Transistors and BJT Fundamentals Flashcards

Introduction to Transistors

A transistor is a semiconductor device primarily utilized to either amplify or switch electronic signals. It functions by regulating the flow of current or voltage, acting effectively as an electronic button or gate. In a standard circuit, a controlling voltage or current applied to one pair of terminals (the base and emitter) dictates a much higher controlled current flowing through another pair of terminals (the collector and emitter). This gain property, where the output is controlled in proportion to the input signal, allows the transistor to act as an amplifier. Alternatively, it can serve as an electrically controlled switch, turning current on or off based on the input signal, with the specific current levels determined by other circuit elements.

• Internal Behavior: The connection between the base and emitter behaves similarly to a semiconductor diode. Consequently, a voltage drop known as the base-emitter voltage ($V_{BE}$) develops when base current exists. This voltage value is specific to the semiconductor material.

• Materials: Most transistors are manufactured from silicon and germanium, though others utilize gallium-arsenide and silicon-carbide.

• Physical Structure: They consist of solid pieces of semiconductor material with at least three terminals for external connections.

• Classification by Specs: Transistors are categorized into low, medium, and high power; high and low frequency; and very high current and/or high voltage ratings.

• Packaging: Some are packaged individually, but most exist in multiples within Integrated Circuits (ICs).

• Major Groups:

  1. Bipolar Junction Transistors (BJTs): These include normal BJTs and phototransistors.

  2. Field-Effect Transistors (FETs).

Phototransistors

Phototransistors are BJTs designed to conduct when exposed to light. Photons absorbed in the base region generate a photocurrent that serves as the base current. These devices feature a transparent window in their packaging to accept light and possess much larger base and emitter areas compared to normal BJTs.

• Terminal Configuration: A phototransistor has collector and emitter physical terminals. The base exists without a physical connection terminal, acting instead as a light sensor to absorb incident light energy ($photons$).

• Current Relationship: The collector current is approximately $\beta$ (common-emitter current gain) multiplied by the base photocurrent. The generated base current is directly proportional to the intensity of the incident light.

• Structural Types:

  • Homo-structure: Provides a gain level between 50 and a few hundred. It has a limited bandwidth of approximately $250\,kHz$.

  • Hetero-structure: Provides a gain level up to 10,000. These are expensive and rarely used, but some can operate at frequencies up to $1\,GHz$.

• Dark Current: This is the small current that flows when no light is present, representing the small number of carriers injected into the emitter, which are then amplified by transistor action.

• Comparison to Avalanche Photodiodes: Phototransistors are often preferred over avalanche photodiodes due to their lower noise levels, despite providing high gain.

• Limitation: They exhibit poor high-frequency response because of the large capacitance associated with the base-collector junction, which is intentionally made large to collect sufficient light.

• Operation Polarity: For operation, the collector of an NPN transistor is made positive relative to the emitter, while it is made negative for a PNP transistor.

Bipolar Junction Transistor (BJT) Construction and Operation

A BJT is a three-terminal device composed of three layers of NPN or PNP semiconductors forming the emitter, base, and collector.

• Construction Types:

  • NPN Transistor: Two large N-type materials separated by a thin P-type material.

  • PNP Transistor: Two P-type materials separated by a thin N-type material.

• Physical Properties: These create two PN junctions: the base-emitter junction and the base-collector junction, separated by the thin base region. The term "bipolar" refers to the movement of both electrons and holes into oppositely polarized materials.

• Doping and Function:

  • Emitter: Heavily doped to emit electrons.

  • Base: Medium doped/thin to eject electrons.

  • Collector: Lightly doped to pass electrons.

  • N-type Doping: Uses group five (pentavalent) elements acting as electron donors.

  • P-type Doping: Uses group three (trivalent) elements acting as electron acceptors (creating holes).

• Mathematical Relationships:

  • Kirchhoff’s Current Law for transistors: $I_E = I_B + I_C$

  • Collector current relationship: $I_C \approx \beta \times I_B$

  • The factor $\beta$ is typically greater than 100 for small-signal transistors but smaller for high-power applications.

