How Does a BJT Work? Device Physics Explained for IC Designers
Let us travel back in time to Christmas of 1947—the point-contact transistor has just been invented, and intense experimentation at Bell Labs has revealed a critical insight: if you inject minority carriers into a semiconductor, you can achieve amplification. But how does it work?
The Invention of the Transistor: A Step-by-Step Breakdown
This new finding was electrifying. At long last, Brattain and Gibney had overcome the blocking effect of the surface states—the practical problem that had for so long caused the failure of our field-effect experiments.
PN Junction
Since band diagrams represent electron potential energy, this is reflected in the upward bending of the conduction band in the p-region. The built-in electric field opposes further electron diffusion from n to p, creating an energy barrier. This results in the conduction band bending downward in the n-region.
Carrier Transport
In lightly doped semiconductors, lattice scattering dominates, and the carrier mobility decreases with temperature. In general, the trend is decrease in mobility with higher temperature and with higher doping.
Intrinsic Carrier Concentration
A key nuance in the derivation of intrinsic carrier concentration is that it assumes thermal equilibrium—meaning the carrier concentration remains constant over time. But how does this make sense? We’ve established that electrons are continually being thermally excited from the valence band to the conduction band due to random thermal processes. If this generation kept happening unchecked, electrons would accumulate in the conduction band and holes in the valence band, driving the system out of equilibrium.
Energy Bands
Intuitively, if two atoms were close enough and their electrons remained at the same energy level, it would violate the Pauli Exclusion Principle, which states that no two electrons can occupy the same quantum state. Therefore, the energy levels split, ensuring each electron has a unique quantum state.
Quantum Physics
The uncertainty principle serves as a fundamental safeguard of quantum mechanics. Heisenberg realized that if it were possible to measure both the position and momentum of a particle with perfect accuracy, the entire framework of quantum mechanics would fail.