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Bipolar Junction Transistor and DC Biasing

Introduction | Definitions and Terms | Bipolar Junction Transistor (BJT) | Operation of an npn and pnp Transistor | BJT Collector Characteristic Curves | BJT Switch Characteristics | BJT Amplifier Characteristics | Applications of a Transistor | Transistor Biasing Circuit

Introduction

This week, you will learn about three kinds of terminal devices, or transistors. Transistors are an essential component of complex integrated circuits, and they ushered in the design and manufacturing of the lightweight, inexpensive electronic devices we now take for granted.

In the next two weeks, you will explore two types of transistors:

• Bipolar Junction Transistors (BJT)
• The structure and operation of a BJT
• The collector characteristic curves and biasing of a BJT
• Junction Field Effect Transistors (JFET)
• The structure and operation of an FET

Without the BJT and FET devices, we would not have CT and MRI Scanners or the Global Positioning Systems (GPS) we have come to depend on today. A BJT device is formed using two p-n junctions connected back-to-back. The term bipolar is attributed to its simultaneous operation of both electrons and holes. As you will see in this week's lecture and labs, the BJT is a minority carrier device. Most of the current flow in the transistor is due to the flow of minority carriers. An FET device is governed by the principle that you can alter the conductivity of the semiconductor when you apply an electrical field.

Definitions and Terms

The following terms are used in this week's lecture:

Term	Definition
pnp transistor	An n-type material sandwiched between two p-type materials
npn transistor	A p-type material sandwiched between two n-type materials
DC current gain or common emitter current gain	Ratio of collector current to base current Symbol: (Greek letter Beta)
Large signal current gain or common base current gain	Ratio of collector current to emitter current Symbol: (Greek letter Alpha)
Cut-off region	Transistor that acts like an open switch
Saturation region	Transistor that acts like a closed switch. Both the B-E and C-B junctions are forward-biased.
Active region	Transistor that can be used as an amplifier in which the B-E junction is forward-biased and the C-B junction is reverse-biased

Bipolar Junction Transistor (BJT)

Typically, a junction transistor consists of a silicon (or germanium) crystal in which layers of p- and n-type semiconductors are etched. In the process of etching, each layer is built on top of the previous layer. The three layers of any transistor are:

A BJT device is formed by using two p-n junctions connected back-to-back. In a BJT, there are two types of junction transistors:

The key to fabricating a BJT is to make the middle layer (the base) as thin as possible without shorting the outer layers, i.e., the collector and emitter. Figures 1.1 and 1.2 represent the structure and symbol of an npn and a pnp transistor in a BJT.

This transistor is constructed as follows:

Note: The collector (C) and emitter (E) regions are always constructed of the same type of material.

Figures 1.1a and 1.2a show that within the structure of each BJT, the npn and pnp transistors have two p-n junctions. Figures 1.1b and 1.2b illustrate, through the symbols of the respective npn and pnp transistors, that the direction of the arrow at each of the emitter terminals is different and corresponds to the respective p-n junction. The arrow follows the direction of the flow of conventional current when the emitter-base junction is forward-biased. You can therefore conclude that the emitter current flows out of the emitter in an npn transistor and flows into the emitter in a pnp transistor.

Note: In both npn and pnp transistors, assume the emitter (E), base (B), and collector (C) currents are positive. Practically speaking, if you do not apply voltage to a silicon transistor, it has no use. Therefore, in the next section, we will apply bias voltages to an npn transistor and analyze the result. We will apply and compare the results to the pnp transistor as well.

Operation of an npn and pnp Transistor

In the following example, you will learn how a transistor operates in contrast to a diode. We start with Figure 1.3, which shows a small forward-bias voltage, V_EE, applied across the emitter-base junction. A large reverse-biased voltage, V_CC, is applied across the collector-base junction.

Based on what you know about p-n junctions, you know that since the emitter-base junction is forward-biased, the electrons from the emitter jump across the junction into the base region (B). (Figure 1.3 indicates the direction of the conventional current, and the electron flow will be in the opposite direction.) The momentum of the electrons carries them across this junction and into the collector (C). Despite the reverse-bias of the collector-base junction, the large positive voltage V_CC at the collector end attracts electrons out of the collector towards the positive terminal of the V_CC.

Note: In the p-type region, the electrons are minority carriers and cross the junction as leakage current. This is typically specified in the data sheet as ICBO. Because of its negligible effects, this current is ignored, and most of the current is diverted through the collector (C).

Figure 1.4 shows how the operation and biasing of a pnp transistor is very similar to the npn transistor. For example, in both the pnp and the npn transistors, the relationship between the three currents is the same. The only difference is the minority carriers in a pnp transistor are the holes, and the conventional current enters the diode through the emitter.

The biasing of an npn and a pnp transistor highlights how a transistor operates in contrast to a diode. In a transistor,

• a small base bias current controls the large collector current and
• if a varying base current is applied, the collector current will vary even more.

In summary, a transistor not only acts as a device that can control current but also as a device that can amplify it. Thus, transistors are commonly used in amplifiers.

