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generally say zenner diode breakdown voltage is some distribution so when you measure the breakdown, youmay meaure the current via anode and cathode and it must be the same value as defined the spec (Iz) changing the series resistor value. Otherwise you cannot distingwish the fructuation on the mass line.
Additionally you may check the tempetature characteristics of breakdown voltage for wide temp range characteristic. Probably it is around -2mV/deg
The temperature characteristics in reverse (normal diode) is around -2mV/K but in forward (zener) the tc is quite different! Values below say 5V have a negative tc and higher values have a positive tc! This however also depends on the biasing current, in old discrete power supplies this was used to generate a constant reference voltage over the temperature range.
To measure zener voltage, it's nice to have a current source. You can make a sloppy one with a lab supply (voltage source) and a pnp transistor. For example, connect the emitter of the pnp (2N3906) through a 600 ohm resistor to +vout of the lab supply. Connect 2 diodes (1N4148) in series between the +vout of the lab supply and the base of the pnp with the cathodes pointing towards the base. Connect a 47k ohm resitor from the base to the -vout of the lab supply. Finally the DUT (device under test), in this case the zener, connects between the collector of the pnp and the -vout of the lab supply with the cathode pointing toward the collector of the pnp. Now increase the lab supply voltage until the collector voltage (voltage across the zener stops rising). Read the zener voltage. The 600 ohm resistor can be adjusted for different currents. Good Luck
Yes you are right. The gamma-z takes from + to - temp coefficient. -2mv/deg is only lower zenner breakdown voltage rage. the tc depends on mamy conditions such as impurity,aneal,ion-implantation temp many. So itis not correct to say -2mV/deg as tc.