The PN Junction and its Biasing: MDCAT Physics notes
The PN Junction and its Biasing for MDCAT: p-type and n-type semiconductors, depletion region, forward and reverse bias, diode curve, LED and transistor.
Semiconductors
Semiconductors such as silicon and germanium (group IV, tetravalent) have conductivity between conductors and insulators. At 0 K no electron has enough energy to cross the energy gap, so a pure semiconductor behaves as an insulator. Heating frees more charge carriers, so its resistance falls with temperature.
Doping
| Type | Dopant | Majority carriers | Minority carriers |
|---|---|---|---|
| n-type | Pentavalent (P, As, Sb): donor | Electrons | Holes |
| p-type | Trivalent (B, Al, Ga, In): acceptor | Holes | Electrons |
Both types are electrically neutral overall.
The PN junction
When p-type and n-type regions are joined, electrons diffuse from n to p and holes from p to n, recombining near the junction. This leaves a thin depletion region, free of mobile carriers, containing fixed positive ions on the n side and negative ions on the p side. These ions set up a potential barrier of about 0.7 V for silicon and 0.3 V for germanium that stops further diffusion.
Biasing
| Feature | Forward bias | Reverse bias |
|---|---|---|
| Connection | p to positive, n to negative terminal | p to negative, n to positive terminal |
| Depletion region | Narrows | Widens |
| Barrier | Reduced | Increased |
| Current | Large (mA), due to majority carriers | Very small (about µA), leakage due to minority carriers |
| Resistance | Low | Very high |
Because it conducts easily in one direction only, a PN junction diode is used as a rectifier to change AC to DC. It is a non-ohmic device.
Characteristic curve
The diode characteristic is a graph of current against voltage. In forward bias the current stays tiny until the barrier voltage is overcome, then rises steeply. In reverse bias only a small, nearly constant leakage current flows until breakdown.
Light emitting diode (LED)
- An LED emits light when forward biased, as electrons and holes recombine.
- It has two leads. The longer lead is the anode and connects to the positive terminal; the shorter lead is the cathode.
- Since it glows with small currents, an LED can indicate weak currents.
Transistor
A transistor has three regions (emitter, base, collector). It is used as an amplifier and as a switch: driven fully on it conducts, driven off it does not.
Key facts
- Barrier potential: Si $\approx 0.7$ V, Ge $\approx 0.3$ V.
- Forward current: majority carriers; reverse current: minority carriers.
Common MDCAT traps
- Forward bias narrows the depletion region; reverse bias widens it.
- Leakage current comes from minority carriers, not majority carriers.
- The shorter LED lead goes to the negative terminal, not the positive.
- At 0 K a semiconductor is an insulator, not a superconductor.
- Reverse current is in microamperes, not large currents.
Quick revision
- Holes are majority carriers in p-type material.
- Silicon is the most common semiconductor.
- Diode curve: current versus voltage.
- Diodes rectify; transistors amplify and switch.