Now, let's take the operation of a BJT transistor one step further by applying Kirchhoff's Current law. When you apply Kirchhoff's Current law, you will notice the relationship between the emitter current (I_E), the base current (I_B), and the collector current (I_C) is I_E = I_B + I_C.

The base region of a transistor is lightly doped; hence, the base current is very small compared to the emitter and collector currents.

The following current gains are specified on a data sheet and used to analyze a transistor:

• The dc current gain of a transistor is the ratio of the collector current to the base current:
• The ratio of collector current to emitter current:

Note: Typically, ranges from 0.95 - 0.99 and is always less than 1, and ranges from 100 to 300.

The following Table 1.1 summarizes the relationship between the current gains and the currents.

Current Gain Formula	Current Gain
IB+IC	IE
IC/IE
IC/IB
	,
	IE

Table 1.1

Review your knowledge on npn and pnp transistor

Given an npn BJT transistor circuit with =100 and a collector current IC = 25mA, determine the emitter current IE, base current IB, and .   (Click to view Answer) Using the relationship = IC/IB , we get IB = 0.25mA. Also, recall IE = IC + IB, therefore, IE = 25.25mA,  = IC/IE = 0.99.

Another important characteristic to note from the biasing circuit is the voltage drop across the base emitter junction of the transistor. Since this p-n junction is forward-biased, there is approximately a 0.7V drop across the junction for a silicon npn and a -0.7V drop for a silicon pnp transistor.

Transistor bias circuits always have collector and base resistors included to ensure that the currents stay within safe limits to prevent any damage. Figure 1.5 illustrates a biasing circuit with these resistors for an npn transistor. The data sheets provided by the manufacturer list various important ratings and parameters for the BJT, including the following:

VCBO	The maximum reverse-biased voltage than can be applied between the collector and base terminals when the emitter terminal is left open. Typical values - 120V to 160V.
VCEO	The maximum voltage that can be applied between the collector and emitter terminals when the transistor is turned off (the base terminal is left open). Typical values - 25V to 100V.
VEBO	The maximum voltage between the emitter and base terminal with the collector terminal left open. This voltage is typically 0V or a very small value since this junction is almost never reverse-biased and cannot handle very high reverse voltages. Typical values 5V to 10V.
IC	The maximum allowable continuous collector current a transistor can withstand. For BJTs designed for small signal applications, IC is less than 1A. BJTs designed for large signal applications can withstand surge currents two to three times their continuous current rating. Power BJTs have IC of 1A or more.
PD	Maximum power that can safely be dissipated by the transistor without overheating. This is calculated as the product of collector current and the collector-emitter voltage, i.e., PD= IC*VCE.

Refer to the document link provided here to learn how to test and identify the terminals of a transistor.

BJT Collector Characteristic Curves

Now that you are aware of the construction of a BJT circuit, let us analyze how the example npn transistor circuit in Figure 1.5 operates. Whether you have an npn or a pnp BJT transistor, there are three operating regions:

These regions can be identified by varying the base-bias voltage, V_BB, and the collector-bias voltage, V_CC, and measuring the resulting voltages and currents. When you vary the base-bias voltage and the collector bias voltage, you produce a set of curves. These curves are referred to as the collector characteristic curves, and they are pictured in Figure 1.6.

The steps to obtain the collector characteristic curves are as follows:

Table 1.2 depicts the operating regions of the transistor that are obtained by following the steps 1 through 6 to produce the set of curves shown in Figure 1.6.

VBB	VCE	VBE	IB	IC	Operating region of transistor
<0.7V	VCC	0V	0A	0A	Cutoff region, transistor acts like an open switch
>0.7V	<=0.3V	0.7V	>0A	>0A	Saturation region, transistor acts like a closed switch
>0.7V	>0.3V	0.7V	>0A	>0A	Active or linear region

Table 1.2

The results in Table 1.2 show

• the base-bias voltage V_BB determines the base current I_B and
• the collector bias voltage V_CC determines the collector current I_C and voltage V_CE.

BJT Switch Characteristics

As you can see from Table 1.2, when V_BB < 0.7V, the base-emitter maintains the following characteristics:

• The voltage V_BE < 0.7V.
• The base-emitter is reverse-biased.
• The current I_C is 0A.

Result:

The transistor acts as an open switch. The voltage drop across the collector-emitter junction V_CE is V_CC. This region of operation is referred to as the cutoff region. In the cutoff region (region in the graph below the line indicating I_B=0  A), both B-E and C-B junctions are reverse-biased. In the cutoff region, the transistor acts like an open switch.

Now, consider the opposite scenario. When the bias voltage V_BB is > 0.7V and V_CE is <= 0.3V, the base-emitter maintains the following characteristics:

• The voltage V_BE = 0.7V and I_B > 0A.
• The B-E junction is forward-biased.
• The collector current I_C is > 0A and increases sharply in small values of V_CE.

Result: The transistor acts like a closed switch. The operating region in which this occurs is referred to as the saturation region. In the saturation region, both the B-E and C-B junctions are forward-biased.

Essentially, when a transistor is used as a switch, it operates in either the saturation or the cutoff region.

