Bipolar Junction Transistors (BJTs)

• Bipolar Junction Transistors (BJTs) are pivotal semiconductors for modern electronics.
• Heterojunction Bipolar Transistors (HBTs) have superior high-frequency operation.
• BJTs remain relevant in analog amplification, digital switching, and RF circuits.

Introduction to BJTs

• Bipolar junction transistors (BJTs) utilize both electrons and electron holes as charge carriers.
• Unipolar transistors, like field-effect transistors (FETs), use only one type of charge carrier.
• A small current injected at one terminal controls a larger current between the other two, enabling amplification or switching.
• BJTs were invented in 1947.
• BJTs have enabled advancements in signal amplification, switching, and analog/digital circuit design.
• BJTs operate via minority and majority carrier injections, providing high current gain and linearity.
• BJTs maintain key advantages in high-speed and high-power applications like RF and microwave systems.
• Heterojunction Bipolar Transistors (HBTs) use heterostructure engineering.
• HBT capabilities extend into the millimeter-wave spectrum, making them important in 5G, satellite communications, and optoelectronics.

BJT Basic Circuit Analysis

• The fundamental BJT circuit has two external bias voltage sources, VBB and VCC.
• The fundamental BJT circuit has two resistors, Rin and RL.
• Three key DC voltages across transistor junctions are VBE, VCE, and VCB.
• Three DC currents are IB, IC, and IE.
• The collector is n-type, while the base is p-type.
• If the collector voltage is higher than the base, the collector-base junction is reverse-biased.
• a collector voltage lower than the base results in a forward-biased collector-base junction.

Transistor Action

• BJTs are three-terminal semiconductor devices with two p-n junctions.
• These junctions amplify or magnify a signal, making it a current-controlled device.
• The three terminals are the base, collector, and emitter.
• a small amplitude signal is applied to the base.
• Amplification is available in the amplified form at the collector of the transistor.
• Amplification requires a DC power supply.

Working Principle of BJTs

• A BJT comprises three regions:
• Emitter (E) - heavily doped to inject charge carriers
• Base (B) - thin and lightly doped to allow carrier diffusion
• Collector (C) - moderately doped to collect carriers.

Charge Flow in NPN BJTs

• A p-type semiconductor is sandwiched between two n-type semiconductors.
• The n-type semiconductors function as the emitter and collector.
• The p-type semiconductor acts as the base.
• Current entering the emitter, base, and collector is positive.
• Current leaving the transistor is negative.

Charge Flow in PNP BJTs

• an n-type semiconductor is sandwiched between two p-type semiconductors.
• The p-type semiconductors act as the emitter and collector.
• The n-type semiconductor acts as the base.
• Current enters the transistor through the emitter, forward-biasing the emitter-base junction.
• The collector-base junction is reverse biased.

BJT Parameters

• DC beta (βDC), or hybrid parameter (hFE), is the DC collector current to DC base current ratio.
• It essentially functions as the transistor's DC current gain.
• βDC is crucial in determining the DC collector current.
• The DC beta (βDC) varies with the collector current and junction temperature.
• A constant junction temperature results in DC beta rising until it hits a max value as collector current increases.
• The DC beta (βDC) declines if the collector current continues to rise past its peak.
• Conversely, kept constant, the DC beta (βDC) will change with changes to the junction temperature..
• The DC Alpha (αDC) is the DC collector current to the DC emitter current ratio.
• αDC is rarely used, compared to DC beta (βDC).
• The value is always less than 1, the collector current is always less than the emitter current.

Operation of Bipolar Junction Transistors

• There are three operating regions:
• Active region: transistors act as amplifiers
• Saturation region: the transistor is fully on, functioning as a switch where collector current equals saturation current
• Cut-off region: transistor fully off, collector current equals zero
• The breakdown region is not advisable for BJTs.

Cutoff Region

• The cutoff region is the nonconducting state of the BJT.
• The BJT operates in cutoff when the base current (IB) is zero.
• In theory, no current flows through the collector.
• In practice, small collector leakage current may happen to thermal build up.
• Minimal leakage current allows VCE to approximate VCC.

