depletion area at the emitter-base junction, current IC is able  to cross the collector base junction, flow through emitter-base  junction, and complete the collector-emitter battery circuit  (hollow arrows).  To some extent, varying the voltage to the base material can  increase or decrease the current flow through the transistor  as the emitter-base depletion area changes width in response  to the base voltage. If base voltage is removed, the emitter- base depletion area becomes too wide and all current flow  through the transistor ceases.  Current in the transistor circuit illustrated has a relationship  as follows: I$_{E}$ = I$_{B}$ + I$_{C}$. It should be remembered that it is  the voltage applied to the base that turns the collector-emitter  transistor current on or off. Controlling a large amount of current flow with a small  independent input voltage is very useful when building  electronic circuits. Transistors are the building blocks from  which all electronic devices are made, including Boolean  gates that are used to create microprocessor chips. As  production techniques have developed, the size of reliable  transistors has shrunk. Now, hundreds of millions and even  billions of transistors may be used to construct a single chip  such as the one that powers your computer and various  avionic devices. Combination of semiconductor materials is not limited to a  two-type, three-layer sandwich transistor. By creating a four- layer sandwich of alternating types of semiconductor material  Some caveats are necessary with this explanation. The  transistors that comprise this four-layer diode must be  constructed of material similar to that described in a zener  diode. That is, it must be able to endure the current flow  without burning out. In this case, the voltage that causes the  diode to conduct is known as breakover voltage rather than  breakdown voltage. Additionally, this diode has the unique  characteristic of allowing current flow to continue until the  applied voltage is reduced significantly, in most cases, until  it is reduced to zero. In AC circuits, this would occur when  the AC cycles. Within a four-layer diode, sometimes known as a Shockley  diode, there are three junctions. The behavior of the junctions  and the entire four-layer diode can be understood by  considering it to be two interconnected three-layer transistors.  [Figure 11-28]  Transistor behavior includes no current flow until the base material receives an applied voltage to narrow  the depletion area at the base-emitter junction. The base  materials in the four-layer diode transistor model receive  charge from the other transistor’s collector. With no other  means of reducing any of the depletion areas at the junctions,  it appears that current does not flow in either direction in this  device. However, if a large voltage is applied to forward bias  the anode or cathode, at some point the ability to block flow  breaks down. Current flows through whichever transistor is  charged. Collector current then charges the base of the other  transistor and current flows through the entire device.  (i.e., PNPN or NPNP), a slightly different semiconductor  diode is created. As is the case in a two-layer diode, circuit  current is either blocked or permitted to flow through the  diode in a single direction. Silicon Controlled Rectifiers P P N N P P N P N N Anode Cathode Four-Layer Diode Transistor Equivalent Equivalent Schematic Schematic Symbol Anode Cathode Figure 11-28.  A four-layer semiconductor diode behaves like two transistors. When breakover voltage is reached, the device conducts  current until the voltage is removed.  11-13