RF/Photonics Lab

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  jalves61 10:58 am on March 22, 2020 Permalink | Reply
  Tags: Analog Electronics ( 8 ), Transistors   

  Transistor IV curves and Modes of Operation/Biasing 

  In the field of electronics, the most important active device is without a doubt the transistor. A transistor acts as a ON/OFF switch or as an amplifier. It is important to understand the modes of operation for these devices, both voltage controlled (FET) and current controlled (BJT).

  For the MOSFET, the cutoff region is where no current flows through the inversion channel and functions as an open switch. The “Ohmic” or linear region, the drain-source current increases linearly with the drain-source voltage. In this region, the FET is acting as a closed switch or “ON” state. The “Saturation” region is where the drain-source current stays roughly constant despite the drain source voltage increasing. This region has the FET functioning as an amplifier.

  

  The image above illustrates that for an enhancement mode FET, the gate-source voltage must be higher than a certain threshold voltage for the device to conduct. Before that happens, there is no channel for charge to flow. From there, the device enters the linear region until the drain-source voltage is high enough to be in saturation.

  DC biasing is an extremely important topic in electronics. For example, if a designer wishes for the transistor to operate as an amplifier, the FET must stay within the saturation region. To achieve this, a biasing circuit is implemented. Another condition which effects the operating point of the transistor is temperature, but this can be mitigated with a DC bias circuit as well (this is known as stabilization). “Stability factor” is a measure of how well the biasing circuit achieves this effect. Biasing a MOSFET changes its DC operating point or Q point and is usually implemented with a simple voltage divider circuit. This can be done with a single DC voltage supply.  The following voltage transfer curve shows that the MOSFET amplifies best in the saturation region with less distortion than the triode/ohmic region.

  

   

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  mbenkerumass 7:13 pm on February 7, 2020 Permalink | Reply
  Tags: Semiconductors ( 14 ), Transistors   

  The Bipolar Transistor, Modes of Operation 

  The transistor is a multifunction semiconductor device that, when used with other circuit elements has the ability to produce a current gain, voltage gain and signal-power gain. The transistor is referred to as a passive device, while the diode is passive. The three basic types of transistor technologies are the bipolar transistor, the metal-oxide-semiconductor field effect transistor (MOSFET) and the junction field effect transistor (JFET). The bipolar transistor most often functions as a voltage-controlled current source.

  The Bipolar Junction Transistor

  The BJT has three separately doped regions and two pn-junctions, which are close enough to interact between each other. The BJT can either be constructed as an NPN or PNP transistor, which stands for the arrangement of positive and negatively doped regions.

  

  The main connections of a BJT transistor are referred to as the collector, base and emitter. Generally, the emitter side is doped to a higher level than the collector. The result of this is that when a supplied a voltage, the electrons will flow in the direction from the emitter to the collector. The direction of current then will be from the collector to the emitter.

  

  BJT Modes of Operation

  There exist three modes of operation for the BJT transistor. In reference to the diagram below, when the Base-Emitter voltage is zero or reverse biased, the majority of carrier electrons from the emitter will not be injected into the base. This mode where all currents in the transistor are zero is referred to as cut-off. When the Base Emitter voltage is positive (forward biasing), an emitter current is generated. As the Base Emitter voltage increases, the collector current will continue to increase until a certain point at which both the Base Emitter and Base Collector junctions become forward biased. This mode is called saturation.

  

   
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  mbenkerumass 7:00 am on January 11, 2020 Permalink | Reply
  Tags: Semiconductors ( 14 ), Transistors   

  BJT vs. FET 

  Transistors are important components that are used in a variety of applications. Some types can be used for switching, some for amplification or both. Other transistors perform exclusive tasks, such as the phototransistor, which responds to light by producing a current.

  The main premise of a transistor is that by feeding a transistor a source voltage or current (depending on the type), the transistor allows for the passage of electrons. This process is accomplished through pnp or npn semiconductor structures. The following diagrams provide a general example of the function of a transistor:

  

  

  

   

   

  Bipolar Junction Transistors (BJT) are controlled using a biasing current at the base pin. This means that they will also consume more current than other transistors such as the FET. One advantage of BJT transistors is that they offer greater output gain than an FET. However, BJT can be much larger in size than FET and for this reason, they are less popular, despite being easier to manufacture.

  

  Field Effect Transistors (FET) are voltage-controlled. For this reason they essentially draw no current and therefore do not pose a substantial load to a circuit. FETs are not as useful for gain as BJT, however if the intent is not for amplification then this is not a problem. FETs can be manufactured very small and this is important in manufacturing integrated circuits that use many transistors. FETs and especially the MOSFET subtype are more expensive to manufacture, but remain more popular than the BJT.

  

  Some FET transistor types are even constructed on the nano-scale. The FinFET for example is about 10 nm, currently used by Intel, Samsung and others.

  

  (2) http://www.learningaboutelectronics.com/Articles/Types-of-transistors.php