The Effects of Switching on Semiconductors
Although integrated circuits (IC) sit atop the world of electronics, the transistors within them do the bulk of the work. Within each IC are millions of transistors constantly switching on and off to allow the IC to complete its functions. Even if just one of the many transistors were to stop working, the IC could lose some or all of its capabilities.
Circuits that handle digital signals use transistors to switch from a high state to low state and vice versa. If a voltage is at a point close to the supply voltage, a circuit is considered high state. If the circuit point is close to zero voltage, it is considered to be in a low state. The time a transistor takes to change from a high to low or low to high state is known as its switching rate. Although transistors do not use a significant amount of energy while in the high or low state, the same cannot be said for the time during the process of switching. In an ideal scenario, a transistor would switch instantly, meaning it takes zero seconds to change its rate. Nevertheless, the transistor does take a small amount of time to switch over.
Transistors are made from semiconductor materials, or materials that have conductive properties between those of conductors (such as metals) and nonconductors/insulators (such as ceramics). As such, each transistor junction has a limited capacitance and resistance. The junction capacitances store energy and the combination of resistance and capacitance slows down switching. This means the capacitance must completely fill up or empty before the transistor can flip. The rate at which the capacitance fills or empties is dependent on the junction resistance.
As switching frequency increases, this problem becomes more preventable. When the transistor is driven to toggle faster and faster, the junction capacitance may not have sufficient time to fully discharge or charge. This limits the maximum switching rate the transistor can achieve. To reduce junction capacitances and resistance and enable semiconductors to switch faster, semiconductor manufacturers employ a variety of methods. Though modern semiconductors such as transistors and diodes are able to switch at MHz or GHz scales, the cumulative effect of the tiny switching losses quickly adds up to increase the junction temperature.
Power is the result of voltage and current. When a semiconductor is in a high state, the voltage is high, but the current is minimal. As a result, the power drawn from the supply is also minimal. In a low state, the semiconductor’s voltage is close to ground level and is once again minimal. Despite this, during switching, when the voltage is between supply and ground levels, the current increases too. This causes the value of the product of voltage and current to increase and the semiconductor’s heat increases due to the heightened power consumption. In higher frequencies, this will happen more frequently and the heat will increase to a higher junction temperature. If the natural process of heat dissipation is able to remove the accumulated heat, the semiconductor will eventually reach a stable temperature. If not, heatsinks or other forced cooling methods are needed to keep the heat steady.
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