Unit 3: Bipolar Junction Transistors

1. Junction Transistor

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that consists of two p-n junctions formed back-to-back. It is a current-controlled device where a small current at one terminal controls a large current at the other two terminals.

Structure and Terminals

A BJT is constructed with three doped semiconductor regions separated by two p-n junctions. The regions are:

1. Emitter (E): Heavily doped. Its function is to inject a large number of majority charge carriers (electrons for NPN, holes for PNP) into the base.
2. Base (B): Very thin and lightly doped. Its function is to pass most of the injected charge carriers from the emitter to the collector.
3. Collector (C): Moderately doped and physically the largest of the three regions. Its primary function is to collect the charge carriers that pass through the base. Its large size is to help dissipate heat.

There are two types of BJTs based on their construction:

• NPN Transistor: A thin layer of p-type semiconductor is sandwiched between two layers of n-type semiconductor.
• PNP Transistor: A thin layer of n-type semiconductor is sandwiched between two layers of p-type semiconductor.

2. BJT Operation (Active Region)

For a BJT to operate as an amplifier, it must be in the Active Region. This requires specific biasing conditions for its two junctions:

• Emitter-Base (EB) Junction: Must be Forward-Biased.
• Collector-Base (CB) Junction: Must be Reverse-Biased.

Operation of an NPN Transistor (Active Region)

1. Forward-Biased EB Junction: The voltage V_BE is applied to forward bias the EB junction. This reduces the depletion barrier, allowing the heavily doped emitter to inject a large number of electrons (majority carriers in the N-type emitter) into the thin, lightly doped p-type base.

2. Carrier Flow in the Base: The base is very thin and lightly doped. Therefore:

   • A very small percentage of the injected electrons (typically < 2%) recombine with the holes (majority carriers in the P-type base). This recombination constitutes the Base Current (I_B).
   • The vast majority of electrons (> 98%) diffuse across the thin base region and reach the CB junction's depletion region.

3. Reverse-Biased CB Junction: The voltage V_CB is applied to reverse bias the CB junction. This creates a wide depletion region with a strong electric field.

   • The electrons that reach this junction are swept across it by the strong electric field and "collected" by the collector terminal. This flow of electrons constitutes the Collector Current (I_C).

4. Current Relationship: The electrons leaving the emitter constitute the Emitter Current (I_E). Since this current splits into the base and collector currents, we have the fundamental BJT equation:
   I_E = I_B + I_C

Key Takeaway: A small base current I_B (caused by recombination) is associated with a much larger collector current I_C (caused by electrons passing through the base). This demonstrates the current-controlling nature of the BJT. The small I_B controls the large I_C.

3. Transistor Current Components

The total current in a transistor is comprised of both majority and minority carrier flows.

• Emitter Current (I_E): Primarily composed of majority carriers injected from the emitter into the base.
• Base Current (I_B): Primarily composed of the recombination of carriers in the base region.
• Collector Current (I_C): Composed of two main components:
  1. The majority of carriers from the emitter that successfully pass through the base (α * I_E).
  2. A small leakage current due to minority carriers crossing the reverse-biased CB junction. This is called the Collector-Base leakage current (I_CBO), where 'O' indicates the emitter is open.

This gives a more precise expression for the collector current:
I_C = α * I_E + I_CBO

Important Current Gain Parameters

1. Common-Base DC Current Gain (α - Alpha):

   • The ratio of the collector current to the emitter current.
   • It represents the fraction of emitter current that reaches the collector.
   • α = I_C / I_E
   • Typically, α is very close to 1 (e.g., 0.95 to 0.998).

2. Common-Emitter DC Current Gain (β - Beta):

   • The ratio of the collector current to the base current.
   • This is the primary measure of a transistor's current amplification capability.
   • β = I_C / I_B
   • β values typically range from 20 to 500. It is also often denoted as h_FE on datasheets.

Relationship between α and β

We can derive the relationship starting from the basic current equation:
I_E = I_B + I_C

Divide by I_C:
(I_E / I_C) = (I_B / I_C) + 1
1/α = 1/β + 1
1/α = (1 + β) / β

This gives us the two key conversion formulas:

α = β / (β + 1)

β = α / (1 - α)

4. CE, CB and CC Configurations of BJT

A BJT has three terminals, but for use in a circuit, we need four: two for input and two for output. This is achieved by making one of the three terminals common to both the input and output. This leads to three possible configurations.

a) Common-Base (CB) Configuration

• Input: Applied between Emitter and Base (V_EB).
• Output: Taken from Collector and Base (V_CB).
• Common Terminal: Base.

Characteristics:

• Input Characteristics: A plot of I_E vs. V_EB for a constant V_CB. It looks like a standard forward-biased diode curve.
• Output Characteristics: A plot of I_C vs. V_CB for a constant I_E. The curves are nearly flat, showing that I_C is almost independent of V_CB and is approximately equal to I_E.
• Current Gain (α): α = ΔI_C / ΔI_E. Less than 1.
• Voltage Gain: High.
• Input Resistance: Very Low (typically 20-100 Ω).
• Output Resistance: Very High (typically > 1 MΩ).
• Phase Shift: 0° (Input and output are in phase).
• Applications: High-frequency amplifiers, impedance matching circuits.

b) Common-Emitter (CE) Configuration

• Input: Applied between Base and Emitter (V_BE).
• Output: Taken from Collector and Emitter (V_CE).
• Common Terminal: Emitter.

