Diodes, Transistors and Solid State Principles

Transistor Construction

A BJT is made of three layers of semiconductor material, either two pieces of N-type material and one piece of P-type material, or two pieces of P-type material and one piece of N-type material. The three pieces are joined to form two PN junctions (the BJT is similar to two lots of diode PN junctions). Transistors follow the operating principles: - When both PN junctions are reverse biased, the transistor is said to be in cutoff; it is not conducting - When both junctions are forward biased, it is said to be in saturation. This is when increases in base current do not increase in collector current. There is maximum current flow in the device - Active operation is the condition when the emitter base junction is forward biased and the collector base junction is reverse biased. The collector base junction is designed much the same as a zener diode, which conducts in reverse direction.

Transistor Operation

A Bipolar Junction Transistor (BJT) is a three terminal device made of three layers of semiconductor material. The terminals are emitter, base and collector. The transistor is a current controlled device. When current is applied to the base, the transistor conducts from emitter to collector. An output voltage can be taken from either the collector or emitter. In analog circuits, transistors are used as amplifiers. Analog circuits include audio amplifiers, stabilized power supplies and radio frequency amplifiers. In digital circuits, transistors function essentially as electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.

JFET Transistors

A Field-Effect Transistor (FET) has only two pieces of material, one making a belt around the other. The FET is a voltage controlled device, whereas the BJT was a current controlled device. There are two types of FET's, the JFET and the MOSFET. The three terminals for this type of transistor are gate, source and drain. In operation, the voltage to the gate is varied to control the current from source to drain. By varying the voltage to the gate-source junction, the effective size of the gate is changed: - Increasing the gate size decreases the current flow from source to drain - Decreasing the gate size increases the current flow from source to drain

Diode

A diode is simply the main building block of semi-conductors. It is a small electronic device that limits current flow to one direction. There are two sides to a diode; the anode (the positive side) and the cathode (the negative side). To turn on a diode, or to "forward bias" it, the anode must be more positive than the cathode. To turn off a diode, or to "reverse bias" it, the anode must be "less positive" than the cathode.

Diode in a DC circuit

A diode will allow current flow in only one direction. When the positive terminal of a DC supply is connected to the P-type anode and and the negative terminal is connected to the N-type material cathode, the PN junction resistance becomes very low, and current will flow through the material. If the leads of the DC supply are reversed, the junction resistance becomes extremely high, so current is effectively zero. The voltage drop across the diode in forward bias can be measured with a meter. This should be approximately 0.7 volts for silicon and 0.3 volts for germanium.

LED

A light-emitting diode is a PN junction device that gives off light photons instead of heat. LED's are constructed with elements such as gallium or phosphorus instead of the more commonly used silicon. By varying the quantities of these elements during manufacture, specific colours can be produced. Red, green, yellow and infrared are the most common colours. LED's must have a resistor in series to limit the current flow since they are very sensitive to excess current. LED's are also sensitive to reverse voltage; reverse voltage generally can't exceed three to five volts. At about 1.5 volts (depending on the colour of the LED), the forward voltage drop for an LED is higher than for a silicon diode.

Zener Diode Voltage Regulation

A zener diode can be placed in parallel with a load to provide voltage regulation: - The zener voltage is chosen to match the voltage needed by the load - If the voltage from the supply varies above the rated load voltage, the zener will begin to conduct - There will be a voltage drop across the zener that is equal to the zener voltage. Since the zener is in parallel with the load, the same voltage will be seen by the load. Excess voltage is conducted through the zener If the supply voltage is below the zener voltage, the zener acts like a high resistance and has no effect on the circuit.

Zener Diode

A zener diode is a PN diode designed to operate as a reverse breakdown diode. Only when a large enough voltage is applied in the reverse bias direction will the diode "break down" and start to conduct. If voltage is continued to be applied, the diode will begin to conduct the voltage above the zener voltage. Zener diodes are designed to break down at a specific voltage. This break down voltage can be set during manufacturing and is usually displayed on the component packaging.

Integrated Circuit

An Integrated Circuit (IC) is a microelectronic semiconductor device consisting of many interconnected transistors and other components. IC's are constructed (fabricated) on a small rectangle (a "die") cut from a silicon wafer. This is known as the substrate. Different areas of the substrate are "doped" with other elements to make them either P-type or N-type materials, and polysilicon or aluminum tracks are etched in one to three layers deposited over the surface. The die is then connected into a package using gold wires which are welded to "pads", usually found around the edge of the die. Integrated circuits can be classified into analog, digital and hybrid (both analog and digital on the same chip). Digital integrated circuits can contain anything from one to millions of logic gates, inverters, AND, OR, NAND and NOR gates, flip-flops, multiplexors etc.- on a few square millimetres! The small size of these circuits allows for high speed, low power dissipation.

