how is emmiter area of a pnp decided for a given current ?

[amic]

tsmc pnp10 area

I am designing a full custom pnp transistor for a emitter current of 100uA, but I don't know how exactly is emmiter area of a pnp decided for the given current ?

Can anybody help me with this ?

thanks

Sachin

 

[vicky]

Re: how is emmiter area of a pnp decided for a given current

hi dear,

The bipolar junction transistor, unlike other transistors, is not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized to forward-mode operation, interchanging the collector and the emitter makes the values of α and β of reverse operation much smaller than those found in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter.

Small changes in the voltage applied across the base–emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled current sources, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base

 

[amic]

Re: how is emmiter area of a pnp decided for a given current

Vicky,

What you said is correct. Usually saturation current value is helpful in deciding the emitter area since its directly proportional to emitter area. Since these values are given by fabs like tsmc only for 3 such devices available ( pnp2, pnp5,pnp10 ), I can't use any of these values to decide the emitter area of my full-custom substrate pnp formed in 65nm TSMC process.

So i am more interested in knowing the exact method to define emitter area.

thanks anyways.

 

[Fom]

Use maximum beta aproach. Change your area until beta will reach maximum value.

 

[amic]

Re: how is emmiter area of a pnp decided for a given current

Fom, Instead of maximum beta I think I am interested in a stable beta over a range of currents ( 1uA to 100uA) since the transistor is used as thermal diode. Any suggestions on this ?

Use maximum beta aproach. Change your area until beta will reach maximum value.

 

[Fom]

If you have spice model of that just simulate it and you will get answer.