I'm the only developer at an early stage startup (think ~5 people). I have no problems with my current job or coworkers, but a much better offer has come along and I'm considering taking it.
How can I leave my current company without causing a major problem for them? I obviously don't wish them any ill will or want to burn any bridges.
First things first, you should probably read your contract and make sure to give the required amount of notice with regard to leaving (if this is stated there, if not, ~2-4 weeks should be the absolute minimum amount of notice to give) and make sure you won't be violating any other terms if you leave.
In theory, ensuring that they can survive without you is not your problem and this shouldn't significantly affect your decision to stay or go. However, they may hold leaving against you (goodbye references..., possibly), especially if you made some verbal or implicit agreement to stay or at least not leave them in a tight spot (i.e. make some arrangements (see below) before leaving so someone can easily take over). There very well may be an implicit agreement if you're part of a start-up with around 5 people, especially if you're the only developer.
Here are a few things you can do to make it easier on them / leave on better terms:
Give / offer a longer notice period.
Assuming your new employer would be okay with you starting a bit later, you could either give longer notice than required (e.g. 4 weeks instead of 2, or even significantly more, if possible), or you could give your required notice and point out that you'd be willing to extend leaving a bit if so desired (although employers may be less inclined to take you up on this one).
Document everything.
You could consider spending a large amount of the time from now until you leave documenting as much as possible in order to allow a new employee to get up and running without your assistance.
While doing development or bug fixes during this time might seem more productive, documentation might make it easier for a new employee to get started, so could be more helpful in the longer run.
You should probably discuss this with your higher-ups if applicable.
You might even consider putting in some additional hours during this time to get more of this done.
Keep the door open for communication after you leave.
Tell them you'd be perfectly willing to answer any questions a new developer might have via phone or email after you've left.
Offer contract work.
If you're up to it, you could tell them that you'd be willing to put in a few hours after hours and on weekends after you've left to tend to any critical issues, or to assist a new developer, for a short time.
Refer someone.
Good developers are hard to come by. If you know someone who might be good for the role, consider recommending them.
Help with the interviewing.
Judging whether a developer is good or not can be difficult for someone not in the industry. It could help them a lot if you help here.
And you could also:
Consider a counter offer.
Since your motivation appears to be largely based on money and/or perks, I imagine you'd be willing to accept a counter offer that matches these.
But you should be careful not to come across as bringing up the other offer just to get more money out of them.
See my related answer regarding counter offers.
Ask for a raise.
If it's just about money and/or perks, asking for a raise should be a consideration.
See this related post, for example - in short, you should highlight what you bring to the company. I suggest avoiding bringing up the other offer, although you could mention how big raise you'd like (or slightly higher, as per usual negotiation techniques), and mention that you think this would be you more in line with the market (assuming that's true enough).
Ask for stock options, partial ownership or similar.
Keep in mind that start-ups tend to have below market salaries and may not have the cash flow to give higher salaries.
One way to make up for this would be to offer stock options to employees.
This could end up with you scoring big time if the company succeeds.
If you're the only software developer in an early stage software company, one of two things is true:
The way you announce your decision to leave needs to be based on your stake in the company.
If you do have a big stake, and you've decided rationally that your new offer has greater potential than this company, that's a very consequential decision. You've decided that the present early stage company isn't worth any further investment of your time. That most likely has been a painful decision. When you announce it, your colleagues and possibly the investors may try to change your mind. You should be open to that.
You should also be prepared for them to be upset: you are likely to be the person confronting them with an unpleasant truth (their company isn't going anywhere).
If you don't have a big stake, get going! Tell your new place you need four weeks to wrap up your present work. Give the present place four weeks' notice, and do your best to finish your work and help hire someone to replace you.
In either case thank them profusely for the opportunity to work with them, and wish them the best.
Many areas have local employment laws that could get involved. It doesn't sound like your leaving or efforts the company may make to keep you violates any of them.
Since you do want to make this right and not cause any problems, think about what their expectations were when they made you an offer. Did you give a strong impression you were there for the long-haul?
Start out with making some offers to stay on maybe more than any customary amount of notice time. Try and open the door to find solutions they may come up with. It could be documentation, reviewing CVs, finishing up that one difficult or crucial part of the app before you leave. Maybe they would like to hire you as a consultant once they get someone onboard to help out if needed.
It sounds like to want to be open to suggestions, so just ask for them. You have a better opportunity, if anyone wants to hold that against you, put in your notice and move on.
