Transistors

TRANSISTOR INTRODUCTION:
The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.

Transistor terminalsTransistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.

TransistorsSemi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940’s a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties.Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: “Three Electrode Circuit Element Utilizing Semiconductive Materials.”

There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice.

JUNCTION TRANSISTORS

PNP and NPN transistorsA junction transistor consists of a thin piece of one type of semiconductor material between two thicker layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be n-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions.

The layers of an NPN transistor must have the proper voltage connected across them. The voltage of the base must be more positive than that of the emitter. The voltage of the collector, in turn, must be more positive than that of the base. The voltages are supplied by a battery or some other source of direct current. The emitter supplies electrons. The base pulls these electrons from the emitter because it has a more positive voltage than does the emitter. This movement of electrons creates a flow of electricity through the transistor.

The current passes from the emitter to the collector through the base. Changes in the voltage connected to the base modify the flow of the current by changing the number of electrons in the base. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector.

Manufacturers also make PNP junction transistors. In these devices, the emitter and collector are both a p-type semiconductor material and the base is n-type. A PNP junction transistor works on the same principle as an NPN transistor. But it differs in one respect. The main flow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.

FIELD EFFECT TRANSISTORS

Field-effect transistorA field effect transistor has only two layers of semiconductor material, one on top of the other. Electricity flows through one of the layers, called the channel. A voltage connected to the other layer, called the gate, interferes with the current flowing in the channel. Thus, the voltage connected to the gate controls the strength of the current in the channel. There are two basic varieties of field effect transistors-the junction field effect transistor(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). Most of the transistors contained in today's integrated circuits are MOSFETS's. 




Bipolar Junction Transistor (BJT):

A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an npn transistor, a thin and lightly doped p-type material is sandwiched between two thicker n-type materials; while in a pnp transistor, a thin and lightly doped n-type material is sandwiched between two thicker p-type materials. In the following we will only consider npn BJTs.
transistors1.gif
transistorBJT1.gif
In many schematics of transistor circuits (especially when there exist a large number of transistors in the circuit), the circle in the symbol of a transistor is omitted.
transistorBJT2a.gif
transistorBJT2b.gif
The three terminals of a transistor are typically used as the input, output and the common terminal of both input and output. Depending on which of the three terminals is used as common terminal, there are three different configurations: common emitter (CE), common base (CB) and common collector (CC). The common emitter (CE) is the most typical configuration:
transistors2.gif

  • Common-Base (CB) Two voltages $V_{BE}$ and $V_{CB}$ are applied to the emitter $E$ and collector $C$ of the transistor with respect to the common base $B$. Te BE junction is forward biased while the CB junction is reverse biased.
    CB.gif
    CBnpn.gif
    The behavior of the npn-transistor is determined by its two pn-junctions:

    • The forward biased base-emitter (BE) junction allows the free electrons to flow from the emitter through the PN junction to form the emiiter current $I_E$.
    • As the p-type base is thin and lightly doped, most electrons from the emitter $\alpha
 I_E$ (e.g. $\alpha \approx 0.99$) go through the base to reach the collector-base junction, only a small number of the electrons are combined with the holes in base to form the base current $I_B=(1-\alpha)I_E$.
    • The reverse biased collector-base junction blocks the majority carriers (holes in the p-type base, electrons in n-type collector), but lets through the minority carriers, electrons in base and holes in collector, including most of the electrons from the emitter $I_{CN}=\alpha I_E$, and the reverse saturate current of the CB junction $I_{CP}=I_{CB0}$,
    The relationship between the output $I_C$ and the input $I_E$ can be found as:
    \begin{displaymath}I_C=I_{CN}+I_{CP}=\alpha I_E+I_{CB0}\approx 
\alpha I_E \end{displaymath}

    The base current $I_B$ is the small difference between two nearly equal currents $I_E$ and $I_C$:
    \begin{displaymath}I_B=I_E-I_C\approx I_E-\alpha I_E=(1-\alpha)I_E 
\end{displaymath}

  • Common-Emitter (CE) Two voltages $V_{BE}$ and $V_{CE}$ are applied to the base $B$ and collector $C$ of the transistor with respect to the common emitter $E$. The BE junction is forward biased while the CB junction is reverse biased. The voltages of CB and CE configurations are related by:
    \begin{displaymath}V_{CE}=V_{CB}+V_{BE} \end{displaymath}

    CE.gif
    CEnpn.gif
    The input current is $I_B$, $I_E=I_B+I_C$, and the output current is
    \begin{displaymath}I_C=\alpha I_E+I_{CB0}=\alpha (I_C+I_B) + 
I_{CB0}
\approx \alpha (I_C+I_B) \end{displaymath}

    Solving for $I_C$, we get the relationship between the output $I_C$ and the input $I_B$:
    \begin{displaymath}I_C=\frac{\alpha}{1-\alpha} 
I_B+\frac{1}{1-\alpha} I_{CB0}
=...
...a I_B +(\beta+1)I_{CB0}=\beta I_B + I_{CE0}
\approx \beta I_B \end{displaymath}

    Here $\beta\stackrel{\triangle}{=}\alpha/(1-\alpha)$ is the current-transfer ratio for CE (e.g., $\alpha=0.99$ and $\beta=99$), and $I_{CE0}=(\beta+1) I_{CB0}$ is the reverse saturation current between collector and emitter. In summary:
    \begin{displaymath}\left\{ \begin{array}{l} I_C=\beta I_B\ 
I_E=I_C+I_B=(\beta+1) I_B
\end{array} \right.
\end{displaymath}