1/21/2024 0 Comments Microchip transistor diagram![]() ![]() ![]() This voltage turns on a transistor by changing its characteristics from that of an open circuit (the 'off' position) to one that can conduct a small current. ![]() "But in a computer chip, the control mechanism is the voltage on the control terminal of a transistor. A flow of animals through a complex maze of gates would require a farm hand at each gate. On the farm, the farmer resets the gate location by making a decision and then moving a physical barrier. But the most important difference is the means of controlling the flow. "Returning to the analogy between the farm and the computer chip, it is obvious that the flow is different (farm animals compared to information) and that the gate itself is different (a physical barrier compared to a transistor in the CMOS technology). So, when a gate is controlling current flow, it is actually controlling the flow of information. In either case, the current flow results in a change in the circuit voltage, and the circuit voltage represents a bit of information. Again, current will flow through the NMOS transistor for a very brief interval, but for the NMOS the current is between the circuit and the negative supply. "The current flow that charges the circuit node to a 0 is steered away from the circuit through a different kind of transistor (NMOS) connected between the circuit and the negative supply voltage, or electrical ground. For the very brief interval that is required for a logic gate to switch (on the order of a nanosecond, or a billionth of a second), current will flow through a PMOS transistor from the positive power supply to the circuit. The supply voltage is usually an industry-standard value such as 3.3 or 5.0 volts. "To establish a 1 on a circuit, the current flow is steered to the circuit (controlled) by 'turning on' a PMOS transistor connected between the circuit and the positive supply voltage. The voltage may either be high (representing the value '1') or low (representing the value '0'). This voltage represents a single 'bit' of information. The current that flows through a gate establishes a voltage at a particular point in the circuit. The gate consists of transistors the transistors are selected by the chip designer from two basic types (PMOS and NMOS transistors) that are found in the ubiquitous CMOS (complementary metal-oxide semiconductor) technology. "In a computer, a gate controls the flow of electric current through a circuit. The farmer makes a decision about the flow of animals and then moves the physical barrier to permit the desired flow. In this case, the gate consists of a physical barrier whose position is controlled by a farmer. "On a farm, gates may be used to control the 'flow' of sheep or goats between pens. But the reason for the usage of the term 'gate' for computer logic can be appreciated by examining the basic function of a gate: to control a flow. The vision of a gate swinging back and forth clearly does not literally represent the structures on a silicon chip. "Those of us who design logic gates for computers seldom reminisce on how the terms we use to describe technology came into use. Larry Wissel, ASIC Applications Engineer at IBM Microelectronics, replies: ![]()
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