does HEMT work with positive or negative gate voltage? - Page 1
Forum section: electronics / beginners

author: fonograph
author_rank: Frequent Contributor
timestamp: April 27, 2017, 07:11:43 am
content: I have read on two places on internet someting that got me confused.They way I interpreted is that depletion mode mosfet or hemt can be operated with either positive or negative voltage at the gate.So its normaly on type of transistor,and if there is positive or negative voltage it will turn off so it doesnt matter what polarity the voltage is,it will modulate the gate either way.Is that how it really works? Can one transistor be really used with both positive and negative voltages at the gate? I was thinking all transistors need specific gate polarity to operate,so this was shocking to read so I am thinking I must got it wrong since I am noob.

author: fonograph
author_rank: Frequent Contributor
timestamp: April 27, 2017, 07:23:20 am
content: does there exist type of transistor that can use both negative and positive gate voltage?

author: fonograph
author_rank: Frequent Contributor
timestamp: April 27, 2017, 07:35:35 am
content: this is what i sawhttps://www.quora.com/Depletion-MOSFET-works-for-both-positive-and-negative-voltage-whereas-enhancement-MOSFET-operates-only-for-positive-gate-voltage-Why#!n=12

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2017, 07:45:31 am
content: Quote from: fonograph on April 27, 2017, 07:23:20 amSorry, that question doesn't make any sense to me.With N-channel MOSFETs, the impedance between the drain and the source remains high (assuming drain is positive, relative to the source), until the gate voltage is taken outside a certain threshold, relative to the source. The threshold voltage can be positive or negative, depending on the type of transistor. In a enhancement mode device, the threshold is positive and with a depletion mode device it's negative.To complicate things further: it's possible to operate some depletion mode devices in enhancement mode. For example, an ordinary depletion mode J-FET will be operating in enhancement mode, if it's gate voltage is 0.2V, but it can't be too high, otherwise current will start flowing into the gate, which is a diode junction. I suspect there may be devices specifically designed for both depletion and enhancement operation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2017, 09:48:53 am
content: An N-type (NPN, N channel) transistor always turns on more as the input (gate/base/grid) voltage rises (goes positive).The voltage required for cutoff (zero collector/drain/plate) may be positive or negative.P-type devices have the signs reversed.Tim

author: filssavi
author_rank: Frequent Contributor
timestamp: April 27, 2017, 10:10:20 am
content: Ad far ad Power Gran HEMT go only n-type existIn this case the gate voltages requires to turn the device on depends whether the device is enhancement or depletion modeFor enhancement Mode HEMT a positive gate voltage turns them on (typically no more than 6-7V)NonFor depletion devices to turno the gate in you have to apply a negative gate voltage to turn them off (-6/-7 volts typically)And to my best knowledge is physically impossible (I stand to be corrected obviously) to have a FET (HEMT are really mosfets with a fancy band strutture that created a channel with a heterojunction type structure) that turns in with either positive or negative? gate voltage, it might be theoretically possible to do with some exotic tyristor type structure but I'm not aware of a similar device being producerObviously It might be done with a multi device module

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2017, 10:26:19 am
content: Quote from: filssavi on April 27, 2017, 10:10:20 amCorrected.

author: TimFox
author_rank: Super Contributor
timestamp: April 27, 2017, 12:38:22 pm
content: Simply put, the difference between "enhancement mode" and "depletion mode" devices is that enhancement mode devices are OFF at zero gate bias, and depletion mode devices conduct at zero gate bias. In both cases, increasing the forward-bias voltage will increase the channel current, once the threshold is exceeded. Vacuum triodes are depletion mode, and can operate with positive or negative grid voltage, with substantial grid current flowing at positive grid voltage.

author: rfeecs
author_rank: Frequent Contributor
timestamp: April 27, 2017, 05:40:23 pm
content: Quote from: fonograph on April 27, 2017, 07:23:20 amYes.Although your question is not very clear.Most RF HEMTs have a Schottky barrier gate.When the gate is at zero volts (Vgs=0), the drain current is Idss.As the gate voltage goes negative, the drain current drops and eventually pinches off.For positive gate voltage the drain current increases.For higher gate voltage, the Schottky barrier gate will start to turn on and gate current will increase. Usually this is the limit of positive gate voltage. At this point the drain current goes to Imax.So the drain current goes from zero at pinchoff to Imax.The pinchoff voltage can be negative for a depletion mode HEMT or positive for an enhancement mode HEMT or it can be set by the processing to anything in between.For example, a HEMT with a pinchoff voltage of -0.5V can operate with a gate voltage range of approximately -0.5V to +0.5V.I would think that a MOSFET could be made similarly. That is, the Vt of a MOSFET can be controlled to be either positive or negative (enhancement or depletion mode). So if the threshold voltage was set to say -1V, then at zero volts it would be partially turned on, and at +1V fully turned on for example.

author: rfeecs
author_rank: Frequent Contributor
timestamp: April 27, 2017, 06:49:26 pm
content: Here's an example for the MOSFET case. Refer to the curves on page 5:http://www.mouser.com/ds/2/196/BSP135_Rev1.33-240630.pdf