1. More on HgCdTe detectors for Astronomy M. Robberto

2. “P on N” design

3. Metallurgy BASE LAYER In-doped CAP LAYER undoped IMPLANT As-doped Gold STOICHIOMETRY (Hg 1-x Cd x Te) vs. DOPING (In, As)

5. Geometry ~10 micron~1 micron REMOVED

6. DLPH: Double Layer Planar Heterostructure - The CAP Layer has more Hg, wider band gap -Passivation is done with CdTe -Passivation needs protection (“overcoating”)

7. Heterostructure REMOVED In concentration ~10 15 cm -3 As is “ion implanted”

8. Built-in field REMOVED N-type Cathode P-type Anode -0.05V +0.05V ( => direction of dark current)

9. Reverse bias REMOVED N-typeP-type -0.250V Vsub Vreset 0.0V

10. Photogeneration REMOVED N-typeP-type -0.250V Vsub Vreset 0.0V

11. Photogenation FORWARD biases the junction REMOVED N-typeP-type -0.250V Vsub Vreset 0.0V

14. Reset

15. End of integration

16. The “unit cell equation” Differentiate: If :

17. Detector capacitance And it is immediate to calculate also the derivative.

18. Detector current I phot is negative because the convention is to assume Idark >0 under forward bias. HEAT GR DIFF

19. Diffusion current Diffusion is a slow process; Last only as long as the carrier lifetime.

20. Shockley Equation (this is A/cm2) depends on mobility and recombination time scales (delicate to control) depends on doping (easier to control) proportional to n i 2

21. Generation-Recombination current The Shockley equation neglects the current due to generation and recombination of charges in the depletion region. Both majority and minority charges are swept away by the electric field, become majority carrier in the neutral layer and generate a current that can be detected.

22. Generation-Recombination current (this is A/cm2) similar to the expression for the diffusion current. G-R proportional to n i Factor of 2 due to the detection of both majority and minority carriers term E 0 – E b accounts for the size of the depletion region (in reverse bias E b =-Erev)

23. Intrinsic Carrier Concentration From Hansen and Schmit (1983) for T>50K. a)Calculate n i for the four cases of Eg (Ex.1). Using ε=(20.5-15.5x+5.7x 2 ) ε 0, calculate b) W (depletion width) c) C jun (junction capacitance) Assuming V builtin =0, V bias =-250mV, N d =1E15cm -3, N a =1E19 cm - 3, and 18micron pix.size.

24. Other dark current sources Band-to-band tunneling Trap-assisted tunneling Surface region tunneling Surface leakage Other leakages a)Tunneling is independent on temperature. b)These phenomena dominate at low temperature.