Transistors - Diodes with a perk?

[Femme_physics]

Are transistors essentially diodes with a perk? I mean, they do exactly what diodes do (one way valves) AND they can amplify current. Doesn't that make them more efficient? Why not make only transistors then? They appear to be always made out of the same basic materials, so I don't see the big difference in the monetary spending.

 

[yungman]

For one, diode only have PN junction and BJT have NPN or PNP junction, one more process to form, it is always more expensive. On top, you put 3 leads in BJT instead of two, all come to $$$! Also, the doping profile for transistor optimized for current gain, not necessary for low parasitic resistance. In fact collector is usually lightly doped meaning higher resistance and base is very thin, look at some of the transistor data sheet, the maximum base current is limited...I better stop here as I am not a semi-conductor expert by any stretch!

 

[Femme_physics]

So, I guess it always comes down to money, doesn't it? Thanks for clearing it up, and with details!

 

[Studiot]

There is a difference between making individual components and components within an integrated circuit.

Within an integrated circuit the cost comparison is

inductor> capacitor>> resistor > diode > transistor.

As a result transistors are often used within integrated circuits to diode and resistor functions which they can do directly and to simulate capacitors and inductors which they can do indirectly.

The actual size on chip also follows the above list so using transistors rather than other components so this is another reason transistors are used in preference.

 

[yungman]

There is a difference between making individual components and components within an integrated circuit.

Within an integrated circuit the cost comparison is

inductor> capacitor>> resistor > diode > transistor.

As a result transistors are often used within integrated circuits to diode and resistor functions which they can do directly and to simulate capacitors and inductors which they can do indirectly.

The actual size on chip also follows the above list so using transistors rather than other components so this is another reason transistors are used in preference.

Inside ICs, it is a totally different story. We tie the B to C and it become a diode. There are no cost difference making a diode and transistor because as long as you have transistors, you need the number of process to make transistor regardless, so you cannot save a process putting a dedicated diode. So cost is not an issue anymore.