Wednesday, April 20, 2011



The transistor is a semiconductor device than can function as a signal amplifier or as a solid-state switch.

In a transistor a very small current input signal flowing emitter-to-base is able to control a much larger current which flows from the system power supply, through the transistor emitter-to-collector, through the load, and back to the power supply.

In this example the input control signal loop is shown in red and the larger output current loop is shown in blue. With no input the transistor will be turned OFF (cutoff) and the relay will be dropped out. When the low-level input from the PLC microprocessor turns the transistor ON (saturates) current flows from the power supply, through the transistor, and picks the relay.


Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The amount of current amplification is called the current gain, symbol hFE.


Transistor Types

There are three main classifications of transistors each with its own symbols, characteristics, design parameters, and applications. Several special-function transistor types also exist which do not fall into the categories below, such as the unijunction (UJT) transistor that is used for SCR firing and time delay applications. These special function devices are described separately.

v Bipolar transistors are considered current driven devices and have a relatively low input impedance. They are available as NPN or PNP types. The designation describes the polarity of the semiconductor material used to fabricate the transistor.

v Field Effect Transistors, FET’s, are referred to as voltage driven devices which have a high input impedance.

Field Effect Transistors are further subdivided into two classifications:

1) Junction Field Effect Transistors, or JFET’s, and

2) Metal Oxide Semiconductor Field Effect Transistors or MOSFET’s.

· Insulated Gate Bipolar Transistors, known as IGBT’s, are the most recent transistor development. This hybrid device combines characteristics of both the Bipolar Transistor with the capacitive coupled, high impedance in the MOS device.

Transistor Fundamentals

Bipolar transistors have the following characteristics:

· Bipolar transistors are a three-lead device having an Emitter, a Collector, and a Base lead.

· The Bipolar transistor is a current driven device. A very small amount of current flow emitter-to-base (base current measured in microamps - mA) can control a relatively large current flow through the device from the emitter to the collector (collector current measured in milliamps - mA). Bipolar transistors are available in complimentary polarities. The NPN transistor has an emitter and collector of N-Type semiconductor material and the base material is P-Type semiconductor material. In the PNP transistor these polarities are reversed: the emitter and collector are P-Type material and the base is N-Type material.

· NPN and PNP transistors function in essentially the same way. The power supply polarities are simply reversed for each type. The only major difference between the two types is that the NPN transistor has a higher frequency response than does the PNP (because electron flow is faster than hole flow). Therefore high frequency applications will utilize NPN transistors.

Note: Bipolar transistors are usually connected in the Common Emitter Configuration meaning that the emitter lead is common to both the input and output current circuits. The Common Collector and the Common Base configurations are sometimes used in the input or output stages of an amplifier when impedance matching is required.


  • The bipolar transistor is a three-layer semiconductor.
  • The base lead connects to the center semiconductor material of this three-layer device. The base region is dimensionally thin compared to the emitter and collector regions. Two PN (diode) junctions exist within a bipolar transistor. One PN junction exists between the emitter and the base region, a second exists between the collector and the base region.

Bipolar Transistor Symbols

  • The arrow is always on the emitter lead and points in the direction of conventional current flow (positive-to-negative). As with the diode, the nose of the arrow points to the negative, or N-Type semiconductor material, and the tail of the arrow is toward the P-Type material.
  • The arrow on the NPN points away from the base.

(Remember as NPN = Not Pointing iN.)

  • The arrow on the PNP points toward the base.