BJT Amplifier Characteristics

One final scenario applies to using the transistor as an amplifier. In this scenario, V_BE > 0.7V and V_CE > 0.3V. When a transistor is used as an amplifier,

• the B-E junction is forward-biased,
• the C-B junction is reverse-biased, and
• the currents I_B and I_C are both > 0.

Note: In this scenario, I_C is reaching steady state and is not increasing significantly.

The operating region in which the transistor is used as an amplifier is labeled the active or linear region.

As usual, exercise caution when applying voltage to the transistor. Exceeding the breakdown values for any of the voltages like V_BE and V_CE will destroy the transistor.

There is an inconstant dc current gain for a transistor. It depends on the operating point, which is also called the quiescent point or Q-point. The operating point is a collection of the dc bias conditions, which include the currents I_B and I_C and voltage V_CE. Figure 1.6 shows the dc load line joining the cutoff and the saturation points for a transistor.

Applications of a Transistor

In general, a transistor operates somewhere between saturation and cutoff regions. However, when used as a switch, it operates exclusively either in the saturation or the cutoff region, but not both simultaneously. Consider the following values for the components in Figure 1.5:

• V_BB = 5V
• V_CC = 20V
• R_C = 2k
• R_B = 100k

The saturation current I_C(SAT) for the circuit is given by . This is because during saturation, the transistor behaves like a closed switch, as shown in Figure 1.7a below.

Hence, I_C(SAT) will be 10mA, and the switch is turned "on."

Similarly, you can calculate the cutoff voltage V_CE(OFF) by having the transistor act as an open switch as indicated in Figure 1.7b. Since the transistor is an open circuit, no current I_C flows through the circuit, and no voltage drops across the resistor R_C. Therefore, all the voltage V_CC appears across the terminals of the transistor. In other words, the switch is turned "off." This property of the transistor assists in turning on an LED or operating relays.

When the transistor is used as an amplifier, it operates between the saturation and cutoff regions. This operating point is dictated by the base circuit. As the base current increases, the operating point of the transistor inches closer towards the saturation point.

Figure 1.8 specifies the component values.

In the base of the circuit in Figure 1.8, the base current can be determined using the formula, . Figure 1.9 illustrates how applying different base bias voltages will result in different operating points for the transistor:

• When V_BB = 4.7V,
  Result: This puts the operating point in the middle of the load line in the active region. Typically, this will be the operating point of the transistor. It is represented by the point Q1 in Figure 1.9.
  
  
• When V_BB is set to 1.7V, the current
  Result: The operating point inches closer to the cutoff region as indicated by the point Q2 in Figure 1.9.
  
  
• When you increase V_BB to 8.7V, then
  Result: The operating point inches closer to the saturation region as indicated by the point Q3 in Figure 1.9.

Transistor Biasing Circuit

So far, we have considered biasing circuits for a transistor with a separate supply voltage at the base and at the collector. While this transistor is certainly operational, it depends heavily on the dc current gain to operate. The range varies by each transistor and introduces significant challenges to the design of the transistor circuit. One way to compensate for this challenge is to use the fixed bias circuit. This type of circuit is very similar to the circuits we've considered so far in this course. A fixed bias circuit would connect both the resistors R_B and R_C to the same collector bias voltage V_CC. However, this biasing method is not recommended, because the operating point of such a circuit still relies on the changing beta value.

Figure 1.10 illustrates the voltage divider emitter-stabilized bias circuit, i.e., the universal bias circuit. A universal bias circuit includes an emitter resistor that acts like a feedback resistor into the emitter. This stabilizes the effect the changing current gain has on the operating point of the transistor.

Note: The emitter resistor R_E is common to both the input and output loops and includes an extra base resistor. While it takes longer to analyze a universal bias circuit, it does stabilize the circuit.

Figure 1.11 shows how a universal bias circuit can be simplified using two separate loops at the input and output ends. Obtain the input loop by applying the voltage divider rule and Thevenin's theorem from the base terminal and ground.

In this simplified design, the Thevenin equivalent resistance between the base and ground terminals is the parallel combination of the base biasing resistors R1 and R2. Thus, for this circuit,

Similarly, determine the Thevenin voltage using the voltage divider rule at the base terminal as follows:

Figure 1.11 shows the Thevenin equivalent of the circuit in Figure 1.10.

Now, Figure 1.11 applies Kirchhoff's Voltage Law around the input and output loops of the circuit to demonstrate how you can solve for various currents and voltages.

Kirchhoff's Law Applied to the Input loop results are

• V_BB = R_BBI_B + V_BE + I_ER_E
• I_E = ( + 1) I_B
• Therefore, V_BB = R_BBI_B + V_BE + ( +1) I_BR_E
• 
• For a silicon npn transistor V_BE = 0.7V.
  Note: assume a =150. Using these results, you can solve for the unknown quantities I_B, I_C , and I_E:

Result:

Kirchhoff's Law Applied to the Output loop results in the following:

V_CC = R_C I_C + V_CE + I_E R_E

Solving for the unknown quantity V_CE, we get

Result: V_CE = V_CC - R_C I_C - I_E R_E = 4.75 V