# Physics Exam Thoughts (1 Viewer)

#### doggian

##### New Member
What did people say for 5, the MCQ about undoped silicon? Pretty sure the answer must have been D, the one that says equal numbers of positive holes and electrons move in opposite directions, but I said A, that more electrons move than positive holes, because there are both free electrons and valence electrons moving in undoped silicon

#### silverdawn565

##### Member
What did people say for 5, the MCQ about undoped silicon? Pretty sure the answer must have been D, the one that says equal numbers of positive holes and electrons move in opposite directions, but I said A, that more electrons move than positive holes, because there are both free electrons and valence electrons moving in undoped silicon
That question confused me a bit. I went with D... but I don't think there should even be any holes (or at least not too many) in undoped silicon? Since it isn't doped with Boron for example to create a p-type semiconductor?

#### Sp3ctre

##### Active Member
That question confused me a bit. I went with D... but I don't think there should even be any holes (or at least not too many) in undoped silicon? Since it isn't doped with Boron for example to create a p-type semiconductor?
There will still be holes in undoped semiconductors as long as an electron jumps from the valence to conduction band. The absence of that electron always leaves a hole behind in the valence band. I think once it's been doped, for example with a group 3 element then the majority of charge carriers will be holes over electrons.

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#### fluffchuck

##### Active Member
What did people say for 5, the MCQ about undoped silicon? Pretty sure the answer must have been D, the one that says equal numbers of positive holes and electrons move in opposite directions, but I said A, that more electrons move than positive holes, because there are both free electrons and valence electrons moving in undoped silicon
It should be D because electrons and holes exist in pairs, so there are equal amounts of electrons and holes.

#### silverdawn565

##### Member
There will still be holes in undoped semiconductors as long as an electron leaves jumps from the valence to conduction band. The absence of that electron always leaves a hole behind in the valence band. I think once it's been doped, for example with a group 3 element then the majority of charge carriers will be holes over electrons.
Oh, true. Thanks.

It should be D because electrons and holes exist in pairs, so there are equal amounts of electrons and holes.
That makes sense.

##### New Member
I'm sorry but what were they smoking for the copper skipping rope question

#### Fizzy_Cyst

##### Well-Known Member
What did people say for 5, the MCQ about undoped silicon? Pretty sure the answer must have been D, the one that says equal numbers of positive holes and electrons move in opposite directions, but I said A, that more electrons move than positive holes, because there are both free electrons and valence electrons moving in undoped silicon
Yeah, I agree with this.

Say 2 electrons get excited from VB —> CB, so you have 2 free electrons in CB, 2 holes in VB, but only way those holes will move is if an electron moves into it, so you would have 2 electrons moving in VB also :/

For it only being Q5 tho, I think they would want D, I don’t know how I feel about that.

#### doggian

##### New Member
It should be D because electrons and holes exist in pairs, so there are equal amounts of electrons and holes.
Free electrons and positive holes exist in pairs. But holes can only move by the movement of valence electrons, not free electrons

#### pikachu975

Free electrons and positive holes exist in pairs. But holes can only move by the movement of valence electrons, not free electrons
Yeah that means both free electrons and valence electrons exist so there is more

#### camstu

##### New Member
paper is out now. Think I'm looking at anything between 80-85 raw depending on how its marked. Is that enough to get a B6? For the proj motion question (29b) I did it all right but I thought that the 10ms-1 was horizontal (idk how I misread it that bad) so my answer was off but my process was right, will I lose 1 or 2 marks?

#### pikachu975

paper is out now. Think I'm looking at anything between 80-85 raw depending on how its marked. Is that enough to get a B6? For the proj motion question (29b) I did it all right but I thought that the 10ms-1 was horizontal (idk how I misread it that bad) so my answer was off but my process was right, will I lose 1 or 2 marks?
Probably lost 1 mark out of 3 I guess since the rest was okay. Also 80-85 would be enough for a b6 I'm just guessing b6 cutoff might be 78-80.