Saturation Region

• The BJT functions in saturation when its collector current is independent of the base current.
• The collector current has reached its maximum value.
• Both the base-emitter and base-collector junctions are forward-biased.

Active Region

• Load line identifies the cutoff point.
• At the bottom of the load line, IC is zero, and VCE equals VCC.
• The saturation point is at the top.
• IC equals IC (sat)
• VCE equals VCE (sat)
• The region between cutoff and saturation is the active or linear region.
• Base-emitter junction: forward-biased.
• Base-collector junction: reverse-biased.
• IC remains nearly constant for a given IB.
• An increase to VCE leads to a slight increase to IC from the base-collector depletion region widening.
• A BJT in the active region ideally has the output serve as a linear input signal reproduction

BJT Applications

• Used in audio and RF amplifiers, and operational amplifiers; gain depends on the circuit configuration where CE provides high gain.
• Utilized as ON-OFF switches in digital logic circuits, such as relay drivers, microcontroller interfaces, and power switches.
• Used in oscillators, modulators, and mixers in radio and TV transmission.
• BJTs are key in voltage regulator circuits, such as zener diode regulators to stabilize output voltage.

Frequency Response of BJTs

• Frequency response impacts performance relative to increasing signal frequency.
• BJT operates efficiently with minimal signal distortion.
• Parasitic capacitances and transit time effects limit amplification at increasing frequencies.
• Cutoff frequency (𝑓𝑇) represents when the current gain (β) drops to unity.
• The transistor no longer functions effectively as an amplifier above this frequency.
• A tradeoff exists between amplification and frequency response.
• High-speed BJTs have small base widths and optimized doping profiles to minimize frequency-dependent limits.

Switching Characteristics

• As a switch, a BJT operates between saturation (fully on) and cutoff (fully off).
• Switching speed is impacted by charge storage in the base, capacitances, and external circuit components.
• Turn-on time: delay and rise times.
• Turn-off time: storage and fall times.
• Storage time: the period for excess charge removal before the transistor fully turns off.
• Schottky clamping and base drive control are used in high-speed digital circuits to improves switching performance.
• Switching speed is critical
• Transistor’s switching times are affected by junction capacitance and electron transit time across the junctions.
• Delay time (td) is the interval between the input pulse application and the start of collector current flow.
• Rise time (tr) is how long it takes for IC to rise from 10% to 90% of its max.
• Overall turn-on time (ton) is both delay time (td) and rise time (tr).
• Turn-on Time (𝑡𝑜𝑛): Time required for switching from cutoff to saturation.
• Storage Time (𝑡𝑠): Excess minority base charge.
• Turn-off Time (𝑡𝑜𝑓𝑓): Switching from saturation to cutoff.
• Schottky clamping and optimized base drive control improve switching speed.

Nonideal Effects

• An ideal BJT: constant base width and current gain.
• Real devices: increased collector-emitter voltage (𝑉𝐶𝐸) expands the collector-base junction's depletion region.
• This reduces base width: Early effect.
• Leads to extra collector current (𝐼𝐶), exceeding expected linear correlation.
• The impact results in finite output resistance.
• Results in reducing transistor voltage gain.
• A measure of this effect is the Early voltage (𝑉𝐴).
• Higher values indicate lower distortion in analog applications.
• Lightly doping the collector region and heavily doping the emitter region can countervail this effect.

Base Resistance (𝒓𝒃)

• Arises from the base material's resistance.
• Resistance introduces voltage drop, reducing effective base-emitter voltage (𝑉𝐵𝐸).
• This affects current gain (β) and frequency response.
• High base resistance leads to power loss, increased noise, and slower digital circuit switching times.
• High doping and narrow base zones lower resistance and improve high-frequency performance.

