need a bit of help with mosfets please - Page 1
Forum section: electronics / beginners

author: KTP
author_rank: Frequent Contributor
timestamp: May 18, 2010, 07:40:42 pm
content: Hi guys,I have a lab assignment on mosfets for my class and one part is to construct a flow chart describing how to identify the characteristics of a 4 terminal unknown mosfet using just a ohmmeter. They want to identify the gate and body terminals, classify as n-channel or p-channel, and determine if it is enhancement mode or depletion mode. Sounds pretty easy but I want to run what I have by you guys and see if I missed something. Also, I made the assumption that there are no protection devices on the part (drain/source diode, gate protection diodes, etc.) to make things easier (not sure if I will get dinged for that, but it seemed reasonable).Ok, here goes:Step 1: Identify gate terminalDoes lead x conduct in either direction to any of the other leads?Yes: Change to lead x = x + 1 and repeat Step 1No: Lead x is the gate terminal, proceed to Step 2Step 2: Identify body terminalDoes lead y conduct in foward direction to both of the remaining leads?Yes: lead y is the body terminal, device is N-channel, proceed to Step 3No: does lead y conduct in reverse direction to both of the remaining leads?Yes: lead y is the body terminal, device is P-channel, proceed to Step 3No: change to lead y = y + 1 and repeat Step 2Step 3: Determine modeTie gate and body terminal together (to short the MOS capacitor)Do the remaining two leads conduct in both directions?Yes: Device is depletion modeNo: Device is enhancement modeI think my logic is flawed on the determination of depletion vs enhancement mode, but I don't have a 4 terminal mosfet of either type to test (crappy lab to not give us these). If you connect the gate to the body, then it would seem there would be 0V across the silicon oxide layer capacitor and thus if the device were enhancement mode there would be no conduction channel. If the device were depletion mode, then the channel would exist since there is no voltage to "quench" it. I am wondering about the drain/body and source/body diode junctions though, and how those might affect the ohmmeter measurements.thanks for reading all of this and any insight you guys might have!

author: thedigitalprincess
author_rank: Contributor
timestamp: May 19, 2010, 10:25:05 am
content: I know how to test BJT's using the diode function, but you say you're only using an ohmmeter? Hmm. *JUST* an ohmmeter?Eh, I made a drawing that might help you with your depletion and enhancement modes, but whether it actually does, it's a mystery to me!

author: Simon
author_rank: Unknown
timestamp: May 19, 2010, 11:52:28 am
content: the gate offers infinite resistance, the drain/source channel will offer infinite resistance until the gate is correctly energized, how are you going to do it with just an ohmmeter ?

author: KTP
author_rank: Frequent Contributor
timestamp: May 19, 2010, 01:28:46 pm
content: Quote from: Simon on May 19, 2010, 11:52:28 amBecause the drain and source form a PN junction (diode) with the body (substrate) and you can use the ohmmeter's small voltage to distinguish them from the gate which is insulated from all other terminals by the oxide layer.