High Electron Mobility Transistor (HEMT) or Modulation Doped FET (MODFET)

In a recent competitive exam that I appeared, there was a question on HEMT - of which I had no clue. It was equally disappointing and exciting - disappointing bcoz I lost two marks there & exciting because I have an opportunity to learn. That's one of the good things considering competitive exams - it gives pretty much idea about what you do not know! 

And the findings of my research on MODFET is here. In the end, I shall gives you the question that stuck me. Lets see if my research is sufficient for answering it !

HEMT / MODFET

HEMT comes under the category of high speed compound semiconductor devices. Its basically a field effect transistor (FET) incorporating compound material with different band gap forming heterojunction.

Commonly used materials for the fabrication of HEMT include GaAs with AlGaAs combination.Other combinations include InGaAs/AlGaAs, AlGaN/InGaN, AlGaN/GaN.

HEMT (GaN based) by Transphorm Inc

To brief the working of HEMT, we may first consider the band structure of GaAs/AlGaAs HEMT.

Band structure of GaAs/AlGaAs

Working in brief - We have a heavily doped wide bandgap donor supply layer and a narrow bandgap non-doped region forming a hetero junction. As observed from the band structure, the heterojunction form a quantum well where most of the electrons reside. Thus it is also called 2-Dimensional Electron Gas. Clearly due to the absence of donor impurities, the mobility of electrons is very high in this region and hence the channel has very low resistivity or "high electron mobility" . The term "modulation doping". is due to the fact that dopants are in a spatially different region than the current carrying electrons.

Applications : HEMTs possess a combination of very low noise figure (due to the presence of 2D electron gas and ability to operate at very high frequencies - thus widely used in RF design (including telecommunication, radar, radio astronomy etc). HEMTs are also used in small signal amplifiers, power amplifiers, oscillators and mixers at high frequencies upto 60 - 100 GHZ - and have shown current gains for frequencies greater than 600 GHz and power gain for frequencies greater than 1 THz. 

HEMT/ MODFET also possess high switching speed hence making them suitable for high speed applications.

Q. The question is an assertion reason type question :

Assertion (A) : A high electron mobility transistor  based upon modulation doped (GaAs - AlGaAs) single heterojunction.

Reason (R) : HEMT shows very high noise figure and very low gain at very high microwave frequencies upto 70 GHz.

(1) Both (A) and (R) are true and (R) is the correct explanation of (A).

(2) Both (A) and (R) are true and (R) is not the correct explanation of (A).

(3) (A) is true, (R) is false.

(4) (R) is true, (A) is false.

Got the Answer :):):) ???..