Charge Storage and Switching Delays

• Excess charge is stored in the base during saturation, leading to switching delays.
• Storage time is the time required for charge to dissipate before the transistor turns off.
• Charge storage effects affect high-speed applications in digital circuits.
• Schottky clamping mitigates charge storage effects in Schottky transistors by preventing deep saturation.

Parasitic Capacitances and Miller Effect

• Inherent capacitances arise from the physical properties of the semiconductor junctions.
• The base-emitter capacitance (𝐶𝑏𝑒) and base-collector capacitance (𝐶𝑏𝑐) impact high-frequency transistor response.
• The Miller effect amplifies effective capacitance seen at the input, limiting bandwidth and gain in amplifier circuits.
• High-frequency BJTs use small junctions and optimized doping to minimize capacitance.

Thermal Effects and Second Breakdown

• BJTs produce heat from power dissipation (P = 𝑉𝐶𝐸 𝐼𝐶) and can cause thermal runaway.
• Higher temperature, the collector current increases.
• Positive feedback causes device failure.
• Second breakdown (localized heating) may occur from hotspots, which lead to irreversible damage in power applications.
• Effective thermal management (heat sinks, negative feedback circuits) helps limit these effects.

Avalanche Breakdown and Punch-Through

• Impact ionization occurs when the collector-base voltage (𝑉𝐶𝐵) passes a limit, which causes avalanche breakdown and excessive leakage current.
• Punch-through can merge the depletion regions of the emitter-base and collector-base junction, which causes a loss of current control.
• Appropriately designed doping profiles can prevent these effects.

Heterojunction Bipolar Transistors (HBTs)

• HBTs enhance BJT performance through heterostructures with dissimilar semiconductor materials for the emitter and base.
• Enables better high-frequency operation, higher efficiency, and thermal stability than homojunction BJTs.

Key Innovations for HBT

• Bandgap Engineering: Varying bandgap materials like SiGe/Si, GaAs/AlGaAs, and InP/InGaAs.
• Higher Speed: microwave and millimeter-wave for 5G and satellite comms.
• Lower Noise: Stronger signal in RF amplifiers.

HBT Structure vs. BJT Key Differences

How HBTs Work

• Emitter-Base Heterojunction: AlGaAs wider-bandgap prevents hole back-injection into the emitter, with improved electron injection efficiency.
• Narrow-bandgap base (GaAs) enables low-resistance conduction.
• Homojunction Base-Collector Junction is reverse-biased for carrier collection.
• Electrons are injected from the emitter to the base with minimal recombination because of Current Flow Mechanism.
• The base can be heavily doped, improving resistance.

Operation of HBTs

• HBT functions as BJT.
• The base-emitter junction: forward biased.
• The collector-base junction: reversed biased.
• System at unbalanced: Electrons are injected through the base.
• They cross and spread across the base to reach the edge of the base-collector depletion area.
• Majority carriers jump the base-collector junction to the collector, becoming part of the collector currrent.
• The base current is from holes diffusing the base to the emitter.

Advantages of HBTs

• Higher Current Gain (β): Lower base recombination, better emitter effciency.
• Superior High-Frequency Performance: More than 300GHz
• Lower Base Resistance (R-B): Higher doping is allowed, without sacrificing gain.
• Better Thermal Stability: Reduced thermal risk.
• High Density: Used in power amps.

Disadvantages and Challenges for HBT

• Fabrication is harder requiring molecular beam epitaxy (MBE).
• Needs metal-organic chemical vapor deposition (MOCVD), and stricter lattice-matching.
• Exotic material make HBT more expensive.
• Reliability is weaker becasue Current and hot carriers have lower reliability.

Types of HBTs

• Silicon-Germanium is used for the SiGe HBTs.

HBT applications

• Wireless comms. in 5G.
• High-speed digital circuits.
• Optoelectronics, like laser drivers.
• Automotive radar.

Future trends for HBTs

• Combination with CMOS is planned.

• THz HBTS re being researched.

• Wide-Bandgap will be used for power and temp.

• HBT evolution is better offering increased speed, power, and function.

• Advancements through SiGe tech will offer a wider scale.