forward bias

forward bias

1. What Forward Bias Is

Forward bias = external voltage applied with p → +, n → - (p higher than n), which opposes built-in potential $V_{bi}$ (~0.7V Si), reduces depletion width $W$ and barrier height ($V_{bi}-V$), allowing majority carriers to diffuse across junction ⇒ large forward current.

2. Operation Details

• Without bias: $V_{bi}$ (≈0.7V Si) creates built-in field pointing n→p, drift balances diffusion ⇒ net zero, depletion width $W_0$.
• Forward bias $V>0$: effective barrier

decreases, $W$ shrinks as

, diffusion dominates: electrons from n diffuse to p, holes p→n, recombine near junction ⇒ current

for $V≫V_T$.

Carrier motion: majority drift? Actually forward current is diffusion of majority across lowered barrier, then drift in neutral regions.

3. I-V in Forward

• Below cut-in ($V<0.3$ Ge, $0.5$ Si): tiny current.
• Above $V_γ$: exponential rise, dynamic resistance

small, static $R = V/I$ larger but drops with $I$.

4. Worked Checks

(a) Si diode V=0.6V: barrier 0.1V left, W reduced by √(0.1/0.7)=0.38×, current ~ I0 e^{23}= 1nA·9.7e9=9.7mA ✓

(b) Need to limit current? Forward diode without series resistor would draw huge current and burn (thermal runaway, I0 doubles with T).

(c) Forward voltage at 10mA ≈0.7V typical.

(d) Why forward current large? Barrier lowered, so many carriers have thermal energy to cross.

(e) Reverse vs forward W: forward W small, capacitance larger (diffusion capacitance dominates forward).

5. Exam Points

• Define forward bias p +, n -, $V_{eff}=V_{bi}-V$, $W$ decreases — 5 marks, very frequent (almost certain to ask forward bias description).
• Show band diagram with barrier lowering — 5 marks, frequent.
• Write $I=I_0(e^{V/V_T}-1)$ and $r_d=V_T/I$ — 3-5 marks, frequent.
• Mention cut-in 0.7V Si — 3 marks.