• Current Components: Collector current consists of majority carrier flow and minority carrier flow (known as leakage current). Leakage current is temperature-dependent and can affect stability at high temperatures.

Transistor Circuit Configurations

Common-Emitter (CE) Configuration

• The emitter is common to both the input (base) and output (collector) terminals.

• Input is applied to the base-emitter; output is taken from the collector-emitter.

• Current gain ($\beta$) is the ratio of output current to input current.

• Phase angle change is $180^o$.

• Input impedance: Medium; Output impedance: High; Voltage gain: Medium; Power gain: Very high.

Common-Base (CB) Configuration

• The base is common to the emitter and collector.

• Input is applied to the emitter-base; output is obtained from the collector-base.

• Phase angle change is $0^o$.

• Input impedance: Low; Output impedance: Very high; Voltage gain: High; Current gain: Low; Power gain: Low.

Common-Collector (CC) Configuration

• The collector is common to the base and emitter.

• Input is applied to the base-collector; output is obtained from the emitter-collector.

• Phase angle change is $0^o$.

• Input impedance: High; Output impedance: Low; Voltage gain: Low; Current gain: High; Power gain: Medium.

Operating Regions and Parameters

Transistors must be biased within specified maximum ratings found in data sheets, including Collector-Emitter Voltage ($V_{CE}$), Collector-Base Voltage ($V_{CB}$), Emitter-Base Voltage ($V_{EB}$), Collector Current ($I_C$), Power Dissipation ($P_{tot}$), and Operating Temperature ($T_j$).

The Three Operating Regions

1. Cut-off Region: The base-emitter junction is not forward biased (V_{BE} < 0.7\,V). $I_B = 0$ and $I_C = 0$. The transistor is "OFF." In this state, $V_{CE} = V_{CC}$.

2. Saturation Region: The base-emitter junction is forward biased (V_{BE} > 0.7\,V). $I_B$ and $I_C$ are at maximum levels. For an NPN transistor, the collector-base voltage ($V_{CB}$) is negative. In this region, the transistor is a perfectly closed switch, $V_{CE} = 0$, and $I_C = \frac{V_{CC}}{R_C}$.

3. Active Region: The region between cut-off and saturation where the transistor acts as a linear amplifier. The emitter-base junction is forward biased and the collector-base junction is reverse biased. For NPN, $V_{CB}$ is positive. The relationship between $I_C$ and $V_{CE}$ is defined by the load line: $I_C = \frac{V_{CC} - V_{CE}}{R_C}$.

Applications of Transistors

The Transistor Amplifier

An amplifier increases the input signal level using a transistor as the active component, typically in the common-emitter configuration.

• Base Current for Biasing ($I_B$): $I_B = \frac{V_{CC} - V_{BE}}{R_B}$

• Quiescent Collector Current ($I_C$): $I_C = \beta \times I_B$

• Quiescent Voltage ($V_{CE}$): $V_{CE} = V_{CC} - (I_C \times R_C)$

• Power Output ($P_{ac}$): $P_{ac} = (I_{C\,rms})^2 \times R_C = \frac{(V_{R_C\,rms})^2}{R_C}$

Transistor Oscillators

An oscillator converts a DC source into an alternating (AC) output. It requires a fixed feedback from the output and a loop gain of exactly unity (1).

• Types: Phase-shift, Colpitts, Hartley, and Crystal oscillators.

• Phase-Shift Oscillator Frequency ($f$): $f = \frac{1}{2 \pi \times R \times C \times \sqrt{4 \times \frac{R_C}{R} + 6}}$

Transistor as a Switch

Electronic switches often drive electromagnetic relays. Transistors used as switches operate only in the saturation region (ON) or the cut-off region (OFF). In a perfectly closed switch (saturation), $I_C = \frac{V_{CC}}{R_C}$. Inputs are chosen to ensure the output is either completely off or completely on, as seen in digital logic circuits.