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November 27, 2008

好久没有写“从硅到搜索引擎”这个主题了,因为我写完前面一篇后,忽然想把这个系列写成对当年发明这些东西的科学家和公司的八卦。但是犹豫之后,还是决定走技术这条路,毕竟八卦不是我的强项。写技术虽然枯燥很多,毕竟不会出大错。

上次说到三极管可以把电流按照比例放大。这对于音响一类需要把小信号放大的应用当然是好事情,但是对于数字信号,就没有这个必要了。数字信号就是0和1,对应的三极管的状态,就是完全没有电流通过(关闭)或者有大量电流通过(开启),中间的状态,比如有100微安的电流进入,导致有0毫安的电流通过,这个状态对于数字芯片里的三极管来说,不仅不必要,反而是有害的,因为:这个状态的三极管就相当于一个电阻,会发热。一个三极管发热不要紧,几百万个三极管一起发热,这个芯片就要融化了。

所以数字芯片里的三极管,从完全关闭到完全开启,速度一定要快,就是因为要减少发热的时间。假如说每次三极管从关闭到开启的过程中,等效的电阻是R,转换的时间是T的话,那么产生的热量就是: W = U^2 / R * T,这里U是芯片的运行电压。

另外一个发热的地方是输入三极管的控制电流,发热的公式也可以写成W’ = U * I * T,这里I是控制电流,所以I越小越好。实际的数字芯片里面的三极管,都不是电流控制的,而是用电压控制的金属氧化物三极管,类似这样:

其中的Gate就是控制三极管开关的管脚。因为Gate和另外两个管脚是绝缘的,所以从Gate到另外两个管脚的电流非常的小,因而产生的热量也很小。当Gate充电/放电的时候,就能控制电流在Source和Drain之间的流动,达到开关的目的。

October 25, 2008

上回说到,吕老板跟林老板去圭老板那里做客,然后林老板身边的兵都跑到吕老板那里去应聘了。本来大家都相安无事,这么一折腾,吕老板身边的兵多了,林老板身边缺人了。为了让兵,也就是电子们回去安心工作,这个外界压力,或者说电场,还不能加反了。

不过这还不是最热闹的,更热闹的,是两个林老板,把一个吕老板挤在中间,大家一起在圭老板的地盘上喝茶。就像这样:

如果你从c到e给个电压,那么是没有什么电流通过的。这个东西的神奇之处在于,如果你从b到e再给一个小的电压,那么从b到e会有一个小的电流,而且这个电流的大小,能控制c到e的电流大小。这个在武林里,叫做四两拨千斤。更神奇的是,这个四两如果变成八两,那么就能拨两千斤。这就是为什么,麦克风里微小的声音,最后能够被变成大喇叭里震耳欲聋的咆哮。因为那个微小的声音所产生的微弱电流,被按比例放大了很多倍。

不过人总是贪心的,既然八两能拨两千斤,那咱们要是给b到e那里放上个10斤的东西,岂不是可以拨两万五千斤了?这种贪心的麻烦,反而产生了今天无处不在的计算机。

October 19, 2008

上会说到圭老板喜欢身边8个手下,吕老板只有三个手下,林老板有5个手下。老板也是人,彼此之间也喜欢串个门啥的。

这天,吕老板到圭老板这边来串门了,俗话说人比人得死,吕老板跟圭老板一比,心里就不平衡了:凭什么圭老板身边那8个手下都这么整齐呢?哦,原来有4个是跟邻居借的。这社区建设的,真好,哪像我们吕社区,雇员整天从这个公司到旁边公司串门。吕老板看看身边的三个兵,心里觉得缺了点什么。

碰巧呢,林老板也来圭老板的大楼串门,林老板也不平衡着呢,凭什么人家圭老板养4个手下,我偏偏要养5个手下?林老板看着5个手下,总觉得有一个多余的。

老板的心思,员工最清楚了。那是,弄不好要丢工作的。于是林老板周围的那些“多余“的员工,就都跑到吕老板这边来找工作了,林老板身边的人反而少了。世间本无事,庸人自扰之。这下吕老板身边的闲人反而多起来了,林老板有事的时候,手下总是在请病假。

左边这张图,就是吕老板跟林老板的状况。吕老板总觉得缺人,于是身边就有很多空位,或者说空穴;林老板总觉得人浮于事,于是身边就有很多闲人,或者说多余的电子。于是这些”多余“的员工都往求贤若渴的吕老板那边跑,反而显得吕老板那边闲人很多,电子多了就形成一个内电场,阻止更多的电子过来。

如果在吕老板跟林老板的这场人才争夺战外面加上一个电场,比如让吕老板这边是正极,那么从林老板那边跑过来找工作的电子们就有了去处,于是电子就可以继续流动了,形成了电流。如果把电场反过来呢,跟电子们说,你们都回去老老实实呆着,这下电子们都不干了,停下来静坐,于是电流也就停止了。从外面看来,林老板和吕老板的这场茶话会,只能让电流按照一个方向流动,这个就是著名的二极管。

October 17, 2008

硅如今是无处不在了。只要你能看到这篇东西,就有无数的硅片在为你服务,你手下面那只鼠标里面就有好多片,你桌子下面的计算机里就更多了。那么,硅是怎么工作的呢?

硅跟好多做生意的人一样,都是一根筋,只认8这个数。硅这个原子最外层只有4个电子,所以为了凑成8这个吉利数,一个硅原子就跟其他4个硅原子一起凑电子,凑成8个。一个老板有了8个兵,吉利了,硅老板就安心了,就好像这样:

硅晶体
硅晶体

不过电子就跟所有的员工一样,就是不安分守己,如果有点风吹草动的,他们就跳槽,从一个硅老板跳到另外一个硅老板。反正给哪个老板打工都一样,老板看员工也是长得都一样,所以跑个把员工也没关系,从别的老板那里挖一个过来就是了。好在没有光照在硅片上的时候,员工的流动性也不是那么强。从整体上来说,一共也没有多少电子在跳槽,于是整个硅的导电性也不是很强,这就是为什么硅被叫做半导体。

如果你看看硅的邻居铝,每个铝老板外层只有3个电子,铝老板也没啥心思控制他们,这些电子就非常不听话,到处跳槽要求涨工资,所以铝就是导体;硅的另外一个邻居磷,外层有5个电子,都被磷老板控制的死死的,户口本身份证都收走,这些电子也没什么跳槽的想头,大家都不跳槽,所以磷就是绝缘体。