//Transistors

Introduction to Transistors

A transistor is a semiconductor with 3 leads:

1. Emitter
2. Base
3. Collector

Transistors allow or restrict current flow with the application of voltage.

There are two types of transistors: bipolar and field-effect transistors (FETs). Unlike bipolar transistors, the leads of FETs are referred to as gate, source and drain.

Bipolar transistors can be divided into two categories: NPN and PNP. NPN transistors act like normally opened switches. PNP transistors act like NC switches.

Function

Transistors can amplify the current or can be used as switches.

Operating Values

These values vary and you must refer to the datasheets.

Basics

Bipolar Transistors are composed of three layers:

• An Input Layer, the emitter
• A Control Layer, the base
• An Output Layer, the collector

Layers can be N-type (negative) or P-Type (positive).

Layers are formed by doping silicon with traces of impurities. Silicon treated with Boron produces negative type material. It is negative because it has a surplus of electrons. Doping the silicon with Phosphorous produces positive material, or material with a surplus of electron holes.

Silicon transistors are considered semiconductors because they conduct only a small amount of current unless a change in voltage is applied at a juncture.

When current is applied to the input layer, or the emitter of an NPN transistor, electrons flow to the P-type base and fill the positive material, or electron holes. When the holes are filled, the material no longer conducts current. But when a small voltage is applied to the control layer, or base, that current will produce additional holes which in turn allows current to flow from the emitter through the base to the collector.



The difference between a PNP transistor and an NPN transistor is the polarity, or proper biasing of the junctions. Transistors restrict the amount of current passed according to a smaller, controlling current. The main current from collector to emitter (PNP) or from emitter to collector (NPN) depends on the type of transistor it is. The small current that controls the main current goes from base to emitter (PNP), or from emitter to base (NPN), depending on the kind of transistor it is.

The arrow always points against the direction of electron flow.


Bipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). Two types of charge carriers: electrons and holes, comprise the main current through the transistor.

All currents must be going in the proper directions for the device to work as a current regulator. The small, controlling current is usually referred to as the base current because it is the only current that goes through the base wire of the transistor.

The large, controlled current is referred to as the collector current because it is the only current that goes through the collector wire.

NPN transistor

• The base-emitter junction behaves like a diode.
  
  
• A base current I_B flows only when the voltage V_BE across the base-emitter junction is 0.7V or more.
  
  
• The small base current I_B controls the large collector current Ic.
  
  
• The collector-emitter resistance R_CE is controlled by the base current I_B:
  • I_B = 0   R_CE = infinity   transistor off
    
    
  • I_B small   R_CE reduced   transistor partly on
    
    
  • I_B increased   R_CE = 0   transistor full on ('saturated')
    
    
  
  
  

Transistors are not a good choice when switching large currents (> 5A). When switching large currents, you need to use relays.

Switches

If there is no current through the base of the transistor, the transistor shuts off the controlled current like an open switch and prevents current through the collector. A base current, turns the transistor on like a closed switch and allows a proportional amount of current through the collector. Collector current is primarily limited by the base current, regardless of the amount of voltage available to push it.

Night lights

Schematic

Materials and Tools

• BC548 or 2N2219 or 2N2906 transistor
• 470Ω resistor
• 1K Ω resistor
• 1 LED
• 1 LDR
• 1 10K potentiometer
• 9V battery
• battery clip

The BC548 is a general purpose silicon NPN BJT transistor.
Which transistor is the BC548:

Night light houses

Instructions

1. Once you have wired up your circuit, design an enclosure.
   
   

   ---

2. You can leave the circuit on on the breadboard or freeform solder it
   
   

/ / Muscle Wire

Self-folding paper from Jie Qi on Vimeo.