cctech Digest, Vol 68, Issue 45

[Jonas Otter]

On 4/26/2009, ard at p850ug1.demon.co.uk (Tony Duell) wrote:


>
>Message: 1
>Date: Sat, 25 Apr 2009 19:27:20 +0100 (BST)
>From: ard at p850ug1.demon.co.uk (Tony Duell)
>Subject: Re: Transistors...
>To: cctalk at classiccmp.org
>Message-ID: <m1Lxmay-000J3RC at p850ug1>
>Content-Type: text/plain
>
>> > If you wire two diodes in series (PN->NP), it isn't the same as a
>> > transistor (PNP), at all.
>>
>> It is. A PNP is just the same thing as a PN-+-NP.
>
>In one sense it is, but I can assure you that if you connect 2 diodes
>together in this way (no matter what sort of diodes you use), you will
>not get a transistor. The resulting circuit will not show any current gain.
>
>IIRC, what you need is a sufficiently thin base region (the 'N' in the
>example you gave) that electron-hole recombination does not occur. You
>can't join 2 n-type pieces with a bit of wire and get this.
>
>> possible space wise anyway, not even considering characteristics. But
>> that don't change the fact that you get a working transistor with just
>> two diodes.
>

Excuse the bluntness, but that is utter and complete bollocks. You do
*not* get a transistor with two diodes. Tony's comment above is
correct, for a transistor to work, the base needs to be thin enough that
electron-hole recombination does not occur (to any significant degree at
least).

Two diodes connected back to back will never be anything but two diodes
connected back to back. No current gain *at all*.

>I am not convinces. If you have got this to work, can you please tell me
>what sort of diodes you used, and I will try to recreate it (if I see it
>working on my bench, I'll be convinced!)
>
>-tony
>

/Jonas