SEMICONDUCTORS (A.T.L)

 SEMICONDUCTORS

THE DIODE

The basic function of a check valve is to allow water to flow in only one direction. Once the force of the spring is exceeded, the plate moves away from the stop allowing water to pass through the pipe. A flow of water in the opposite direction is blocked by the solid stop and plate. If it took a pressure of 0.7lb to exceed the spring force, the flow of water versus pressure might look like Figure 27. In electronics, this curve would represent the typical silicon diode if pounds per square inch equaled volts and gallons per minute equaled amperes. Of course, the amount of current that flows through the diode must be limited or the device could be damaged. Just as too much water through the check valve could destroy the plate (shorted diode). If the diode is made of Gallium Arsenide, it would take approximately twice the voltage to produce a flow of current . The energy level required to “turn on” a Gallium Arsenide diode is so high, that light is generated when current starts to flow. These diodes are called “Light Emitting Diodes”, or simply LEDs.

THE TRANSISTOR

The transistor is best described as a device that uses a small amount of current to control a large amount of current (Current Amplifier).A small amount of “Base Current” pushes on the L1 portion of the lever arm forcing check valve D1 to open, even though it is “reverse biased” (pressure is in direction to keep check valve shut). Keep in mind the base current would not start to flow until the check valve D2 allowed current to flow (0.7lb). If the current ratio through D1 and Base was equal to the lever arm advantage, then I1 / Ib = L1 / L2. Call this ratio Beta (b) and let L1 = 1 inch and L2 = 0.01 inch. Then b = 100 and I1 will be 100 times Ib. Since both currents must pass through D2, I2 = I1 + Ib. These same principles apply to a silicon NPN transistor. I1 becomes collector current (IC), and I2 would be emitter current (IE). b = IC / IB and IE = IB + IC.

THE PNP TRANSISTOR

The emitter releases current that splits into two paths. The base current “forces open” the collector check valve which collects all the current except the small amount that goes into the base. The direction of current in the PNP transistor is opposite that of the NPN transistor. Because of these differences, the emitter of the PNP is usually referenced to the power supply voltage and the emitter of the NPN is usually referenced to ground or zero voltage. In both transistors, the current amplification factor (Ic/Ib) is called Beta (b).

THE FIELD EFFECT TRANSISTOR

0 the center of a small section of a pipe is made of thin, flexible rubber and that rubber is surrounded by water from a third pipe called the gate. When pressure is applied to the gate, the rubber pinches off the current from the source to the drain. No current flows from gate to drain or source. This device uses a change in gate pressure to control the current flowing from source to drain.

Since there are no check valves, the current can flow in either direction. In other words, this device acts like a variable resistor. The Field Effect Transistor (FET) also controls current between source and drain by “pinching off” the path between them. The level of voltage on the gate controls the amount of current that will flow. Since no DC current flows in or out of the gate (only momentarily a small amount will flow to adjust to new pressures as in a capacitor), the power used by the gate is very close to zero. Remember, power equals voltage times current, and if the current is zero, the power is zero. This is why FETs are used in the probes of test equipment. They will not disturb the circuit being tested by removing power during a measurement. When a second gate section is added (pipe and rubber) between the source and drain it is called a Dual Gate FET. In our water pipe analogy of the FET transistor, the rubber must be very thin and flexible in order to “pinch off” the current from the source to the drain. This means it could be easily damaged by a small “spike” of high pressure. The same is true of an electronic FET. A high voltage “spike” (Static Electricity) can destroy the gate and ruin the FET. To protect the FET, they are sometimes packaged with metal rings shorting their leads, and a fourth lead may be added to the metal case containing the transistor.