Transistor Diode Identification

Connect the right leads of a transistor to form a diode.

A diode is similar to a water valve in operation; when open it lets electrical current flow through a wire and when shut it prevents current flow. Transistors, because they contain two diodes, can be configured to operate like a diode. Just tie together the right two transistor leads and you are done. A transistor diode has the same electronic schematic symbol as a transistor. However, the transistor diode schematic will have two of its leads connected together. Be aware that there are two transistor symbols to identify, the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and the Bipolar Junction Transistor (BJT) symbol. Notation on these schematics will also indicate the type of MOSFET and BJT transistor.

Instructions

Required

• Transistor schematics
• Diode schematics
• Electronics textbook
• Electronic circuit schematic

1. 1
   Find a schematic for an electronic circuit. Consider asking an electronics manufacturer for a schematic. Application notes that manufacturers publish often contain schematics. Specifically, ask a transistor manufacturer (the applications or marketing department) for an application note that contains a schematic of a circuit that has transistors connected as diodes. Ask the manufacturer if there is a schematic available that contains both MOSFET and BJT transistors diode connections.
2. 2
   Look for the bipolar and MOSFET transistor schematic symbols in the schematic. Circle each transistor symbol. If you are not familiar with transistor symbols, examine the schematic symbols for MOSFETs and bipolar transistors in elementary electronics textbooks or look in resource 1 and resource 2. Understand that the MOSFET has a drain, gate and channel lead and that the bipolar transistor has a base, emitter and collector lead. Also realise that the MOSFET and the BJT have three leads. Know how to identify each of the leads for these two types of symbols. Know that there are two types of MOSFET symbols, a NMOS, also known as an n-channel MOSFET, and a PMOS, also known as a p-channel MOSFET. Know how to recognise the symbols for each type. Note that their symbols are very similar. A NMOS transistor has a small circle on its gate lead. Also note that there are two types of bipolar transistor symbols, the NPN and PNP. Know how to recognise the symbols for each. Note that their symbols are very similar. A NPN transistor has an arrow on the one lead that points outward. A PNP transistor has an arrow on one lead that points inward toward the base lead. Examine each circled bipolar transistor and check to see if the base and the collector leads are wired together or if the base and emitter leads are wired together. If they are, mark them as bipolar diodes. Examine each circled MOSFET transistor and check to see if the gate and source leads are wired together or if the gate and drain leads are wired together. If they are, mark them as MOSFET diodes. 

   
   
3. Mark the anode and cathode of each MOSFET transistor diode. Consider that a diode has an anode (positive) and a negative (cathode) lead. Identify each MOSFET as either a PMOS or NMOS MOSFET. The anode of the PMOS transistor diode will be the gate lead. Mark that lead with an "A" for anode. The cathode of the PMOS transistor diode will be the lead of the transistor that isn't connected to the gate of the PMOS transistor. Mark that lead with the letter "C" for cathode. The cathode of the NMOS transistor diode will be the gate lead. Mark that lead with a "C" for cathode. The anode of the NMOS transistor diode will be the lead of the transistor that isn't connected to the gate of the NMOS transistor. Mark that lead with the letter "A" for anode. 

   
   
4. Mark the anode and cathode of each bipiolar transistor diode. Identify each bipolar transistor diode as a PNP or NPN transistor diode. For PNP transistor diodes, the cathode of the PNP transistor diode will be the PNP's base lead. Mark that lead with a "C" for cathode. The anode of the PNP transistor diode will be the transistor's lead that isn't connected to the base of the PNP transistor. Mark that with the letter "A" for anode. For NPN transistor diodes, the base lead is the anode. Mark the base lead of a NPN transistor with an "A" for anode. The NPN transistor's lead that isn't connected to the base is the cathode. Mark this lead with the letter "C: for cathode.