Zener Diode

This section studies several special-purpose junction diodes: the Zener diode, the photo diode, the solar cell, and the light-emitting diode (LED). Unlike the Zener diode -- which works on the principle of controlled JUNCTION BREAKDOWN -- the other three exploit photosensitivity, a very different and equally useful property of semiconductor junctions.\n\nJunction breakdown: in Class XI we saw that if the reverse bias voltage across an ordinary p-n junction diode is increased beyond a critical value (the reverse breakdown voltage), the reverse current suddenly jumps to a large value -- and an ordinary diode is normally DAMAGED when this happens. Electrical breakdown, in general, can occur by one of two mechanisms: AVALANCHE breakdown or ZENER breakdown. Zener breakdown occurs when the reverse voltage across the junction becomes large enough that the resulting strong electric field at the junction directly pulls electrons out of the covalent bonds holding the semiconductor lattice together (rather than relying on carrier multiplication by collision, as in avalanche breakdown); these newly freed electrons become available for conduction, and as the applied reverse voltage increases further, progressively more bonds are broken and the current increases rapidly. Crucially, Zener breakdown occurs specifically in HEAVILY DOPED diodes, whose depletion layer is correspondingly narrow -- and, unlike an ordinary diode's uncontrolled breakdown, Zener breakdown does NOT damage the diode, because the diode is specifically designed (via its heavy doping) to operate safely and repeatably in this regime.\n\nA ZENER DIODE is exactly such a p-n junction diode, purpose-built to work in its own reverse breakdown region without damage, and used as a voltage regulator or voltage stabiliser (its circuit symbol has a distinctive bent/Z-shaped cathode bar, Fig. 16.7a). Its I-V characteristic (Fig. 16.7b) behaves like an ordinary diode when forward biased, but shows a sharp breakdown when reverse biased, at a voltage called the ZENER VOLTAGE $V_Z$: beyond $V_Z$, current increases suddenly and substantially while the voltage across the diode stays essentially PINNED at $V_Z$. $V_Z$ itself depends on the amount of doping -- heavier doping gives a thinner depletion layer and hence a LOWER breakdown voltage, while lighter doping gives a higher one. Diodes with $V_Z$ below about 6 V operate mainly through the true Zener mechanism; those above 6 V operate mainly through avalanche breakdown instead, but both types are still conventionally called 'Zener diodes'. A Zener diode's datasheet typically specifies: its V-I characteristic; the Zener voltage $V_Z$ (the reverse voltage at which it is designed to operate); the maximum Zener current $I_{Z(max)}$ (or $I_{ZM}$) it can safely carry at $V_Z$; its power rating (the maximum power $P_Z=I_{Z(max)}V_Z$ it can dissipate); and its Zener (or dynamic) resistance $R_Z$ (also called Zener impedance $Z_Z$), which is the reason the voltage across a real Zener diode is not PERFECTLY constant -- $R_Z$ behaves like a small resistance in series with the ideal Zener, so changes in $I_Z$ do cause small corresponding changes in $V_Z$.\n\nUsed as a VOLTAGE REGULATOR (Fig. 16.8), a Zener diode of breakdown voltage $V_Z$ is connected in REVERSE bias across an input source $V_{in}$ (with $V_{in}>V_Z$), with a series resistor $R_S$ limiting the current through it, and the load resistor $R_L$ connected in PARALLEL with the Zener diode, so the voltage across $R_L$ always equals $V_Z$. If the input voltage $V_{in}$ increases, the current through $R_S$ and the Zener both increase, but this extra current increase drops entirely across $R_S$ -- the voltage across the Zener (and hence across $R_L$) stays fixed at $V_Z$, since a Zener in breakdown holds its voltage constant across a wide current range. If instead $V_{in}$ stays fixed but $R_L$ decreases (so the load draws more current), that extra load current is simply diverted away from the Zener branch (the Zener current $I_Z$ decreases correspondingly), again keeping $V_L=V_Z$ unchanged. In the extr …