Semiconductor basics — PN junction
Pure semiconductor barely conducts, but doping adds carriers to control conductivity. Join N-type (free electrons) and P-type (holes) and the carriers recombine at the boundary, forming a depletion region and a built-in barrier (~0.7 V) — that barrier is the threshold voltage you see in diodes and BJTs. Forward bias narrows it to conduct, reverse bias widens it to block; this one-way conduction is the shared foundation of the diode (1 junction), BJT (2 junctions) and MOSFET (voltage-formed channel).
Doping — N-type and P-type
- Intrinsic silicon barely conducts. A pentavalent dopant (P, As) leaves free electrons (−) → N-type; a trivalent one (B) leaves holes (+) → P-type. These majority carriers carry the current.
- A hole is 'a missing electron'; as neighboring electrons fill it, it looks like a positive charge drifting the other way.
| Type | Dopant | Majority carrier |
|---|---|---|
| N-type | Pentavalent (P, As) | Free electrons (−) |
| P-type | Trivalent (B) | Holes (+) |
PN junction & depletion region — what the 0.7 V threshold is
- Join N and P and electrons/holes recombine at the boundary, leaving a carrier-free depletion region that sets up a built-in potential barrier.
- Current only flows once you overcome that barrier — which is exactly the ~0.7 V (Si) threshold seen in diodes and BJTs.
Forward vs reverse bias — one-way conduction
- Forward (P to +, N to −): pushes the barrier down, depletion narrows → conducts once past 0.7 V.
- Reverse (P to −, N to +): depletion widens → blocked (tiny leakage only). Past breakdown it conducts abruptly and is destroyed.
- This behavior is the diode's I-V curve itself (forward conduction · reverse block · breakdown).
| Bias | Depletion | Result |
|---|---|---|
| Forward (P+) | Narrows | Conducts past 0.7 V |
| Reverse (P−) | Widens | Blocks (surges at breakdown) |
The shared root of active devices
- Diode = 1 PN junction; BJT = 2 junctions (NPN) where a small base current controls a large collector current; MOSFET = a voltage-controlled device that forms a channel via gate voltage.
- Once this clicks, the #9 diode / #10 BJT / #11 MOSFET lectures stop stalling on 'why one-way' and 'why 0.7 V'.
Lectures skip this floor (doping, depletion) and start at 'the 0.7 V forward threshold'. If diodes/BJTs didn't click, this is usually the missing piece — read PN junction + bias first and 'threshold/blocking' suddenly makes sense. Deeper physics (bandgap, Fermi level) can wait until needed.