Leakage?

[fuzzy645]

So what exactly is "Leakage" in a transistor?

1.  what is leaking?

2.  how does one measure what is leaking?

3.  what are the ill effects of too much leakage on the average circuit?

[R.G.]

So what exactly is "Leakage" in a transistor?

In a bipolar transistor there are two semiconductor junctions. The collector-base junction is run reverse biased and the base-emitter is run forward biased in normal amplifier operation. "Leakage" refers to current leaking through the collector-base junction and being amplified by the base action.

1.  what is leaking?

Current through the collector-base junction.

2.  how does one measure what is leaking?

One applies a voltage to the collector-base junction and measures the current that flows. That's called Icbo (current flowing from collector base with the emitter open). One can also measure Iceo (current flowing from collector to emitter with base open).  Iceo is Icbo times the current gain of the device at that current and temperature.

3.  what are the ill effects of too much leakage on the average circuit?

It's current going through the transistor that can't be controlled by what happens on the base. It's a loss of control of the operations of the device to some extent. That extent may be trivial for silicon (low leakage) devices especially at low gain. It may make the transistor useless if it's high leakage, especially for a high gain device. Leakage is usually temperature sensitive, so the device bias can wander around with temperature.

[fuzzy645]

Awesome answer RG.  Thanks for taking the time.

[boogietone]

Leakage is usually temperature sensitive, so the device bias can wander around with temperature.

Is this the reason for the temperature sensitivity of Ge transistors?

[petemoore]

  Setting the circuit up for best temperature range...
   Starting with a cold box, opening, then placing a thumb on the Q to warm it up, Some drift [Ge transistors] should be noticed, the voltage bias at Q2c will drift up, and shutoff or gating may be the result.
  55 through 95 degrees is the 'window' of temperature I like to play in, so these numbers were 'followed' with a thermometer, 98.6 being close enough...any colder or hotter and this player would be seeking refuge anyway.

[Perrow]

  Setting the circuit up for best temperature range...
   Starting with a cold box, opening, then placing a thumb on the Q to warm it up, Some drift [Ge transistors] should be noticed, the voltage bias at Q2c will drift up, and shutoff or gating may be the result.
  55 through 95 degrees is the 'window' of temperature I like to play in, so these numbers were 'followed' with a thermometer, 98.6 being close enough...any colder or hotter and this player would be seeking refuge anyway.

He doesn't say, but I suspect farenheit.

[R.G.]

Is this the reason for the temperature sensitivity of Ge transistors?

It is. Actually both silicon and germanium have about the same % rise in leakage per degree. It's just that silicon's base leakage is over 1000 times less than germanium's.  It's one reason why thermal issues with germanium are not to be ignored. Even small signal germaniums can get into thermal runaway. And germanium transistors used for switching have to have their base-emitter junction actively reverse biased to turn them off. A germanium device with base shorted to emitter is still amplifying the base leakage, and still conducting some.