Characteristics:

• Input Characteristics: A plot of I_B vs. V_BE for a constant V_CE. It also resembles a forward-biased diode curve.
• Output Characteristics: A plot of I_C vs. V_CE for a constant I_B. This is the most important characteristic graph.
  • Cutoff Region: Below I_B = 0, the transistor is OFF.
  • Active Region: The region where the curves are relatively flat. I_C is strongly controlled by I_B (I_C = β * I_B). This is the region for amplification.
  • Saturation Region: The region where V_CE is very small (approx. 0.2V). Here, I_C is no longer controlled by I_B and is instead limited by the external circuit. The transistor is fully ON.
• Current Gain (β): β = ΔI_C / ΔI_B. High (20-500).
• Voltage Gain: High.
• Input Resistance: Moderate (typically 1-5 kΩ).
• Output Resistance: Moderate (typically 40-100 kΩ).
• Phase Shift: 180° (Output is inverted with respect to the input).
• Applications: The most widely used configuration for general-purpose amplifiers due to its high current and voltage gain, resulting in the highest power gain.

c) Common-Collector (CC) Configuration

• Input: Applied between Base and Collector (V_BC).
• Output: Taken from Emitter and Collector (V_EC).
• Common Terminal: Collector.
• Also known as an Emitter Follower.

Characteristics:

• Current Gain (γ - Gamma): γ = ΔI_E / ΔI_B = β + 1. High.
• Voltage Gain: Slightly less than 1 (no voltage amplification).
• Input Resistance: Very High (typically > 100 kΩ).
• Output Resistance: Very Low (typically < 100 Ω).
• Phase Shift: 0° (Input and output are in phase).
• Applications: Buffer amplifiers, impedance matching (to connect a high-impedance source to a low-impedance load), digital logic circuits.

5. Comparisons of Transistor Amplifier Configurations

6. BJT as an Amplifier

For a transistor to amplify a small AC signal, it must first be biased to operate in the active region. Biasing establishes a DC operating point, also known as the Quiescent Point (Q-Point). The AC signal is then superimposed on this DC bias.

DC Load Line and Q-Point

Consider a simple CE amplifier circuit. The output loop equation (applying KVL) is:
V_CC = I_C * R_C + V_CE
This is the equation of a straight line, called the DC Load Line, on the transistor's output characteristics (I_C vs. V_CE).

The two endpoints of the load line are:

1. Saturation Point: When V_CE = 0 (ideal switch ON), I_C(sat) = V_CC / R_C. This is the y-intercept.
2. Cutoff Point: When I_C = 0 (switch OFF), V_CE(cutoff) = V_CC. This is the x-intercept.

The Q-Point is the specific DC operating point (V_CEQ, I_CQ) on the load line, determined by the chosen DC base current (I_BQ). For best amplification without distortion, the Q-point is typically set near the center of the load line.

Amplification Process

1. A DC base current I_B establishes the Q-point (I_CQ, V_CEQ).
2. A small AC input signal v_in is applied to the base. This causes the total base current to vary sinusodially around I_BQ.
3. This variation in base current causes a much larger, proportional variation in the collector current I_C around I_CQ (since ΔI_C = β * ΔI_B).
4. The change in collector current causes a corresponding change in the voltage drop across R_C (ΔI_C * R_C).
5. This results in a large AC voltage variation at the collector terminal (v_out), which is an amplified and inverted version of the input signal v_in.

7. Transistor as a Switch

A BJT can be used as an electronic switch by driving it between its cutoff and saturation regions.

1. Cutoff Region (OFF State)

• Condition: The input voltage is low (e.g., 0V), so V_BE < 0.7V.
• Junctions: Both EB and CB junctions are reverse-biased.
• Operation: No base current (I_B ≈ 0). This means the collector current is nearly zero (I_C ≈ I_CEO ≈ 0).
• Result: The transistor acts like an open switch. The output voltage V_out = V_CE is approximately equal to V_CC.

2. Saturation Region (ON State)

• Condition: The input voltage is high enough to supply sufficient base current to drive the transistor into saturation (I_B > I_C(sat) / β).
• Junctions: Both EB and CB junctions are forward-biased.
• Operation: The collector current I_C rises to its maximum possible value, limited by the external load resistor R_C. I_C(sat) = V_CC / R_C. The collector-emitter voltage V_CE drops to a very small value, V_CE(sat) (typically 0.1V - 0.3V).
• Result: The transistor acts like a closed switch. The output voltage V_out = V_CE(sat) is approximately 0V.

8. Transistor Switching Times

When a BJT switches between ON and OFF states, the transition is not instantaneous due to internal capacitances and charge storage effects. The total switching time is divided into turn-on and turn-off times.

Turn-On Time (t_on)

This is the time required for the transistor to switch from the OFF state to the ON state.
t_on = t_d + t_r

• Delay Time (t_d): The time from the application of the input pulse until the collector current begins to rise. This is the time required to charge the EB junction depletion capacitance to the forward-bias voltage (approx. 0.7V).
• Rise Time (t_r): The time it takes for the collector current to rise from 10% to 90% of its final saturated value, I_C(sat). This is determined by the transistor's internal characteristics and the time constant of the collector circuit.

Turn-Off Time (t_off)

This is the time required for the transistor to switch from the ON state to the OFF state.
t_off = t_s + t_f

• Storage Time (t_s): The time from the removal of the input pulse until the collector current begins to fall. When a transistor is in saturation, the base region is flooded with excess charge carriers that are not needed for recombination. t_s is the time required to sweep these stored charges out of the base region. This is often the longest component of the switching time.
• Fall Time (t_f): The time it takes for the collector current to fall from 90% to 10% of its saturated value. During this time, the EB junction capacitance discharges.