construction

construction

1. What Construction Means (BJT)

Construction of BJT = physical realization of n-p-n/p-n-p sandwich with ohmic contacts, doping, geometry to make two junctions work together (emitter injects, base thin to let carriers reach collector).

2. Physical Structure (planar process, exam diagram)

Layer/Part	Construction Detail	Doping / Geometry
Emitter	Heavily doped ($n^+$ for NPN, $p^+$ for PNP) large area? Actually emitter small but high doping to emitter inject efficiently: $N_E ≫ N_B$	$10^{19}$ cm⁻³, thin
Base	Very thin ($0.5$–$5$ µm), lightly doped ($p$ for NPN), between emitter and collector	$N_B≈10^{16}$–$10^{17}$, width $W_B$ narrow so minority diffusion length $L_n$ > $W_B$ ⇒ most carriers cross without recombination
Collector	Moderately doped, large area (to collect heat)	$N_C≈10^{15}$–$10^{16}$, largest volume

Fabrication steps (planar):

1. Start with n-type collector substrate (for NPN).
2. Diffusion/ion implantation to create p-base region.
3. Diffusion to create n+ emitter inside base.
4. Oxide ($SiO_2$) grown, windows etched, metal (Al) contacts deposited for E, B, C.
5. Passivation, packaging (TO-92, etc.) with leads.

Cross-section:

Top: Emitter metal (n+) | Base metal (p) contact on side | Collector metal (n) bottom
Inside: n+ (E) - p (B) thin - n (C) substrate
Junctions: E-B and B-C clearly defined, depletion widths.

3. Why Base Thin and Lightly Doped

• Thin ⇒ transit time short, recombination low ⇒ α≈1.
• Lightly doped ⇒ emitter injection efficiency high ($γ≈ N_E/(N_E+N_B)≈1$).

If base were thick/heavily doped, recombination would kill $β$.

4. Exam Points

• Draw cross-section (E-B-C sandwich, metal contacts) and label doping — 5-7 marks, very frequent (almost certain construction diagram).
• State emitter heavily doped, base thin/light, collector moderate/large — 5 marks, very frequent.
• Mention planar process with diffusion — 3-5 marks, frequent.
• Show $N_E ≫ N_B > N_C$ and $W_B$ small — 3 marks.