This article is our 11th in a series intended as an overview for those who are not technical specialists in the semiconductor processing field. The following is a brief over view of some of the basic structures that make up a semiconductor devices, or integrated circuits (IC): resistors, capacitors, diodes, transistors, fuses, and conductors. These components are made in the course of standard semiconductor processing and are typically contained in an IC.
Resistors limit current flow by using dielectric materials or high-resistivity portions of a semiconductor wafer surface. In the semiconductor industry, resistors are formed from isolated sections of the wafer surface, doped regions, and deposited thin films. The value of a resistor is (in ohms) a function of the resistivity, a material property of the resistor, and its physical dimensions, and can be expressed in the following relationship:
R = ρ L/A (1)
In the above equation, ρ is resistivity, L is length of resistive region, and A is cross-sectional area of the resistive region.
A majority of the resistors in integrated circuits are formed by a sequence of oxidation, masking, and doping operation. A typical resistor is in the shape of a dumbbell, with the square ends acting as contact regions and the long skinny region serving the resistor function. After doping and reoxidation, contact holes are etched in the square ends to contact the resistor into the circuit. A resistor is a two-contact, no-junction device. The term no-junction device means that the current flows between the contacts without crossing an N-P or P-N junction. The junctions serve to confine the current flow in the resistive region. Doped resistors can be formed during any of the doping steps during semiconductor processing. Those formed by ion implantation have more controlled values than those in diffused regions. Resistors generate heat according to the ohm’s law and the power law: V= IR and P = IV, yielding P= I2R. The voltage and current in a resistor do not have a time lag relative to each other, unlike capacitors and inductors.
A capacitor is a device that stores an electrical charge. A junction capacitor is formed at every junction in a semiconductor device. When a voltage is applied across any junction, carriers on each side move away from the junction, leaving a depleted region that acts as a capacitor. The value of this junction capacitance must be taken into account when the circuit is designed.
One of the issues with oxide-metal capacitors is their large area. Trench capacitors help to solve this problem by creating a capacitor in a trench etched vertically into the wafer surface. Etching is done either isotropically with wet techniques or anisotropically with dry techniques. The trench sidewalls are oxidized and the center is filled with deposited polysilicon. Stacked capacitors offer another alternative to conserving surface area. The need for small high dielectric capacitors for dynamic random access memory (DRAM) circuits has driven this development. The storage portion of a DRAM cell is a capacitor and can be in planar, cylindrical, or fin shaped. Similarly, FLASH memory has large numbers of capacitors. The voltage and current in a capacitor have a time lag relative to each other.
A diode is a two-region, two-contact, device separated by a junction, which either allows current to easily pass or acts as a current block. A diode is the simplest junction device.
Voltage polarity, called biasing, determines which function the diode performs. When the current voltage is the same as the diode region, the diode is in forward bias and the current flows easily. When the polarities are reversed, the diode is reverse-biased and the current is blocked. Diodes are used in circuits to steer the current around the circuit and are usually formed along with transistor doping steps. In MOS circuits, most of the diodes are formed with the source-drain doping step.
A transistor is a three-contact, three-part, two-junction device that performs as a switch or an amplifier. A NPN transistor is where a P-type region lies between two N-type semiconductors. A PNP transistor is just the opposite, where an N-type region lies between two P-type semiconductors.
There are two common types of transistors: bipolar (amplifier) and MOSFET (switch). A bipolar transistor has three terminals: the base, collector, and emitter. The current flows from the emitter region, through the base and into the collector. Bipolar transistors feature fast switching speeds, which are governed by a variety of factors including base width. The shorter an electron, or hole, has to travel, the less time it will take. To achieve fast speeds, the transistor is maintained in the on position, requiring that the base always have power. This will lead toward a buildup of heat in the transistor and will eventually affect the circuit.
On the other hand, a MOSFET transistor is a field effect device. It has three terminals: source, gate, and drain. The middle terminal (gate) does not require current to turn it on, but instead an electrical field. The field effect is due to a voltage applied across an insulator (gate oxide) on top of the gate, the middle terminal. The gate turns the transistor “on” or “off.” (Note that off is not used as a preposition at the end of the last sentence, so the usage meets with the approval of persnickety grammarians.) MOS is an acronym for metal oxide semiconductor, because the gate stack was originally a metal trace applying a voltage to the gate oxide, which communicated with the underlying semiconductor.
Fuses and Conductors
A fuse is a short piece of conducting material (wire) that is designed to melt and separate in the event of excessive current. Semiconductor devices are especially sensitive to over voltages and over currents and require fast acting protection. Fuses used in semiconductor devices have specially designed necks that allow for rapid melting. Once the fuse melts, the entire circuit opens and current is prevented from flowing through the components.