Physics · Ch 14 — Semiconductors
Depletion region and the potential barrier
Depletion region and the potential barrier
The diffusion of carriers across the junction, and the resulting build-up of fixed positive (donor) ions on the n-side and fixed negative (acceptor) ions on the p-side, sets up a potential difference across the junction -- this is the POTENTIAL BARRIER (Fig. 14.17). Its magnitude is about 0.6-0.7 V for silicon and about 0.3-0.35 V for germanium, and it exists PERMANENTLY, even with no external power source connected at all; it is exactly this barrier that prevents diffusion from simply continuing until the two sides are identical, and instead brings the junction to a state of electrostatic equilibrium.
Free charge carriers simply cannot survive within a region where this potential barrier exists -- any that wander in are swept back out by the barrier's field -- so the region on either side of the junction becomes essentially devoid of any mobile charge carriers at all. This carrier-free region straddling the junction is called the DEPLETION LAYER or DEPLETION REGION. Its width, and the size of the potential across it, can both be externally controlled, which is precisely the useful, exploitable property that biasing (the next sub-section) makes use of.
Because the n-side near the junction has lost electrons and been left with exposed positive donor ions, it sits at a HIGHER potential than the p-side near the junction, which has lost holes and been left with exposed negative acceptor ions. This impurity-ion charge on both sides of the junction therefore establishes an internal ELECTRIC FIELD across the depletion region, pointing from the n-side toward the p-side (Fig. 14.18). …
What this figure shows. A diagram of the p-n junction showing a narrow region straddling the junction line, explicitly marked as the DEPLETION LAYER (or depletion region), inside which no free charge carriers (no free electrons and no free holes) are shown -- only the fixed, immobile ions left behind by diffusion: a row of positively-charged DONOR ions is shown on the n-side edge of this narrow region, and a row of negatively-charged ACCEPTOR ions is shown on the p-side edge of this narrow region. Because free carriers cannot survive inside a region containing this net exposed ionic charge, the layer is shown as depleted of any mobile carriers, and the accumulated ionic charge on both sides is what sets up the potential barrier that opp …
What this figure shows. A diagram of the depletion region (as in Fig. 14.17) with a single electric-field arrow drawn pointing FROM the n-side TOWARD the p-side, across the depletion layer. This field direction is consistent with the n-side, having lost electrons and being left with exposed positive donor ions, sitting at a higher (positive) potential relative to the p-side, which having lost holes is left with exposed negative acceptor ions and sits at the lower potential of the two; the field arrow visually represents this internal potential difference established purely by the impurity ions on either side of …