Diode packaging

Diodes come in all shapes and sizes. They are often marked with a type number. Detailed characteristics of a diode can be found by looking up the type number in a specification book. Diodes are polarized, which means that they must be inserted into the PCB the correct way. This is because an electric current will only flow through them in one direction. Diodes have two connections, an anode and a cathode. The cathode is always identified by a dot, ring or some other mark.

Doping

Doping is the process of adding impurities to intrinsic semiconductors such as silicon or germanium to improve their conductivity. Elements such as aluminium (Al), arsenic (As), gallium (Ga), phosphorus (p), or boron (B) are added to silicon or germanium to create materials that are either N-type or P-type. Both materials are electrically neutral (electron charge balances proton charge), but N-type material will allow easier movement of electrons (they have "spares") and P-type more easily accepts the free electrons (they have "holes")

Solid State Atomic Theory

Solid-state electronic devices are made from a category of atomic elements called semiconductors. The atoms of such elements have four valence electrons in their outer shells. When combined, adjacent semiconductor atoms share their four valence electrons with each other. This covalent bonding creates an effective outer ring with eight valence electrons. An atom with eight electrons in the outermost ring is stable and doesn't conduct electricity well. There are two main types of semiconductor materials: -Intrinsic, where the semiconducting properties of the materials occur naturally - Extrinsic, where the semiconducting properties of the materials are manufactured (called doping) Impurities are added to alter the resistive properties of the material to become a resistor, diode or transistor. These three electronic components are used to make nearly all electronic circuits.

Diode in an AC circuit

The PN junction diode in an AC circuit will allow current to flow on only half of the AC cycle. This will provide voltage to the load for the half cycle of conduction. The voltage drop across the diode in an AC circuit will be 0.7 volts during the half cycle of conduction and full voltage during the non conducting half cycle.

PN Junction Diode

The PN junction is the foundation of all electronic components, where N-type and P-type materials are joined to form the junction. At this junction, negative charge builds up in the P-type material, and positive charge builds up in the N-type material. This build up of charges creates a "barrier potential" (difference of potential) of approximately 0.7 volts for silicon and 0.3 volts for germanium. Forward bias means applying a DC voltage of the proper polarity to the PN junction (DC negative to the N-type and positive to the P-type). This causes the resistance of the junction to be minimal, and current will flow through the PN junction. If the power supply polarity is reversed, the resistance is extremely high, and the junction will not conduct current. This is called reverse bias.

Transistor

The transistor is a solid state semiconductor device used for amplification and switching. One advantage of transistors is their switching speed. A transistor can turn on and off billions of times per second. In essence, it has three terminals. A current or voltage applied through/across two terminals controls a larger current through the other terminal and the common terminal.

Trasistor Ratings

There are many important transistor ratings which give information about the capabilities of a transistor. This is especially important to know if there is a need to replace the device. These ratings include the following: - DC beta ratio of collector current to base current represents the gain possible - Vcc is the power voltage applied to the collector - Vce is the DC voltage measured between collector and emitter - Vcb is the maximum collector to base voltage - ICmax is the maximum collector current -Fmax is the maximum frequency that can be applied to the transistor - PD is the total power dissipation capability - BVceo is the maximum reverse voltage from collector to emitter

Transistor Configurations

There are three basic ways in which a transistor can be connected for common amplifier applications. The configuration is based on the connection to the base by the other terminals: - The common-emitter has a DC voltage between the base and emitter. This circuit is used when high current gain or voltage gain are needed - The common-collector has a DC voltage between the base and collector. This circuit has high current gain but low voltage gain - The common-base has a DC voltage between the base and emitter and between the base and the collector. This circuit is used where high frequency signals are used.

MOSFET Transistors

There are two types of MOSFET transistors: depletion and enhancement. The main difference between the two is in the construction of the source and drain. There is no direct channel between the source and drain in the enhancement type of MOSFET. When operating in depeltion mode, MOSFET characteristics are very similar to the JFET. A reverse bias voltage on the gate effectively increases the size of the gate, decreasing the current flow from source to drain. In the enhancement mode, a positive voltage is applied to the gate. This positive voltage forms the channel between the source and the drain, allowing current to flow. Varying this voltage changes the size of the source-drain channel, changing the amount of current that flows.