Minority-carrier injection

Minority-carrier injection is the physical mechanism that makes a BJT work. Forward-biasing the emitter–base junction pushes carriers from the emitter into the base, where they are minority carriers; they then diffuse across the thin base and are collected. This is fundamentally different from how a MOSFET conducts (surface inversion under a gate), and understanding it explains every structural choice in the BJT.

The mechanism, step by step

Take an npn in active mode.

1. Injection. The EBJ is forward-biased. Lowering that junction’s barrier lets a large current of electrons pour from the emitter (n-type, electron-rich) into the base (p-type). Inside the p-type base, electrons are the minority carrier — hence “minority-carrier injection.” The injected concentration is exponential in $v_{BE}$, which is exactly where the BJT collector current law $i_C=I_S{e}^{v_{BE}/V_T}$ comes from.

2. Diffusion across the base. Those injected electrons now sit at high concentration at the emitter edge of the base and near-zero concentration at the collector edge (the reverse CBJ field sweeps them away there, pinning the concentration low). That concentration gradient drives them to diffuse straight across the base — the same diffusion physics as in any PN junction.

3. Recombination loss = base current. While crossing, a small fraction of the electrons meet holes in the p-type base and recombine. This is the only loss. Because the base is deliberately thin (under a micron) and lightly doped, the transit time is short and there are few holes to recombine with, so the lost fraction is tiny. The carriers lost to recombination are continuously replenished through the base terminal — that replenishment current is the base current $i_B$. A thinner, more lightly-doped base means less recombination, hence a larger Common-emitter current gain $\beta$.

4. Collection. The electrons that survive reach the reverse-biased CBJ. Its strong field, pointing the right way to pull electrons toward the collector, sweeps them across with essentially 100 % efficiency. They exit as the collector current. This is why $i_C\approx i_E$, differing only by the recombination loss $i_B$.

Why this is different from a MOSFET

A MOSFET conducts by field-effect: a gate voltage inverts the silicon surface and forms a conducting channel of majority carriers, with no junction being forward-biased and no DC gate current. A BJT conducts by injecting minority carriers across a forward-biased junction and letting them diffuse. The consequences trace directly back to this difference:

• The BJT base draws a real DC current (the recombination/replenishment current), so its input resistance is finite — unlike the MOSFET’s insulated gate.
• The current is exponential in the control voltage, giving the BJT a higher BJT transconductance per unit current than the square-law MOSFET.
• Carrier mobility matters: electrons diffuse faster than holes, so npn devices are faster than pnp ones.

The carriers involved are exactly the minority carriers discussed in Majority and minority carriers, injected into a region where they are outnumbered by the opposite type.