(Remember as PNP = Pointing iN pointer)


High-voltage transistors in SO1 that coexist with traditional low-voltage transistors enable the development of mixed-voltage (high-voltage and low-voltage) systems-ona- chip. The parasitic back-channel transistor, however, is a critical issue in these mixed-voltage single-chip systems. The presence of high-voltage can create a situation in which the parasitic back-channel device turns on and “shorts-out” the top device inducing functional failure of the system. An active substrate driver has been designed that automatically adjusts the substrate bias voltage to a level insuring the back-channel devices remain off. The active substrate driver should also help compensate for shifts in back channel transistor threshold voltages induced by temperature, aging, or irradiation effects

Fully integrated systems-on-a-chip that include, for example, MEMS-based sensors and actuators, analog-to-digital interfaces, and digital signal processing can require multiple supply voltages. These supplies will range from low voltages, 3.3V or lower, to high voltages, 40V or higher. High voltage transistors have been developed in SO1 that can withstand the high supply

voltage requirement . A challenge arises when implementing high voltage transistors that coexist with low voltage transistors on the same silicon substrate. These high voltage transistors can withstand drain to source voltages as high as 90V. With such high voltages, a parasitic backchannel device of low-voltage transistors can turn on. Figure illustrates a low-voltage transistor with its parasitic back-channel device.

Parasitic Back-channel Transistor

If the source or drain of the low-voltage FET is connected to a high voltage node, an inversion layer can form just above the buried-oxide (BOX) allowing current to flow. For the technology used in this work, a partially depleted SO1 process, the threshold voltage for an n-type backchannel FET (Vm-Bc) is approximately 28V and the threshold voltage for ap-type back-channel FET (V,,,,) is approximately -18V. With these threshold voltages, if an appropriate maximum power supply voltage is chosen, the substrate can be biased to a voltage insuring that both the ntype andp-type back-channel devices are always off. SO1 technology is attractive for those applications requiring radiation hardness such as space exploration. However, as radiation exposure causes positive charge to become trapped in the buried oxide, the parasitic back-channel threshold voltages will lower. If the substrate is biased with a fixed voltage, the back-channel threshold voltages can shift enough such that the back-channel V,, is greater than the new Vm-BC, causing the n-type back-channel device to turn on. To compensate for this an active substrate driver has been developed that will track the change in V and adjust the substrate voltage bias to follow this change.


The simplified architecture is illustrated in Figure 2. A 1pA current is forced through an n-type back-channel transistor (BCT). The source of this BCT is biased to -5V, insuring that its gate-source voltage is 5V greater than any other n-type BCT on the chip.

Feedback will force this V, to a level allowing 1pA current to flow in this one BCT. With a large aspect ratio , this V,, will be slightly larger than V,,, (approximately 30V), which will force the substrate voltage to be approximately 5V less than the V, of all other n-channel BCTs on the chip. With high levels of irradiation, both V?N-BCan d V,-, will shift in the same direction by approximately the same amount. The active substrate driver will shift the substrate voltage by the same amount, keeping all n-type andp-type back-channel devices off. By design, the amplifier’s input common-mode range provides ground sensing. The amplifier’s output stage can provide a high-voltage output (approximately 2V to 38V). Note also that the amplifier directly drives the substrate. Since the substrate to ground capacitance can vary significantly with buried oxide thickness and die size, the stability requirements of the amplifier take into account substrate capacitance ranging from lOpF to 100pF. A simplified schematic of the active substrate driver is shown in Figure 3. A chargepump is used to provide the -5V supply. The amplifier has a low-voltage input stage (5V) and a

high-voltage output stage (40V). The low-voltage input stage is used to reduce the number of high-voltage devices needed in the design. This minimizes the circuit’s required silicon area since the high-voltage transistors occupy significantly more area than low-voltage transistors. A p-channel source-coupled input pair is used to provide the ground-sensing capability. Devices M5, M8, and MI0 are high voltage transistors. These transistors are not self-aligned and have

poor matching; therefore, low voltage transistors M6, M7, M9, M11 and M12 are used to enhance input/output current matching in the current mirrors.


While consuming 61pA of current, the active substrate driver maintains a phase margin between 80 degrees and 95 degrees for the capacitance loads given above. With a lOpF load, the active substrate driver requires approximately 17Ops to reach its quiescent point after startup. With a lOOpF load, 220ps is required. After the startup quiescent point is reached, changes in the back-channel threshold voltages are very slow respect to time (essentially DC) and the active substrate driver can easily track to compensate for such changes. The active substrate driver enables the implementation of mixed-voltage systems-on-a-chip in partially-depleted SO1 technology.

