Lecture 12. Semiconductor devices

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1 Lecture 12 Semiconductor devices

2 Intrinsic and extrinsic semiconductors The density of ac9ve carriers is for electrons and for holes, respec9vely. The product of the two is This product is independent of the Fermi level! Hence, the product of electron and hole carrier densi9es is constant (for a given band gap and temperature) no mager what the doping concentra9on is. ( ) T k E E o e o B e B F g e T m m m k n )/ ( 3/ 2 * 3 2 3/ = π π ( ) T k E o h o B h B F e T m m m k n / 3/ 2 * / 8 2 = π π ( ) ( ) T k E h e o B T k E T k E E h e o B h e B g o B F B F g o e T m m m m k e e T m m m m k n n / 2 3/ 2 * * / )/ ( 2 3/ 2 * * = = π π π π

3 2) a metal to a doped semiconductor

4 p- type n- type Donors Acceptors

5 p- type n- type Donors Acceptors

6 p- type n- type Donors Acceptors

7 p- type n- type Donors Acceptors

8 p- type n- type Acceptors Donors Charge x

9 p- type n- type Depleted of free carriers: Deple,on zone Acceptors Donors Charge x

10 p- type n- type Charge x ield x Poten9al U leads to energy difference eu A = B 2 x nd n k T ln ni

11 p- type n- type Charge x ield x Poten9al U leads to energy difference eu eu nd n k T ln ni A = B 2 x

12 Power supply U p + - U p =0V p- type n- type pu p eu

13 Power supply U p + - U p >0V p- type n- type pu p eu pu p Deple9on zone shrinks Pull electrons out of lev p- type side! Reduce barrier Conduc,on

14 Power supply U p + - U p <0V p- type n- type pu p eu pu p Deple9on zone grows Pull electrons out of right n- type side! Increase barrier Blocking

15 Get a diode

16 2) A metal with an n- type semiconductor connect gold to n- type silicon Vacuum level electron workfunc9on ΦM = 5.1 ev 4.05 ev electron affinity Φ S = χ S + ( E E ) cond F =- 5.1 ev Δ E =0.49eV +E d E g =1.12eV energy gap E D E D = shiv of from intrinsic Si value Gold Separa9on of Fermi levels in instrinsic Si Silicon

17 2) A metal with an n- type semiconductor connect gold to n- type silicon Vacuum level electron workfunc9on ΦM = 5.1 ev Φ S = χ S + ( E E ) cond E D F E g =1.12eV energy gap =- 5.1 ev E D = shiv of from intrinsic Si value Gold Silicon Electrons flow into the metal The Fermi level drops to the metal value. Electrons diffuse back and forth: the diffusion current

18 2) A metal with an n- type semiconductor connect gold to n- type silicon Vacuum level ΦM Φ S electron Φ = 5.1 ev workfunc9on M 4.05 ev ΦS = χs + ( Econd EF ) electron affinity E D =- 5.1 ev E g =1.12eV energy gap Gold E D = shiv of from intrinsic Si value Silicon The silicon side becomes more posi9ve The deple9on layer builds up on the Si side

19 2) A metal with an n- type semiconductor connect gold to n- type silicon electron workfunc9on Vacuum level Φ Φ eu M S p Φ M ΦS Φ B = χs + ( Econd EF ) E D U p >0V posi9ve bias Barrier smaller Gold Silicon The silicon side becomes more posi9ve The deple9on layer builds up on the Si side

20 2) A metal with an n- type semiconductor ΦM Φ S is key: connect gold to n- type silicon 1) >0 : ShoGky recaier 2) <0 : Ohmic contact Here: ShoGky rec9fier! Posi9ve bias on metal: smaller barrier and electrons flow Nega9ve bias on metal: larger barrier and electrons are blocked

21 2) A metal with an n- type semiconductor ΦM Φ S is key: connect gold to n- type silicon 1) >0 : ShoCky receier 2) <0 : Ohmic contact Here: ShoGky rec9fier! Posi9ve bias on metal: smaller barrier and electrons flow Nega9ve bias on metal: larger barrier and electrons are blocked

22 2) A metal with an n- type semiconductor Φ is key: M Φ S 1) >0 : ShoGky recaier 2) <0 : Ohmic contact electron flow electron flow posi,ve bias nega,ve bias Here: Ohmic contact! Posi9ve bias on metal: smaller barrier and electrons flow Nega9ve bias on metal: larger barrier and electrons are blocked

23 2) A metal with an n- type semiconductor depending on ΦM Φ S >0 get a ShoGky diode <0 or an Ohmic contact In a similar fashion the metal p- type contacts work, where electrons flow into the semiconductor and remove holes.

24 Example Aluminum on p- type Silicon The Si Fermi level rises to match that of Al. The bands bend downwards. Applying a nega,ve bias to the Al raises the metal side. Holes move into the metal and combine with electrons. Electrons from the metal flow into the empty hole states in the semiconductor. Applying a posi,ve bias to the Al removes electrons from the metal but at the contact. Holes are created in the semiconductor, which can be filled with electrons from the power source. Current flows as well.

25 Above the breakdown bias and below the knee the current vs. bias is NOT zero. Thermally generated electron hole pairs flow anyway. Further, the discussion above was about the majority carriers. They are the ones due to doping or electrons in a metal. There are minority carriers, i.e. from the intrinsic system. For them the opposite trend is observed, and they generate small leakage currents. Finally, the reverse breakdown happens very rapidly, a much sharper turn on in current occurs than at the forward diode knee. This Zener region is useful.

26 2) A metal with an n- type semiconductor 3) Deple9on regions Posi,ve bias ShoGky contact metal or Nega,ve bias The deple9on region increases with reverse bias. Electron- hole pairs which are thermally created in the driv region contribute to a small current, the drio current. The diffusion current, on the other hand, is the sta9s9cal mo9on of electrons across the barriers. The total current is the sum of both.

27 2) A metal with an n- type semiconductor 3) Deple9on regions gets Solar Cells In reverse bias, the deple9on region is increased. The band gap is engineered to be just below the solar light wavelengths of interest. Light excites electrons into the. Holes are lev in the. A bias builds that can power devices. Without an external load a bias builds up from the net excita9on and random losses. This is the open circuit bias. Typically the p- type side faces the light. It is very thin (1 nm) or transparent. Defects are very important.

28 2) A metal with an n- type semiconductor 3) Deple9on regions gets Avalanche photo diodes Here the photo- excited electrons are accelerated by a large external bias. These electrons can then generate more electron- hole pairs to trigger as current explosion, i.e. the avalanche. Typically the devices are operated near breakdown bias.

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