Transistors

1.3.1 Common Features of Transistors

The word “transistor” was coined to describe the operation of a “transfer resistor”. First, a point-contact transistor was produced. It included two diodes placed very closely together such that the current in either diode had an important effect upon the current in the other diode. By the proper biasing the diodes, it was possible to obtain power amplification of electric signals between the diode common layer, which lead was called abase, and other layers. One of the leads of this device was designated as anemitter, the corresponding diode was biased in the forward direction, the other was acollector and its diode was biased in the reverse direction. Power amplification was obtained by virtue of the fact that the few variations in the base current caused a large variation in the emitter-collector current. The point-contact transistor had certain drawbacks:

- high sensitivity to temperature, either ambient or self-generated;

- production problems, i.e., a difficulty to reproduce the same electrical qualities in close tolerance for mass production;

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Intensive research has been done to diminish or remove these drawbacks. As a result, developers have produced semiconductor materials that are not so sensitive to temperature, inexpensive, operate at high frequencies, have low power dissipation, and internal noise of the transistor. A device, which is more stable both mechanically and electrically, has been constructed by forming junctions rather than point contacts. General classes of transistors that are used in electronics today are as follows:

- bipolar junction transistors (BJT); - junction field-effect transistors (JFET);

- metal-oxide semiconductor field-effect transistors (MOSFET) up to some kilowatts, hundreds amperes, and tenths gigahertz;

- insulated-gate bipolar transistors (IGBT) up to thousands of kilowatts, some kiloamperes, and hundreds kilohertz.

More powerful devices have been built on the thyristors though IGBTs have the potential to replace them.

1.3.2 Bipolar Junction Transistors (BJT)

A junction transistor has three doped regions as shown in Fig. 1.18. The bottom region is the emitter, the middle region is the base, and the top one is the collector. This particular device is an npn transistor. Transistors are also manufactured as pnp transistors, which have all currents and voltages reversed from their npn counterparts. They may be used with negative power supplies and with positive once in an upside-down configuration. Fig. 1.18 Collector (n) Base (p) Emitter (n)

– – –

+ + +

– – –

+ + +

– – –

+ + +

– – –

+ + +

– – –

+ + +

– – –

+ + +

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Structure. A transistor has two junctions on opposite sides of a thin slab of semiconductor crystal − one between the emitter and the base, and another between the base and the collector. Because of this, a transistor is similar to two back-to-back connected diodes. The emitter and the base form one of the diodes, while the collector and the base form the other diode. From now on, we refer to these diodes as the emitter diode (the top one) and the collector diode (the bottom one). Accordingly, a bipolar transistor

has three terminals: a collector, an emitter, and a base. Before diffusion has occurred, the depletion layers with the barrier potentials are at both junctions. The most common low-frequency transistor is the alloy type. The collector junction is made larger than the emitter one to improve the collector action.

After connecting of external voltage sources to the transistor, some new phenomena will occur. For normal operation, the emitter diode is forward biased and the collector diode is reverse biased (Fig. 1.19). Under these conditions, the emitter sends free electrons into the base. Since the base is lightly doped and thin, most of these free electrons pass through the base to the collector, which collects, or gathers, electrons from the base.

Basic topologies. Fig. 1.20 presents schematic symbols of npn and pnp transistors. There are three different currents in a transistor: emitter current I_E, base current I_B, and collector current I_C. Accordingly, the three

basic schemes of the transistor connection in electronic circuits are usually discussed: common emitter

(CE) connection, common base (CB) connection, and common collector (CC) connection.

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Download free eBooks at bookboon.com 40 Uout Uin Uout Uin Uout Uin Fig. 1.21 B E C B E C Fig. 1.20 + + – – U UCE RB UBE Fig. 1.22 RC UC IB IC + – – + E C UB B UC Fig. 1.19 n p n IE

In the first, shown in Fig. 1.21, the common node is an emitter and it is known as a grounded emitter circuit. Here, the input signal drives the base whereas the output signal occurs between the collector and the emitter. It is the most popular circuit because of its high flexibility and gain.

The second variant is a grounded base circuit because it has a common base node. Here, the input signal drives the emitter whereas the output signal occurs between the collector and the base. This connection is known as a low-gain circuit with high frequency selectivity Q. The common node of the third circuit is a collector. That is why this is a grounded collector circuit. Usually, this circuit is called also an emitter follower. Its input signal drives the base, and the output signal comes from the emitter. When connected between the CE transistor device and the small load resistance, the emitter follower can drive the small load under the stable voltage gain with no overloading and little distortion.

Beta andalpha gains.In Fig. 1.22, the common side, or groundside of each voltage source is connected to the emitter. Because of this, the circuit is an example of a CE connection with the base circuit to the left and the collector circuit to the right. Current from the energy supply enters the collector, flows through the base, and exits via the emitter. The collector current approximately equals to the emitter current. The base current is much smaller, typically less than 5 percent of the emitter current. The ratio of the collector current I_C to the base current I_B is called a current gain or static gain or dc beta of the transistor, expressed as

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This parameter is also called a forward-current transfer ratio. It is the main property of the transistor in the CE connection. For small-signal transistors, this is typically 100 to 300. The current gain of a transistor is an unpredictable quantity and may vary as much as a 3:1 range when changing in the temperature, the load, and from one transistor to another.

The dc alpha of a transistor indicates how close in value the collector current and the emitter current are; it is defined as

a ₌ I_C / I_E.

Alpha gain is the main property of the transistor in the CB connection. Consequently, a formula of alpha in terms of betais

a = b / (b + 1) and vice versa

b = a / (1 – a).

Alpha gain is always less than unity and is near unity. Both gains depend on the signal frequency. In the data sheets, the limit frequency is shown, which reduces dc beta to unity.

Input characteristic. Fig. 1.23 displays an input characteristic or transconductance (base) curve of the CE connection. This graph of I_B versus U_BE looks like the graph of an ordinary rectifier diode. The maximum value of U_BE is limited in the transistor data sheets.

U

I

breakdown

saturation

U

I

active region

_off

on

U

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Output characteristics. Fig 1.24 shows the output characteristic known here as a collector curve that is the collector current I_C as a function of the collector-emitter voltage U_CE. The collector curve has three distinct operating regions. First, there is the most important region in the middle called an active region. When the transistor is used as an amplifier, it operates in the active region. Another region is a breakdown region. The transistor should never operate in this region because it is very likely to be destroyed. The rising part of the curve, where U_CE is between 0 and approximately 1 V is called a saturation region or

ohmic region. Here, the resistance of the device is very low and it is fully open. When it is used in digital circuits, the transistor usually operates in this region in a long time.

The idealized output characteristic of BJT operating as a switch is given in Fig. 1.24 as well.

Fig. 1.25 illustrates the set of collector characteristic curves under the different values of I_B. The bottom curve when there is no base current limits a cutoff region of the transistor where resistance is high, and the small collector current is called a collector cutoff current. As usual, a designer never allows voltage to get close to the maximum breakdown voltage U_CE, which is given in the data sheets for the transistor with an open base (I_B = 0).

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