Bipolar Junction Transistor (BJT) Basics
A bipolar junction transistor controls a larger collector current using a smaller base current. It is called bipolar because both electrons and holes participate in conduction. BJTs remain important in switching, analog amplification, current mirrors, bias circuits, and simple interfaces.
Learning Objectives
By the end of this lesson, you should be able to:
- identify emitter, base, and collector terminals;
- explain NPN and PNP current directions using conventional current;
- use current gain as an approximate design tool;
- distinguish cutoff, active, and saturation regions;
- calculate base resistor values for simple switching;
- recognize thermal and biasing limitations.
Structure and Symbols
A BJT has three terminals:
| Terminal | Role |
|---|---|
| Emitter | heavily doped terminal that injects carriers |
| Base | thin control region |
| Collector | collects carriers and handles most voltage |
For an NPN transistor, conventional collector current flows from collector to emitter when the base-emitter junction is forward biased. For a PNP transistor, polarities are reversed.
Current Gain: Beta and Alpha
A BJT has two common current-gain symbols. They describe current ratios, not fixed component constants.
| Symbol | Name | Definition | Typical use |
|---|---|---|---|
beta, \beta, or hFE |
common-emitter current gain | beta = IC / IB |
estimating base current for switches and common-emitter amplifiers |
alpha or \alpha |
common-base current gain | alpha = IC / IE |
understanding emitter-to-collector carrier transfer and common-base amplifiers |
The terminal currents are related by:
$$
I_E = I_C + I_B
$$
Because emitter current is the sum of collector current and base current, alpha is always slightly less than 1, while beta can be much larger than 1. The two gains are connected by:
$$
\alpha = \frac{\beta}{\beta + 1}
$$
and:
$$
\beta = \frac{\alpha}{1 - \alpha}
$$
For example, if beta = 100, then alpha = 100 / 101 = 0.990. That means about 99.0% of the emitter current reaches the collector, while the remaining current is supplied through the base. In real datasheets, beta or hFE is usually given as a wide range because it changes with collector current, collector-emitter voltage, temperature, and device lot.
Operating Regions
| Region | Base-emitter junction | Collector-emitter behavior | Use |
|---|---|---|---|
| Cutoff | not forward biased | nearly off | open switch |
| Active | forward biased | collector current roughly beta x IB |
amplifier |
| Saturation | strongly driven | low VCE, no longer controlled by normal beta |
closed switch |
In active region:
$$
I_C \approx \beta I_B
$$
Use this equation as an approximation for active-region operation. Do not design precision circuits around an exact beta value.
Base-Emitter Voltage
A silicon BJT base-emitter junction behaves like a diode. It is often near 0.6 V to 0.8 V in normal operation, but it is not a fixed constant. It depends on current and temperature.
The small-signal emitter resistance is approximately:
$$
r_e \approx \frac{25\ mV}{I_E}
$$
with IE in amperes at room temperature. This is useful in amplifier analysis.
Worked Example: LED Switch
A 3.3 V microcontroller drives an NPN transistor that switches 20 mA through an LED branch. Use forced beta of 10 for saturation.
$$
I_B = \frac{20\ mA}{10}=2\ mA
$$
Assume VBE(sat)=0.8 V:
$$
R_B = \frac{3.3\ V - 0.8\ V}{2\ mA}=1.25\ k\Omega
$$
A standard 1.2 kOhm or 1.3 kOhm value is reasonable if the GPIO can source the current.
Bias and Thermal Behavior
BJTs are temperature sensitive. As junction temperature rises, VBE falls for the same current, and leakage increases. In power circuits this can cause thermal runaway unless emitter resistors, feedback, derating, or current limiting are used.
For amplifier biasing, a voltage divider and emitter resistor are commonly used to reduce dependence on beta.
Common Mistakes
- Forgetting that base current must be limited with a resistor or driver.
- Assuming beta is fixed.
- Using active-region gain equations for a saturated switch.
- Reversing collector and emitter.
- Ignoring base-emitter reverse-voltage limits.
- Omitting thermal checks for power BJTs.
Summary
A BJT is a current-controlled semiconductor device with cutoff, active, and saturation regions. Base current controls collector current in active operation, while saturated switching requires deliberate overdrive. Real BJT design must account for beta spread, base resistor sizing, temperature, terminal orientation, and power dissipation.
Further Reading
- ON Semiconductor: BJT switching and safe operating area application notes.
- Texas Instruments: transistor biasing and low-side switch notes.
- Horowitz and Hill, The Art of Electronics, BJT chapters.