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Q-point of diode; avalanche vs Zener breakdown

Short answer

The Q-point is the dc operating point (VDQ,IDQ) at the intersection of diode curve and load line. Avalanche breakdown (>~6 V) is impact ionization, positive temp co; Zener (<~5 V) is field emission, negative temp co.

Define Q-point in pn junction diode operation. Show it graphically with necessary derivations. Differentiate between avalanche and zener break down.

Q-point

The Q-point (quiescent or operating point) is the dc voltage and current (VDQ, IDQ) of the diode when no signal is applied. It is the point where the diode curve and the dc load line intersect.

Graphical derivation

For a series circuit of VDD, R and the diode, KVL gives

VDD = ID R + VD   →   ID = VDD/R - VD/R

This is the load line, with intercepts ID = VDD/R and VD = VDD, slope -1/R. The diode equation ID = IS[exp(VD/(η VT)) - 1] is the second relation. Their intersection is the Q-point.

 ID
 VDD/R |\
       | \      diode curve
  IDQ  |  \ _.-Q
       |   \|
       +----+\------- VD
          VDQ  VDD

Steps: draw the characteristic, draw the load line with the two intercepts, read the intersection. For VDD = 5 V and R = 1 kΩ, the line runs from 5 mA to 5 V, and the Q-point is about 0.75 V, 4.25 mA.

Avalanche vs Zener breakdown

PointAvalanche breakdownZener breakdown
CauseImpact ionisation: carriers accelerate, knock out more carriers (multiplication)Strong electric field pulls electrons directly from covalent bonds (field emission)
Junction dopingLightly doped, wide depletion regionHeavily doped, very narrow depletion region
VoltageAbove about 6 V (typically 6 to hundreds of V)Below about 5 V
Temperature coefficientPositive (VB rises with temperature)Negative
Characteristic kneeLess sharpVery sharp

✓ Answer checked by Claude Opus 5.5 (high effort)

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