.1. Optoisolator

It is known as an optical coupler or optocoupler, which is a semiconductor device that allows signals to be transferred between circuits or systems, while keeping those circuits or systems electrically isolated from each other. An IRED used as a controllable light source and a phototransistor function as the detector element.The light emitted detected by phototransistor form the LED and output the current. The input device is isolated from the circuit connected to the output side. Optoisolators are used in a wide variety of communications, control, and monitoring systems.

9.2. Coin Counters

It is a device that provide for counting and sorting coint of various sizes. The coins are supplied from a container for unsorted coins to a guide channel adjacent there to for guiding the coins individually and successively. The guide channel has a continouus reference edge and a base provided with openings which correspond in size to the sizes of the types of coin. A counter detects coins in the guide channel without contact. Spring loaded levers from a container and converyor belts are provided for mechanically urging the coins against the reference edge.

9.3. Camera shutter control (Mechanical Shutter)

It’s very straightforward to measure the interval of a mechanical shutter. A phototransistor is connected to a resistor and DC power supply. It is then placed in the body of the camera, behind the shutter. The camera is aimed at a source of light. The output of the phototransistor is connected to a digital oscilloscope, such as the Syscomp CGR-101. The shutter is triggered and the resultant pulse of light displayed on the oscilloscope. Compared to the speed of a mechanical shutter, the response of the phototransistor and oscilloscope are instantaneous. The oscilloscope captures the phototransistor pulse and display is an accurate rendition of the opening and closing of the shutter.

9.4. Light pens

Light pen is an input device that is used with a cathode-ray tube display to point at items on the screen or to draw new items or modify existing ones.

9.5. Printer

A printer comprising:

(a) a carriage supporting a print head and movable across print paper for enabling the print head to print on the print paper; control the position of paper

(b) a fixed frame; control the margin of paper

10. CURRENT RESEARCH

Light-sensing circuits fabricated using new phototransistors

Since the demonstration of the first organic field-effect transistor (OFET) in the early 1980s, organic semiconductors have been seen as an alternative to conventional inorganic microelectronics for various low-end applications. Properties such as solution processability allow organic semiconductor devices to be fabricated at low temperatures using high-throughput fabrication methods. This makes the realization of certain organic devices potentially easier and cheaper than their inorganic counterparts.

The field of OFETs has been growing rapidly. Bifunctional ambipolar light-sensing OFETs, or phototransistors, are a new addition to the family of organic devices, and could one day lead to the production of novel, low-cost image sensors. Unfortunately, all organic phototransistors demonstrated to date operate at relatively high voltages (>20V). This characteristic renders the technology unsuitable for portable, low-power applications. For widespread commercial use of the technology, devices with low-voltage and low-power dissipation would need to be developed.

The operating voltage of organic transistors depends on the thickness and type of gate insulator used. The application of self-assembled monolayer (SAM) molecules as gate dielectrics has proven effective in reducing the gate dielectric's thickness while retaining good insulating properties.By combining such SAM dielectrics with hole/electron (p- and n-type semiconductors) transporting organic heterojunctions, we have made organic ambipolar phototransistors with operating voltages below 3V. A unique advantage of our approach is that hole and electron transport can be individually controlled and optimized. The spectral response of these ambipolar phototransistors can also be tuned through the use of suitable semiconductor systems.

We integrated a number of low-voltage ambipolar organic phototransistors, to fabricate light-sensing optoelectronic circuits, such as complementary-like inverters. By varying the incident light's intensity, we found that the inverter's operating characteristics change. This is due to the difference of each organic phototransistor's responsivity under dissimilar biasing conditions. In particular, by maintaining the device at constant supply (V_DD) and input (V_IN) voltages, we noted that the inverter's trip voltage can be modulated and the circuit can be used as an optical sensor.

In summary, by using self-assembled monolayer gate dielectrics and p-n-type, bilayer organic heterostructures, we have demonstrated low-voltage (<3V) ambipolar phototransistors and used them to fabricate light-sensing integrated circuits. The present work is a significant step towards low-cost and low-power light-sensor arrays. Our future work in this field aims to demonstrate organic phototransistors that are able to resolve incident light color, hence paving the way toward full-color image sensors.

11. CONCLUSION

The Phototransistor has a high power output for a photo-electric device and gives good response to a rapidly fluctuating light source. It is particularly sensitive to the wavelengths of light given off by ordinary incandescent light bulbs, and is well suited to operate with these easily available sources with good fidelity. Another virtue is the device’s low impedance.The spectra lresponse factor is normalized to 1.0 at the wavelength of peak response. The sensitivity of the phototransistor is measured at the wavelength of peak response. The response at other wavelengths is then relative to this.

A key fact to remember in using phototransistors is that all phototransistors are most

sensitive in the infrared region and thus work best with infrared emitting diodes (IRED).

There is no requirement that the wavelength of the IRED exactly coincide with the

phototransistor peak response wavelength but the response factor for any deviation should

be known and accounted for in the design. Generally, any spectral response factor greater

than about 0.1 is considered useful.

12. REFERENCES

1. http://www.johnloomis.org/ece445/topics/egginc/pt_char.html

2. The Phototransistor

Bell Laboratories RECORD May 1950 Reprinted from SMEC 'Vintage Electrics vol#2 1990

3. John Labram, Donal Bradley and Thomas Anthopoulos

31 August 2009, SPIE Newsroom. DOI: 10.1117/2.1200908.1768