Junction Transistor — Revision Notes

⚡ 30-Second Revision

Types: — NPN (N-P-N), PNP (P-N-P)
Terminals: — Emitter (E), Base (B), Collector (C)
Doping: — Emitter (Heavy) > Collector (Moderate) > Base (Light, Thin)
Current Relation: — $I_E = I_B + I_C$
Current Gains:

- Common Base: $alpha = \frac{I_C}{I_E}$ ($alpha < 1$) - Common Emitter: $\beta = \frac{I_C}{I_B}$ ($\beta gg 1$)

Relation between $alpha, eta$: — $alpha = \frac{\beta}{1+\beta}$, $\beta = \frac{alpha}{1-alpha}$
Biasing for Active Region (Amplifier):

- E-B Junction: Forward Biased - C-B Junction: Reverse Biased

Biasing for Cut-off (Open Switch): — Both junctions Reverse Biased
Biasing for Saturation (Closed Switch): — Both junctions Forward Biased
CE Configuration: — High current/voltage gain, $180^circ$ phase shift.

2-Minute Revision

Junction transistors (BJTs) are three-terminal semiconductor devices (Emitter, Base, Collector) that come in NPN and PNP types. The Emitter is heavily doped to inject carriers, the Base is thin and lightly doped to control flow, and the Collector is moderately doped and larger to collect carriers and dissipate heat.

The fundamental current relationship is $I_E = I_B + I_C$. For amplification (active region), the Emitter-Base junction must be forward-biased, and the Collector-Base junction reverse-biased. Current gains are quantified by $alpha = I_C/I_E$ (common base, < 1) and $\beta = I_C/I_B$ (common emitter, typically 50-500).

These are related by $alpha = \beta/(1+\beta)$. Transistors can also act as switches: cut-off (both reverse-biased) for OFF, and saturation (both forward-biased) for ON. The Common Emitter configuration is most popular for amplification due to its high gain and $180^circ$ phase shift.

5-Minute Revision

A Bipolar Junction Transistor (BJT) is a three-layer, two-junction semiconductor device, either NPN or PNP. Its three terminals are the Emitter (heavily doped, injects carriers), Base (thin, lightly doped, controls carriers), and Collector (moderately doped, collects carriers, dissipates heat). The arrow on the emitter in the circuit symbol indicates the direction of conventional current flow: out for NPN, in for PNP.

For a transistor to function as an amplifier (in the active region), the Emitter-Base (input) junction must be forward-biased, allowing majority carriers to flow from the emitter into the base.

The Collector-Base (output) junction must be reverse-biased, creating an electric field that sweeps these carriers from the base into the collector. A small base current ($I_B$) then controls a much larger collector current ($I_C$).

The total emitter current is the sum of base and collector currents: $I_E = I_B + I_C$.

Two key current gain parameters are $alpha$ (common base current gain, $I_C/I_E$, always < 1) and $\beta$ (common emitter current gain, $I_C/I_B$, typically 50-500). These are inter-related by $alpha = \frac{\beta}{1+\beta}$ and $\beta = \frac{alpha}{1-alpha}$.

Transistors can also operate as switches. In the cut-off region (both junctions reverse-biased), virtually no current flows, acting as an open switch. In the saturation region (both junctions forward-biased), maximum current flows, acting as a closed switch.

Three common configurations exist: Common Base (CB), Common Emitter (CE), and Common Collector (CC). CE is most widely used for amplification due to its high current and voltage gain, and it introduces a $180^circ$ phase shift between input and output voltage. CC is an emitter follower, used for impedance matching, with voltage gain near 1. CB is used for high-frequency applications, with current gain less than 1.

Prelims Revision Notes

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Transistor Types: — NPN (N-P-N) and PNP (P-N-P). The middle layer is the Base.
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Terminals: — Emitter (E), Base (B), Collector (C).
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Doping Levels: — Emitter (Heavily doped) > Collector (Moderately doped) > Base (Lightly doped). Base is also very thin.
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Emitter Function: — Inject majority carriers.
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Base Function: — Control carrier flow from emitter to collector.
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Collector Function: — Collect carriers, dissipate heat (physically largest).
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Current Relationship: — $I_E = I_B + I_C$. (Emitter current is the sum of base and collector currents).
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Common Base Current Gain ($alpha$): — $alpha = \frac{I_C}{I_E}$. Value is always less than 1 (typically 0.95 to 0.99).
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Common Emitter Current Gain ($eta$): — $\beta = \frac{I_C}{I_B}$. Value is typically large (50 to 500).
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Relationship between $alpha$ and $eta$: — $alpha = \frac{\beta}{1+\beta}$ and $\beta = \frac{alpha}{1-alpha}$.
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Operating Regions & Biasing:

* Active Region (Amplifier): E-B junction forward-biased, C-B junction reverse-biased. * Cut-off Region (Open Switch): Both junctions reverse-biased ($I_C approx 0$). * Saturation Region (Closed Switch): Both junctions forward-biased ($I_C$ is maximum).

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Transistor Configurations:

* Common Emitter (CE): Most common for amplification. High current gain, high voltage gain, moderate input/output impedance. **$180^circ$ phase shift** between input and output voltage. * Common Base (CB): Low input impedance, high output impedance.

Voltage gain, but current gain ($alpha$) < 1. No phase shift. Used for high-frequency applications. * Common Collector (CC) / Emitter Follower: High input impedance, low output impedance. Current gain $approx \beta+1$.

Voltage gain < 1 (no voltage gain). No phase shift. Used for impedance matching.

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NPN vs. PNP Symbols: — Arrow on emitter points OUT for NPN, IN for PNP (conventional current direction).

Vyyuha Quick Recall

EBC - HLM (Doping Levels): Emitter is Heavy, Base is Light, Collector is Moderate.

NPN - No Pointing iN: For NPN, the arrow on the emitter symbol points Not Pointing iN (i.e., out). For PNP, it points in.