Low-Voltage Transistor Employing a High-Mobility Spin-Coated Chalcogenide Semiconductor

In2Se3 thin films are spin-coated using a hydrazinium-precursor approach to yield thin-film transistors (TFTs, see Figure). The highly toxic and explosive solvent hydrazine, previously employed for spin-coating SnS2-xSex films, has also been replaced with a more convenient solvent mixture of ethanolamine and dimethyl sulfoxide. Low-voltage operation (< 8 V) of TFTs based on spin-coated In2Se3 yields mobilities as high as 16 cm2 V-1 s-1 and on/off ratios of 106.

Low-voltage ambipolar organic transistors for microelectronics and optical-sensing applications

Over the past decade advancement in the area of organic transistors and organic microelectronic circuits has been driven mainly by progress in materials science rather than development of new device and circuit concepts. As a result, use of organic transistors has been restricted in applications where the devices are used purely as unifunctional active elements (e.g. current switches), notably in optical displays based on fielddriven technologies,[1] and organic integrated circuits.Recently, however, new types of organic field-effect transistors (OFETs) have emerged. Specifically, OFETs with additional functionalities, i.e. bifunctional OFETs, have been designed and demonstrated with most notable examples the light-emitting (LE-OFETs) and light-sensing (LS-OFETs)[3] organic transistors. Bifunctional OFETs are of particular interest and significance since design and fabrication of a new breed of organic opto-electronic circuits, in which the electrical and optical functionalities are combined, can be

envisioned. To this end of primary importance is the development and demonstration of high-performance bifunctional organic transistors and their use in novel applications and in particular optical sensors. Herein, we report on ambipolar light-sensing OFETs based on different device architectures. In particular, we fabricate LS-OFETs based on ambipolar single semiconductor layer (i.e. films comprising blends of hole and electron transporting materials) and

heterostructure semiconductor layers (i.e. pentacene/PC60BM). By carefully tuning the ambipolar transport character of the LS-OFETs, the photosensitivity can be controlled and optimized. By going a step further and integrating LSOFETs with conventional unipolar OFETs we are able to demonstrate various optoelectronic circuits including

switches and logic (i.e. NOT, OR) gates. A unique characteristic of these opto-electronic gates is that by slightly modifying their circuit layout the input signal(s) can be designed to be either purely optical or a combination of electrical and optical. An additional advantage of this technology, and unlike any other organic-based photodetector device, is that LS-OFETs

could be integrated with the driving electronics (i.e. unipolar OFETs) side by side using the same number of processing steps, hence reducing manufacturing costs. By combining heterostructure type ambipolar semiconductor layers with ultra-thin self-assembled

monolayer (SAM) gate dielectrics, we have also been able to realize LS-OFETs operating at voltages below |1.5|V. A representative example of the operating curves of a low-voltage LS OFET is shown in Figure 1(a). Here, the transfer curves of a LS-OFET, measured in the dark and under different light intensities, are shown. The enhancement in the electron current under n-channel operation is attributed, partly, to photoinduced electron transfer from the pentacene layer to the PC60BM. To our knowledge, this is the first demonstration of photoinduced charge transfer measured in a heterostructure field-effect transistor structure.

Figure 1(b) displays the power conversion efficiency (PCE) for a low-voltage heterostructure LSOFET as a function of gate (VG) and drain (VD) potentials. It can be seen that PCE depends both on VG and VD with a maximum values reaching 0.5%. It is expected that the latter value could be improved by using new materials and/or via device structure optimization.

In summary, the first low-voltage LS-OFETs have been demonstrated. This new type of bifunctional transistors could one day be explored both for fundamental photophysical studies but also in various technological applications such as electro-optical transceivers and large area optical sensor arrays.


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