How transistors work

Transistors as Electronic Components

• Transistors are the most important electronic component in computing.
• Transistors are used as an electronic switch.
• They have helped computers reach their modern state.
• Automated computer devices need to control internal circuits by switching them on or off.
• Before semiconductors, vacuum tubes performed this function, but were large and inefficient.
• Transistors have allowed computer systems to be miniaturized.
• Transistors also have response times that allow for complex operations to be carried out quickly.
• The transistor was first invented in 1947.
• Operation has fundamentally remained the same, but transistors have been miniaturized since the initial invention.
• "Transistor" stems from "Transfer Resistance".
• Controlling current through a transistor is like controlling resistance.
• A transistor is a semiconductor device, like a diode.
• The main difference is transistors have three external connections, while diodes have two.

Bipolar Junction Transistors (BJTs)

• Two external connections connect to a circuit and allow current in and out.
• The third connection is used to control the flow of current through the transistor.
• BJTs have two diodes connected back-to-back.
• This leads to three distinct regions of doping, N-type or P-type, like diodes.
• Two P-type regions separated by an N-type region are PNP transistors.
• Two N-type regions separated by a P-type region, are NPN transistors.
• The three connections are known as the emitter, base, and collector.

Transistor Operation (NPN)

• The emitter connects to the low (or zero) voltage.
• The collector connects to the high voltage.
• The base connects to a voltage between the low and high level and controls the flow of current through the transistor.
• The doping of the two N-regions is not equivalent.
• The emitter region is heavily doped, while the collector region is relatively lightly doped.
• Without any voltage applied to the base, the collector-base junction is reverse-biased, blocking current flow.
• If the base is at the same potential as the emitter (e.g., low or zero), the base-emitter junction is not biased.
• Base-emitter region becomes forward biased when applying a high voltage to the base with respect to the emitter (e.g. 0.7V).
• High voltage leads to the electrons from the heavily doped emitter region flowing into the P-region.
• Since the collector region is relatively lightly doped, electrons sweep across the collector-base depletion region.
• Only a few of these electrons move out of the base connection.
• A small current flows through the emitter-base circuit.
• A large current flows through the emitter-collector circuit.
• The current flowing through the emitter-collector circuit is proportional to the current flowing through the emitter-base circuit.
• The current through the emitter-base is proportional to the bias voltage applied to the base-emitter connection.

Transistor Operation (PNP)

• Operation is analogous to NPN transistors, but with everything reversed.
• The emitter connects to high voltage.
• The collector connects to high (or zero) voltage.
• The base connects to a voltage between the low and high level, and will control current flow through the transistor.
• The transistor activates by forward-biasing the emitter-base junction.

NPN vs PNP Transistors

• NPN transistors are more common in electronics.
• The simplicity of circuit design regarding the voltages required for a forward-biased emitter-base junction, is one of the reasons why NPN is more common.

BJT Symbols

• Symbols for transistors are easily recognised.
• The arrow head always points between the base and the emitter
• The arrow head always points from the P-region to the N-region
• In a NPN transistor, the arrow will point towards the emitter.
• The arrow will point towards the base in a PNP transistor.
• Connections are given as letters in some schematics:
  • E = Emitter
  • B = Base
  • C = Collector

Field Effect Transistors (FETs)

• FETs control current through a semiconductor by applying (or removing) a restriction.
• This is similar to controlling water flow in a hose.
• FETs have a distinct channel through which current flows.
• The channel is surrounded by a region which can be enhanced or depleted to increase or decrease flow, respectively.
• Connections of a FET are source (S), gate (G), and drain (D).
• The source and drain connect to the primary circuit.
• The gate is used to control current flow through the transistor.
• Polarity of source and drain connections depends on channel doping (P-type or N-type).
• P-type for P-channel and N-type for N-channel FETs.
• Two categories of operation:
  • Junction Field Effect Transistors (JFETs)
  • Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)
• Both restrict the channel, but achieve this in slightly different ways.

Junction Field Effect Transistors (JFETs)

• JFETs control flow through the channel by applying a reverse bias to the gate.
• Without the reverse-bias voltage applied to the gate, the p-regions are in their minimum state.
• Current flow through the N-channel is at a maximum.
• To reduce current flow, the gate is then reverse biased.
• Applying a reverse bias causes the depletion region between the P-N junction to grow.
• This restricts flow through the N-channel.
• The current through the JFET is proportional to the current flowing into the gate.
• JFETs always operate in reverse bias, relying on increasing the depletion region.

JFET Symbols

• Arrow always points towards "N" and away from "P".
• For N-channel JFETs, the arrow points towards the gate and the channel.
• For P-channel JFETs, the arrow points away from the channel (toward the circuit).
• The arrow (head or tail, as appropriate) touches the gate in the schematic.

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

• MOSFETs have a similar overall layout to JFETs.
• The gate does not make electrical contact with the channel.
• Instead, controls the channel through an electrostatic field.
• When the gate has a positive voltage, electrons tend to pull through the N-channel and repel positive charges in the P-type substrate.
• Applying a negative voltage at the gate will tend to repel electrons from the N-channel and attract positive charges from substrate, enhancing the depletion region.
• The same operation occurs in reverse with a P-channel MOSFET

MOSFET Symbols

• Symbols are similar to JFETs.
• Disconnect between the gate and the channel.
• Symbol that indicates electrical contact between gate and channel.
• The arrow always points toward "N" and away from "P".

MOSFETs in Computing

• MOSFETs are the most common transistor used in computers.
• They are extremely energy efficient with minimal current flowing into the gate.
• This occurs since the gate operates on an electrostatic field.
• Susceptibility to electrostatic discharge (ESD) damage, is the drawback to MOSFETs.
• Precautions are necessary when handling electronic devices.